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Part 4 of The Popular Science .Educator will be on sale next Thursday October 24thT may be interesting to discuss theplan of the POPULAR SCIENCEEDUCATOR so that readers who areta king it in order to give themselvesan all-round kn owledge of the sc iencesmay know how the course will proceed.The Physiography section beginsnatural ly wi th Astronomy, describingth e uni verse at large, the so lar system,and the va rious planets that are th eEarth s companions in Space. H ere alsowe learn about the Moon, comets,meteorites, and distant uni verses whoseexistence has only become known inrecent years.The section will then go on to dealwith Geology, th e structure of th eEarth, its rock formations and-minerals,and lead up to Physical Geographydescribing the world as it is today. Weshall learn a ll ab out the land and th esea, the mountains and volcanoes,earthquakes a nd cur rents, and then wesha ll come to Meteorology, th e story of

t he weather. This will t ell us aboutwind s and storms and rain and snowan d ha il and lightning and thunder,a nd climate generally.The Physics section began with theconstitution of matter, and is now

dealing with the subject of H ea t . I twill go on to L ight and it s m ys t eries,Sound and music, and th ose fascinatingsciences of Magneti sm and E lectricitywith th eir developments in the modernworld .THE BOY'S BOOK OF WONDERAND INVENTIONAnybody who wants to give anintelligent boy a present that he willthoroughly enjoy and will getpleasure from for months together,should give him a copy of theEditor s new six shilling book,THE BOY'S BOOK OF WONDERAND INVENTION, which is packedfull of illustrations, explanatorydrawings, thrilling scientific articles,and experiments that can be performed in the home. It is cer ta inlyone of the cheapest six shillings 'worth ever provided.In the Chemistry section we arestu dy ing th e va rious elements one byone and learning how th ey combine t oform compound substances, many ofwhich are of the ut most importance toma nkind. Vve sha ll learn all about acids

and a lkalis and so lids, metals and nonmetals, va lence an d ions, and we sha llbe shown how important the chemist isin modern industry.The Bio logy section will lead up fromlow ly forms of life to the mo re complex

creatu res in both the n i m ~ and plantkingdoms. We shall learn how li vingcreatures reproduce th eir kind and allthe wonders of the hu ma n body withits structure of bone and its networkof nerves a nd blood vessels, an d itsmuscles t o move th e va rious pa rt s.n th e Mechanics secti on th e principles on which mo dern machinerywork s will be described and illustrated.We shall see the va rious important lawswhich -have to be observed and how byi11gen iou s dev ices man is ab le t o ca rryout t asks that otherwise would beimpossible to him. In th is section weshall have explained t o us by largedo uble-page drawing;; such complicatedmachines as locomoti ves and motor

cars and marine engines and pr intingmachines, and so on.Throughout, the book will depend asmuch upon its illustrations as upon itsletterp ress for making, clear to th ereader th e great wonders of science.

HERE RE SOME OF THE PRINCIP L FE TURES IN P RTTHE MARVEL OF COLOUR THAT IS FOUND IN THE WONDER OF WATERTHE ANIMAL WORLD A full account of this important sub stance 1th outA fine plate in full colour showing how br illiant are the which none of us could live for more than a day or two .hues of some of the creatures in every department of We learn about it s ch emical composition, its behaviourthe an im al kingdom . We see the magnificen t macaw . under va rying circumst ances, the strange facts abouta brilliant butterfly of P olynesia and , in additi on , its fr eezing, and why the sea keeps our climat e eq uab le.rich ly co loured fish , and sea anemones of th e Britishwaters .THE SIMPLEST OF ALL ANIMALS AND PLANTSAn in teres ting account of some very lowly creatureswhose bodies consist of only a sing le cell, with adescription of how th ey feed and mo ve.THE AMOEBA LIVES ITS STRANGE LIFEA full-page drawing showing th e life st ory of anamoeba, th e simplest of all an imal forms.THE GREAT CARBON CYCLEA full-page drawing, .showing how plants breathe incarbon dioxide gas, se ize th e carbon in it with wh ich tomake their t issues, and give off th e oxygen. Th e oxygenis then taken in by anima ls and used for burning theplant sub st ances th ey ta ke as food in th e course ofwhich carbon dioxide is formed and brea th ed out .

'THE EARTH S NEAREST NEIGHBOURA graphic account of the Moon an d its scenery with adescript ion of its giant craters and the strange condi tions that reign on the Earth s sa t ellite. Illust rated.A GIANT JUMP ON A LITTLE WORLDA fine full-page illustration combining photograph y anddrawing and show ing how athletes could multiplytheir records by six if they lived on th e Moon.

HOW PURE WATER IS PROVIDED FORGREAT POPULATIONS OUR

A fine double-page drawing showing th e whole story ofwater supply, its purification a'na storage.THE INCLINED PLANE AND HOW IT COMES INTOEVERYDAY LIFEThe story of t his import ant mechanical dev ice with itsdevelopments in the wedge and t he screw. Pi ctu rediagrams a re given showing th e enormous advantageth at it gives to man in th e ra ising of heavy loads or theap plication of great pressure. Illustrated .MANY EXAMPLES OF THE INCLINED PLANEA double-page of drawings showing the different waysin wh ich this device is ut ilised in everyday life.THE HEAT THAT MYSTERIOUSLY VANISHESAn account of how things are made t o burn, with an

explanation of ignition point , why some things burnmore easily than others, slow and rapid combu stion,and how different sub stances absorb and hold differentquantit iP s of heat. Illustrated.THE MANY DIFFERENT WAYS IN WHICH MANMAKES FIREA full page of draw ings showing some of th e inge\iifousmethods by which fire is produced in different partsof th e world today.

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THE GLORY OF THE SUN WHEN E LIPSED

T he Sun throws out from i ts surface vast flames of incandescent hydrogen gas, and these flames rise up to a great heightsometimes half a m i ll ion miles. T hey are best observed during a total eclipse but they can also be examined and photo -graphed by means o f an instrument called the Spectrohel iograph even when the Sun is not eclipsed. In addition the Sunwhen total ly ec l ipsed is seen to be su rrou nded by a hal o of lig h t. It i s of a pearly tinge and contrasts strongly with the scarletflames or prom i nences. It is m ade up of fil am ent s w h ic h d iverge l i k e rays and ar e in some cases intertwined. T he natureof t h is co rona is not defini tely kn ow n. T h e pa rt s remote from the Sun are believed to be due to reflected l ight on pa rt iclesof m att er p ro bably elect r ons, bu t nea r er t h e Sun 's surface, wh er e th e corona is b righter it is thought the ight must be dueto in candescent gas. Som e scien tists think t hat t he corona is of electri cal origin an d is l ike a vast auroral disp lay


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CHEMISTRY . The Science of the Elements of which all the matter in - theUniverse is made upThe Wonders of COMBUSTION and CHEMICAL COMBINATION

THE COMMONEST OF ALL ELEMENTSMore than half the Earth's crust, a third of all the water in the world, and a fifth of theatmosphere is made up of oxygen gas, an interesting account of which is given hereW E have seen that when an electriccurrent is passed throughwater, the water graduallydisappears and in its place two gasesare produced the volume of one .beingtwice as great as that of the other.The gas of the double volume ishydrogen, about which we read onpage 55, and the other gas is oxygen,which is nearly sixteen times as heavyas hydrogen.This gas, oxygen, is perhaps the mostwonderful of all the elements. As Mr.Ellwood Hendrick says, 'The heathen

in his blindness bows down to woodand stone,' but if he had .studiedchemistry he surely would haveworshipped oxygen. We can im;i,ginehim engaging in -genufiexions beforean effigy with wings to indicate theair, and the tail of a .fish to show forthe water and withas many otherattributes as thedevout sc u l p t orcould affix to it .Oxygen is, infact, the substancethat gives us lifeand without whichwe should perish ina very fewminutes.t is the commonest of all the e ements, formingone half of thecrust of the Earth,one-fifth of theatmosphere, andone third bv volume, but not 'byweight, of all thewater in the world.We breathe it inas an elen1ent, we breathe i t out asa compound in the .carbon dioxide gaswhich all animalsexhale ; we drink.it as a liquid, and eat

large quantities of it when we consumeour foods which are made up largely ofoxygen gas.More than 3 5 per cent of a loafconsists of water, 87 per cent of milkis water, 73 per cent of an egg, 53 percent of mutton, 19 per cent of beef,62 per cent of bacon, 70 per cent of afresh herring, 35 per cent of a bloater,34 per cent of cheese, 78 per cent of apotato, 90 per cent of a cabbage, 82 percent of an apple, 74 per cent of abanana, 95 per cent of a cucumber,and nearly per cent of a nut. And ofany quantity of water it must. be

remembered that eight-ninths byweight consists of oxygen." t appears from researches madein physiological laborafories, says Mr.Hendrick, t ha t oxygen plays a veryimportant part in what we call themystery of sleep. Although we continue both asleep and awake to inhalethe free oxygen and to exhale oxygenthat has done its work of oxidationand is in combination with carbon ascarbon dioxide, there is a difference inthe comparative amounts of oxygeninhaled and exhaled a t such times.\Vhile we are awake it seems weexhale more .oxygen than we inhale.When we sl.eep we inhale more thanwe exhale. We deolete the store awakeand increase it while we sleep. Nowwe cannot live without it. t providPs,by its reactions, by processes ofoxidation within us, for bodily heat andfor the chemical processes of life. Howneedful it is we are reminded when

Making oxygen gas by heating chlorate of p'otash and black oxide ofmanganese-. The potassium chloratebecomes potassium chloride, givingup its oxygen which is co lec_ted underwater. On the right a piece of char-coal is seen burning with a bri l l iantf lame in a ja r of oxygen

we consider how quickly' we aredrowned.In the history of torture one ofthe most cruel methods of puttingculprits to death was simply to keepthem awake until they died. Withoutsleep they could not make up for theoxygen lost, and so they died of oxygenstarvation.What kind of a substance is thiswonderful oxygen on which so. muchof our welfare and our very existencedepends? \Vell, it is a colourless,-61

odourless and tasteless gas slightlyheavier than air and nearly sixteentimes as heavy as hydrogen __ 159 tobe exact. At a temperature of rrS-8below freezing point, that is, at -1188centigrade and under a pressure of497 atmospheres, it becomes a paleblue liquid with magnetic properties.v cooling the liquid oxygen stillfurther with liquid hydrogen the gasbecomes a pale blue snow-like solid.This solid melts at -219 centigradeand boils at - 1825 in an_ open vesselat ordinary atmospheric pressure.

Oxygen is slightly soluble in water,a fact that is of great importance, forfish are dependent for their supply uponthe dissolved gas,being unable tobreathe it in fromthe air as do mammals and birds.All these are thephysical propertiesof the gas, but itis its chemical properties that makeoxygen of suprnmeimportance. withone or two exceptions it will combine with all theother e I em en t s,forming what areknown as oxides.The process of combining is calledox ida t i on andsometimes t h i stakes place rapidly, as when weburn magnesiumwire and a powderwhich is magnesium oxide is formed, while at othertimes it takes placeslowlv, as when aknife- blade or aniron fender or ad o o r - k e y getsrusty. The red rustis reallv oxide of iron or, as chemistscall it, ,ferric oxide.

Generally oxidation is .accompaniedby heat and light. We notice this inthe burning of the coa.l_in the grateand the combustion of tbe food in ourbodies, which is really oxidation andis the source of the warmth of the body.There are various ways of preparingoxygen gas. We have seen on page 59that it is obtained when water is brokenup by having an electric current p s s ~ dthrough i t ; and on page 15 that it isc


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THE OMMONEST ELEMENTreleased when mercuric oxide or. redprecipitate is heated.A cheaper way of obtaining largequantities is by heating potassiumchlorate, when the whole of the oxygenis given off and potassium chloride isleft. Chemists express the action bythe equation :

2 K Cl 0 3 = 2 K Cl 3 0 2K is the initial of kalium, the Latinname for potassium, and the equationmeans that two molecules of potassiumchlorate yield two molecules of potassium chloride each containing one atomof potassium and one of chlorine, andrelease three molecules of oxygen eachconhining two atoms of that gas.The chlorate is known as a salt, andits white crystals melt at 357 centigrade. At 380 bubbles of oxygenappear. By mixing with the potassiumchlorate about a quarter of its weightof black manganese dioxide the production of the oxygen is greatlyaccelerated. The manganese dioxideundergoes no change ; it merely accelerates the speed of the chemical actionof the potassium chlorate and enablesit to take place at a temperature below200.As manganese dioxide is sometimesadulterated with organic matter, whichmay. cause a violent explosion, itshould be tested before attempting toprepare oxygen, by heating a verysmall quantity in a test tube.There are other ways of preparingoxygen in the laboratory, as for exampleby allowing water to fall upon sodiumperoxide contained in a flask. Oxygenis released and sodium hydroxide left.But for the preparation of oxygenon a large scale for commercial purposes it is obtained either by the

oxygen is set free and passed into theair, where it is breathed in by animals,while the carbon is absorbed by theplant and used in its growth.

If the oxygen in foe .itmosphere were notdiluted with nitrogen gas, not only the fire butthe stove itself would also burn

The chemical change which takesplace in an animal s body throu ghthe action of the oxygen is ictenhcalwith that which goes on when charcoale e c t r o y s i s ofwater, which is expensive, but givesa pure product;or by liquefyingair, 1e t t n g thenitrogen evaporatefirst, and then collecting the oxygenin cylinders. Thisis e a s y because,w h i 1 e nitrogen sboiling point, thatis, the temperatureat which it changesfrom the liquid tothe gaseous form,is - 194 that ofoxygen is -1825.t is interestingto know that anatural process bywhich oxygen is

When a coal fire s burning brightly rapid combustion s goingon but combustion s going on -equally in the coal cellar amongthe loose coal. In that case however the combustion s veryslow and the heat generated is carried off by the air so that thecoal does not catch fire

prepared is seen in the plant world.The gas is set free by the action ofsunlight upon carbon dioxide gas contained in the air. This is done bymeans of the green colouring matterof plants. Sunlight has the power inthe presence of this green colouringmatter of breaking up the carbondioxide breathed in by the plant. The

is burned in oxygen gas or in the air,which is merely diluted oxygen.A simple experiment will prove this.Into a jar of oxygen in which a pieceof charcoal has been burned pour someclear lime water This will becomemilky owing to the formation of chalkby the combination of the lime in thewater and the carbon dioxide formed62

by the combination of the oxygen andcharcoal.Now pour some more clear limewater into a clean glass jar and breatheinto it through a straw. The waterwill become milky in the same way.The carbon dioxide breathed outthrough the straw will combine withthe lime and form chalk. f the jarsare left for a time the chalk particleswill sink to the bottom.Just as the getting of atoms ofoxygen into a chemical compound isknown as oxidation, so the getting ofatoms of oxygen out of the compoundmolecule is called reduction. Thesmelting of iron in a. blast furnace isreduction being carried out on a largescale.At one time there must have beenmuch more free oxygen in the worldthan there is now, but as time went onall the silicon and most of the metalswere oxidized. t is interesting to lookround and see the substances which arealready oxidized and those which arenot. The former will not burn as theyhave already combined with as muchoxygen as they can possibly take, butthe latter are all able to burn, that is,at a certain temperature to combinewith oxygen.Earth, sand, chalk, granite areexamples of substances already oxidized, while paper, wood, coal and fatare examples of substances waitingto be oxidized, that is, burnt.t must be explained at this pointthat though oxidation is combustion,all combustion is not necessarilv oxidation. Combustion in the true scientificsense is any act of chemical combination which is accompanied by the generation of heat and more or lesslight, and oxygen need not necessarilytake part in it. Thus chlorine gas willburn in an atmosphere of hydrogen,p r o d u c i n g bothheat and light, andf o r m i ng hydrochloric acid.

t was the Frenchchemist Lavoisierwho first gave atrue explanation ofcombustion. Beforehis time the Phlogiston theory wasb e 1 e v e d by allscientists. When afire burns flamesare seen leaping up,and this suggestedthe escape of somet h i n g . Men ofscience b e l i e v e dthat a l l bod ieswhi ch b u r n o rundergo changes like rusting containa substance which they called phlogiston, from a Greek word meaning toset on fire. When charcoal heatedin air burnt away leaving very littleash it was supposed that this substance was nearly pure phlogiston.Then in 1774 Joseph Priestley, anEnglish divine, discovered oxygen by


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heating red precipitate or mercuricoxide confined in a glass vessel overmercury, by means of the Sun's raysconcentrated by a burning- glass.What surprised me more than I canwell express, says Priestley, wasthat a candle burned in this air with aremarkably vigorous flame. He couldnot understand his discovery, but calledthe gas dephlogisticated air, that is,air deprived of its phlogiston.Lavoisier, however, began to studyoxygen and soon found that it was oneof the constituents of the atmosphere.He named it oxygen, which meansacid-producer, believing that it was a

oxygen a peculi.ir odour resulted, andthe substance produced, which wascapable of oxidizing various substances,he named ozone, which means to smell.t has since been found that ozoneis only another form of oxygen inwhich the molecule contains threeinstead of two atoms. t is a blue gas,and can be condensed to an indigo-blue

liquid. t is much more soluble inwater than is oxygen, and is used forpurifying water supplies and the air oftube railwavs.t is rea ily a more active form ofoxygen and is used for bleaching oils,waxes, ivory, flour and starch. t is

0 ' ' ""0 c ' 0 ..0

00 x q _ q ~ n ~ a s i n q into o

0the air throuqh stomata-' rmr;uths o leaveso

constituent of all acids. In that,however, he was wrong, for someacids, like hydrochloric acid, haveno oxygen.Oxygen is used for many purposes-for artificial respiration,for disposing of sewage and theoxidation of waste matter ingeneral, for welding metals bymeans of the oxy-acetylene and oxyhydrogen blowpipe, for the oxidationof linseed oil, for the blast furnaceand steel converter, and so on. Hundreds of tons of oxygen are producedevery day in Europe and America forcommercial uses.Oxidation may take place so slowlyas to escape detection by ordinarymeans, or it may happen very rapidly,and be accompanied by an explosion.The speed of the oxidation dependsupon many factors, such as temperature, fineness of material, concentration, and so on.In 1785 a . Dutch chemist, VanMarum, noticed that when an electricspark was passed through air or

prepared on a large scale by the dis-charge of electric waves in oxygen orair. In Nature, ozone is formed in theupper dust-free atmosphere by theaction of the ultra-violet rays onoxygen, and it is also produced insmall quantities during thunderstormsby the action of the lightning on theoxygen in the air. This is due to theaddition of energy to the oxygenmolecules. Any ozone that finds itsway into the lower atmosphere, however, is soon changed back into ordinaryoxygen by giving up its energy owingto contact with the minute dustparticles. Whenever ozone changesinto oxygen energy is liberated so thatozone is really oxygen plus energy.

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THE OMMONEST ELEMENTt is sometimes said that there isozone in sea air, but if this is so thequantity must be very small indeed.In describing the preparation ofoxygen from potassium chlorate onpage 62, it was mentioned that thechemical action was helped by adding asmall quantity of black manganesedioxide to the potassium chlorate. A

chemical thus added, not for the purposeof changing itself, but to assist thechemical action of other bodies, isknown as a catalyst, from a Greek wordmeaning to loosen.The action of a catalyst is curious andinteresting. Mr. Ellwood Hendrick

The green leaves of plants are greatmanufacturers of oxygen. Theybreathe in from the atmosphere thecarbon dioxide gas which has beenbreathed out by animals and give 1off by fires. Then by means oftheir green chlorophyll or o l o u r ~ n gmatter acted upon by the Sun theyextract the carbon to build : JP theirtissues and breathe out the oxygenthrough little mouths called stomata,as shown in this drawing

has described it ratheramusingly. Let ustake, he says, twobodies that accordingto all the rules of ourreasoning should combine when webring them together. They should,but they don't. Of course, there is areason for this, but what we are afteris the reaction.

f you strike a match the reactionwill begin from the heat of the rubbing.Hold the match to the prepared surfaceand nothing will happen. Heat makesthe atoms move around in the moleculeat a livelier pace. They seem to swingin a larger orbit and are more easilycaught up by some other matable atomthat is swinging round in its molecule.But in these cases nothing happens.We have, let us say, two bodieswhich should combine, but don't,dissolved in water in one vessel. We


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THE COMMONEST ELEMENTheat it and shake it and still nothinghappens. Then we add the catalyst,which is a foreign body, and to allappearances it has no relation to whatwe have or the combination we want ;nevertheless, as soon as. it is added,sometimes even in an absurdly slightamount, presto the solution straightway froths up, there is a grand commotion, and a reaction involving everymolecule in the solution takes place.

What we wanted to happen doeshappen, and the little catalyst may befound at the bottom of the beaker justas it was when we put it in, to allappearances, chemically and otherwise

oil lamps all use up the oxygen of theair and give out into it carbon dioxide.So in a matter of ventilation these haveto be taken into consideration.One of the difficulties of mountainclimbing at great heights is the rarityof the atmosphere and consequentlythe diminution of the supply of oxygento the body. Those who climb to greatheights or rise very high in openballoons suffer from what is known asmountain sickness. This is due chieflyto the cutting down of the supply ofoxygen.Men who are climbing and therebyusing much energy need a greater

atmospheres of pure oxygen. At suchhigh pressures the blood contains abouttwenty-eight volumes of oxygen to each100 cubic centimetres of blood, insteadof twenty volumes, which is the usual.The additional eight volumes of oxygenare contained in solution.Fish are killed when the oxygenpressure in the water they swim in isincreased to the extent of ten volumesof dissolved oxygen in each 100 cubiccentimetres of water.The demand for . oxygen varies agreat deal in different creatures. Mammals and birds must have a constantsupply, and if it is cut off they become

Many substances l ike those shown here will not burn because they are already burnt that is are already combined with oxygenunchanged. There are volumes andvolumes written about this, and theretre theories galore, but I shall notdevelop them, The fact is the catalyst,which is the active agent in the processof catalysis, behaves generally like ahuman trouble-maker.

t is interesting to know that anaverage person breathes into his bodyin twenty-four hours over 346 cubicfeet of air, or nearly thirty pounds, andout of this air the lungs take up dailyjust over twenty cubic feet of oxygen,or twenty-nine ounces.Now as at every breath some oxygenis taken from the air and a quantityof carbon dioxide gas given back to it,

supply of oxygen than is normally thecase. t is true that the atmosphere,one-fifth of which is composed ofoxygen, supplies us with an excess ofthat gas over the actual needs of thebody in normal circumstances, but inperforming heavy muscular work as,for example, in climbing, the musclesuse up more oxygen, and if the workeven at sea level is heavy and is maintained for a considerable time, thequantity of oxygen supplied by the airmay be insufficient. High up in themountains, w h r ~ the supplyis less than normal, it bee o mes insufficient, eventhough the rate of respiration

e ves

rapidly exhausted and very soonperish. The lower backboned creatures, however, can survive asphyxiafor many hours if. they are kept inplaces of low temperature.Cats apparently must have an adequate supply of oxygen or they die.There are no cats, it is said, living atheights above I I , 5 00 feet. f they aretaken into the lofty villages of theCordillera Mountains of America, theyvery soon become dejected. Thenthey have convulsions like those ofepileptic fits and finally die.On the other hand, the condorsapparently need less oxygen, for theyfly from the sea level to the tops of the

Substances l ike those shown here are combustible that is they will burn readily because they are able to combine with oxygenif the atmosphere around a person werenot renewed he would at last besuffocated because he would be unableto get from the air the oxygen herequired. In other words, he would dieof oxygen starvation. His death wouldbe of exactly the same kind as if hishead were- placed under the receiverof an air-pump and all the air aroundhim removed.The amount of fresh air required ina given room is determined by thenumber of persons who are gatheredin it. A supply ample for one personwould naturally be insufficient fortwo. Of course, fires, gas-burners, and

be increased so as to take in more air.Too much oxygen may act as apoison, and experiments have shownthat the maximum th


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THE STORY OF OZONE GAS AND ITS VARIOUS USES

Ozone ,gas, which We often hear exists at the Seas.id_e morethan elsewhere, though this is doubtful-, is r ~ l l y a formof oXygen, but instead of having two atoms in the molecule,as is the case with o?'ygen, ozQne has three _ l t is -formedf r ~ m oxygen by _ultra-violet rays f rom the 81; m iri the_upper atmosphere, and in the- lower, atmosphere bylightning. Ozone molecules have more energy than thoseOf ox.Ygen; but when theY come in coiitact_ with_ dust'particles. in the lower air they give up thei r energy andbecome oxygeti molecules. Ozone cart be prepared_a rtifi-cially . by PaSsing an 'electric disch_arge throughoxygen in an apparatus l ike that shown at the bottom.This has two concentric tubes, the inner tube being coatedOn its inner surface .with t infoi l , and the outer tube-c:oatedon its outer surface with tinfoi l . The latter is placed indi lute sulphuric acid, and the terminals are arranged asshown. Most of _the oxygen . s by the electric currentchanged _into ozone;- and cari be collected. Ozone iss_ed_ for p u r i f y ~ i l g the _air in underground railway tubesand the water in reservoirs. As can be seen, the moleculeof ozone iS not VerV sta':ble. t very soon changes backinto oxygerl, as for, example when i_t comes in contactWith the dust pa rticles in the lower atmosphere. TheseabsOrb itS er1ergy and leave it oxygen once more


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TH WONDERFUL ELLS TH T M KE UP OUR BODIES

Our bodies are made up of millions of tiny units called cells. They vary according to the work they have to do. We see someof the different kinds of cells in our bodies greatly magnified on page 69. Here is a cell enormously magnified so that we cans what it is like. In principle all the cells in our body are of this character. The cell itself has a thin membrane for a walland is filled with a jel ly- l ike substance called protoplasm. In this is a nucleus which plays an important part in growth.We grow by the cells dividing and forming new cells and this division begins with the nucleus which divides into two andafterwards the protoplasm arranges itself round the two divisions spreads out and eventually the whole cell divides intotwo. The nucleus itself has a centre called the nucleolus or little nucleus. The centrosome helps in cell division


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The Story of Life and Living Things and how these are constantly being affectedy their surroundings

PHYSIOLOGY ZOOLOGY EMBRYOLOGY and BOTANY

LL LIFE BUILT UP OF ELLSThe body of every living creature, whether it be animal or plant, largeor small, simple or complex, is, as we read here, built up of tiny cells

F we take a thin slice of any part ofa plant, leaf, stem or root, andexamine it by means of a powerfulmicroscope, we find that it has a honeycomb-like appearance. The same thingis true if we take a thin slice from anypart of an animal.t was an Englishman, Dr. RobertHooke, who first drew attention to thisfact, and he gave the name of cellto each of the little compartments whichhe saw in his botanical specimens. Thestructure of the plant and animal partsseen through the microscope suggestedto him the cells in a bee's comb.

Years afterwards it was found thatthe cells of both animals and plantswere filled with a jelly-like substance,to which a German botanist, Hugo vonMohl, gave the name protoplasm, . aword made up from two Greek words,and meaning the first formativematerial.' 'This substance is thickand semi-fluid and is filledwith innumerable whi t egranules which fis the cellexpands flow 'in streamsfrom the centre to the circumference in never-ceasingactivity. The name protoplasm was given because thisis.the simplest form of livingmatter which is known to us,and out of it all the life ofthe plant or animal springs.Living Protoplasm

to have what appear to be manyminute bubbles in it.In all important respects the protoplasm seems to be alike in both plantsand animals. It is the protoplasm thatis sensitive to light, and that moves.There is a large amount of protoplasmin, say, a man's body, but in man, aswell as in all other animals and inplants, the profoplasm is shut off intiny Jumps, as it were, by definite cellsor membranes. t is the single bit ofprotoplasm within its membrane that

forms the cell.In larger plants and animals the outerlayers of cells in the body are usuallydead. Protoplasmis no longer there,and only the deadcell remains. Thisis true of the outside of our skins, of

different kinds of cells in the leaf, thestem iJ.nd the root of a plant. .The cells of a body have been likenedto the bricks and tiles that go to thebuilding up of a house. But while thesimile is interesting and useful up toa point in helping us to understandmatters, we must bear in mind thatcells are not added to cells as bricks areadded to bricks to make a house. Asone scientist has said : t is not thatthe cells make the plant ; it is the plantthat makes the cells, and the same thing is true about animals.What really happens is this. Foodmaterials are gradually changed byprotoplasm into living substance likeitself, and the process is known asassimilation, a word which tneans tobecome like. As a result the amountof protopiasm increases and the cellgrows. f this process continued in-definitely the cell would intime become very large.But that does not happen.When the cell has reachedits normal size, the nucleusdivides, and very soon thehalves separate and formtwo nuclei. Then the bodyof the cell also divides andcell walls form between theparts till at last two cellsexist where formerly therewas only one.

Growth Goes OnChemists have found thatthis living protoplasm ismade up chiefly of the fourelements, hydrogen, oxygen,nitrogen and carbon, and

Why can a living creature made up of tiny jelly-like cells stand erect? Itis because the fluid in the cells presses out in the enclosing skin just asthe gas in a balloQn presses outward and enables it to stand up stiffly

These cells assimilate anddivide, and so the growth ofthe plant or animal goes on.The simplest of all plantsand animals consists of onecell only, but trees andplants have the power of manufacturing it out of non-living matter in theair and in the soil in which they grow.Exactly how this is done by the plantis not known. But it is a curious factthat animals whose bodies are made upof this same kind of living matter areunable to manufacture it out of noneliving matter. We, and all animals,can only obtain it by feeding uponplants, or on other animals which havefed upon plants. The plants makeprotoplasm from lifeless matter, andpass it on to living animals.

Now it is the protoplasm of livingthings that seems to be the matterthat can grow. Seen through a microscope of small power it looks somethinglike white of egg, but under a morepowerful microscope it is seen to consist of an exceedingly fine network, and

the hide of a horse, and the bark of atree.Now though all living creatures aremade up of cells of protoplasm, . thecells are not all alike in form. Evenin our own bodies there ,are manydifferent kinds of cells. They varyin size and in shape. Some have thickwalls and some thin. Some havevarious kinds of solid bodies floatinginside them, while others have few ornone at all. Some have small bubblesof a clear liquid, while others havelarger bubbles. In some plants' cellsthe protoplasm can be distinctly seenmoving about.

The different kinds of cells make updifferent parts of our bodies. We findone kind of cell in the skin, another inthe liver, another in the bones, anotherin the brain, and so on. Then there are67

horses and men have many millions.Even the tiny wheel animalcule thatcan easily pass through the eye of aneedle has nearly a thousand cells.Yet even the most complex livingcreature like a human being begins life as a fertilised egg-cell, which dividesand divides again till with the multiplication of cells the embryo is formed.The whole living plant or animal iscalled an organism. This is made upof parts called organs, and the organsare made up of tissue, which is itselfbuilt tip of a number of cells, the cellsbeing built up of protoplasm.

We must remember, however, thatin the cell there may be other substancesthat cannot be regarded as protoplasm.These are of various kinds, such asstarch grains, globules of oil, pigmentgranules, crystals, and so on.c*


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THE LIFE CELLSSome scientists speak of the wholeqf the matter in a cell as cytoplasm, aword which simply means cell substance. They call the granulesglobules, and other unessential elementsmetaplasm, which means " sharing

" In the great fairs of long ago, hesays, thete used to be a grouping ofsimilar booths together-all theclothiers in one place, all the toolmakersin another ; and when in the course oftime a town grew up like a fair all the

year round, there were streets compose(of similar shops. These correspond towhat are called tissues in livingcreatures, for a tissue is a.combinationof similar cells doing the same kind ofwork. Thus the skin of a leaf or of aroot is a tissue, and the wood and pithof a stem are tissues.matter, and then theydescribe the protoplasm asthe cytoplasm minus themetaplasm.Sir J A. Thomson usesan apt illustration to explain how different kindsof cells are grouped to-gether to do different workin a plant or animal.

" Numerous cells, similar or different,are often compacted together so thatthey form an organ, like a leaf or a root,a tendril or part of a flower. Plants havenot so many organs and tissues as_animals, where the division of labour isgreater.Cells are verysmall and it needsa microscope to seethem. t wouldtake at least fivehundred ordi.narycells laid side byside to make aninch, and it can beunderstood therefore that it needs ave r y power f u lm i r o s ope and

One of the marvels of all life is that the t iny cells of which a plant or animal is made up performs many functions. The cells vary a good deal in form yet in principle they are alike consistingof a membrane containing living matter -known as protoplasm. It is this protoplasm thatenables the animal or plant to grow for it causes the cells to divide and multiply. I t seemsmarvellous that such soft and jel ly- l ike units can build up in one case a delicate and fragileplant like the toadstool shown here on the left and in another a massive solid elephant likethe great African specimen shown above. The elephant stands firm and erect with_ hard rigidtusks and yet it is made up of millions of soft cells. The rigidity is obtained chiefly by the pressure of the protoplasm on the walls of the cells. In addition each cell builds up for itself a kind offramework or scaffolding out of non-living matter and the combined result of the myriad frame.works helps a man or elephant or tree to stand erect instead of flopping on the ground like a jelly

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Eve ry l iv in gcreature s bodyis made u oftiny eel Is but inhigher creaturesthere are differ'ent kinds of cellsfor the differentt i s su s t-hatmake u theo rg an s doingdifferent kinds ofwork Here resome of t h edifferent kinds ofce l l s in t h eh u m a n body.The striated cellsare in the-volu11tary muscl.es likethose of the armsand Jegs ; theother m uS I ecells are in partslike the heart

special methods to examine theirminutest parts.When an object has a diameter lessthan 1/125,oooth of an inch it cannotbe seen distinctly by merely magnifyingit. t is too small for the wavelengthsthat are visible t6 our eyes to .reveal it.But modern science makes it possibleto see such minute objects by what .isknown as the ultra-microscope.The working .of this device may bemade clear by a simple illustration.We know that an ordinary room isfilled with myriads of dust particlesfar too small to be seen by our eyes inordinary daylight. Yet when a strongbeam of sunlight enters the mornthrough a crevice, the particles of dustare seen in the beam. Why is this ?Well, the invisible has not becomevisible in the ordinary sense. What wesee is not the particles themselves butthe rays of light from the surfaces of thebrilliantly illuminated particles. Soin the ultra-microscope, first used in1903 a powerful horizontal beamreveals shining points of diffractedlight from the very minute particles inthe protoplasm of a cell. The particlesare not seen directly ; they are detectedindirectly by the scattered light rays.All the renewal of tissue in a bodywith the healing of wounds and growthgenerally depends upon the abilityof cells to divide and form new cells.And the marvel is that cells becomespecialised for the different kinds ofwork they have to do in the plant oranimal.As Dr. Logan Clendening says:Muscle cells have a special structurewhich gives them the property of contractility. Thisistheirworkin the body,and they cannot be made to .do any

other kind of work. They cannot, forinstance, assume the functions of glandcells, which are endowed with theproperty of secreting or manufacturingmaterial, usually a juice, useful in thecarrying on of the bodily activities.All cells, however, have certaingene.ta] activities in common-all areable to maintain life in themselves byconverting food and air into energy andinto protoplasm.There are five principal kinds oftissue in animals formed by th( ) differentvarieties of cell. There is the epithelialtissue of the skin, the covering of theinside of the mouth and the coat of thestomach and intestines. Then there isthe connective tissue found in tendons,bones, cartilage and gristle. Anothertissue is the blood, made up of twodistinct types of cells-the white andred blood corpuscles-:-which swim in a

voluntary muscles,as of the arms andlegs, those of the in voluntary muscles,such as are foundin the intestines andthe heart muscle

liquid kriown as theserum.Next there is themuscle-tissue, madeup of three kinds ofcells, those of the

cells. Finally, there is thetissue made up of veryspecialised cells.All these different kinds of tissue arefound together in various parts of thebody, and they work together as a unit.f protoplasm is a fluid or a jelly itmay well be asked how animals andtrees can stand up rigidly. The sequoiaor eucalyptus tree rises vertically to aheight of 3 0 0 feet or more and standsagainst the winds and storms, and thegiraffe raises its long neck twenty feetabove the ground and the elephantstij.nds as firm as a rock. How can thesethings happen if the cells making up thetrees and animals are mere collectionsof soft jelly ?Well, the rigidity is o b t i m ~ d in twoways-partly by the pressure of the

69

THE LIFE CELLSprotoplasm on the cell walls, which isknown as turgidity or the quality ofbeing swollen, just as a child's balloonwill stand up when it is blown out ;and secondly by a kind of scaffoldingor supporting framework which thecell builds up for itself out of nonliving matter.Living cells have the property ofbeing irritable, that is, reacting or changing in response to a change of theirenvironment. Some cells ate sensitiveto chemicals, others to light, heat,pressure, and so on. All cells indeedare laboratories in which chemicalreactions supply energy.

t is a very remarkable fact thatisolated pieces of tissue detached fromthe plant or animal may continueto live for long periods. A tissue fromthe embryo of a chicken, for example,kept at a suitable temperature andsupplied with oxygen, has gone on livingfor ten years just as though it were stillpart of a living hen. Throughout allthat time, says Professor PatrickGeddes, there was growth as well aslife, and the growth-rate was practicallyuniform throughout,:.' How long can a cell lfve when itforms part of a living plant or animal ?A giant sequoia tree can live .for 3 0 0 0years, but this does not mean that anyindividual cells have lived all that time.Much of the tree is dead skeleton andthe living cells are constantly beingrenewed.

From the continuance of the lifeof the organism as a whole, ,saysProfessor Geddes, one cannot argueto the longevity of particular cells,except in cases where no replacementoccurs and the individual cells remainalive. Thus it is generally believed thatwhen the brain of a backboned animalhas once reached its normal size there


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M E C H N I C S ~ Howthemighty Forces of Nature are applied and made to work for thebenefit of Mankind. ~ ~ STATICS HYDROSTATICS KINEMATICS and ENGINEERINGTHE GRE T DV NT GE OF THE PULLEY

The pulley, which is described here, is really only an adaptation of the lever.It is certainly one of the. greatest mechanical inventions man has devisedW E have already seen how veryvaluable a device the lever is inits various forms. Well, thereare two developments of the lever,the wheel and axle and the pulley,which are almost equally useful to mankind. Although these are generallyspoken of as distinct mechanical devices or simple machines, they are, ascan be seen from the picture-diagramson page 75, only levers in an unusualform.In the wheel and axle there are twocylinders with the same axis, but oneof the cylinders is larger in diameterthan the other. The larger cylinder isspoken of as the wheel and the smallerbne as the axle. There is no definite proportion be-tween the two any more than there isin the arms of a lever. The diametersare varied according to the kind ofwork the machine is required to perform. Sometimes the wheel is verylarge and the axle small, while at othertimes there is not so much differencegetween them.

A Handle for a WheelA simple form of the wheel and axlein ope,ration is the windlass by whichwater is drawn up from the well. In

this . case a handle is generally substituted for a complete wheel. Anotherfamiliar example is the Elomestic manglewhere the complete wheel is retainedand is turned by a .handle attached toits circumference for convenience.Let us see how this device operates.In the case of the windlass the rope

supporting the pail is wound round theaxle and the axle is rotated by turningthe. handle which corresponds with thewheel. We can see at once that thehandle, as it turns, passes through amuch greater distance than a point onthe circumference of the axle. Wetherefore get the same effect as in anordinary lever when one arm is muchlonger than the other.Mechanical Advantage

The arm turning the handle. (orwheel) part of the device passesthrough a much greater distance thanthe weight or pail is raised by thewinding of the rope round the axle.According as the diameter of thewheel is increased in proportion to thatof the axle, so a correspondingly smallerpower is needed in turning the wheelto raise a weight on the axle.Suppose the radius of the wheel tobe six times that of the axle, then apower equal to one pound at the wheelwill raise a weight equal to six poundson the axle. Other familiar examples of the wheeland axle in daily life are the pedal of anordinary push bicycle and the capstanof a ship.The law relating to the wheel andaxle is practically the same as for thelever and may be expressed thus : thepower multiplied by the radius of thewheel will balance the weight multipliedby the radius of the axle.

t is not only in the raising of aweight that the wheel and axle isuseful. t is also of great service where

it is desired to exert a large force at theaxle by applying only a small force atthe wheel. The domestic mangle is agood example of this. A woman canturn the wheel withouJ undue exertionand apply great force at the axle,which will enable her to wring the waterout of wet clothes or smooth dry onesin a way that she would find impossiblewithout the advantage which theapplication of the wheel and axle givesher.In the bicycle pedal it is also of greatuse and the principle is applied in ourwatches and clocks, where changesin velocity or power are obtained bytrains of toothed wheels working on thewheel and axle principle. The device isalso used in many forms of cranes, thepower being applied sometimes byhuman hands and at other times bymeans of a leather band round thewheel worked by steam or electricpower.A Genius of the Past

While the lever was probably achance discovery the wheel and axlemust have been a definite invention,for it does not exist ready-made inNature as does the lever. No one,however, knows. who the genius waswho first thought of the clever device.A still more useful d p t ~ t i o n of thelever is the pulley; va.qomi_ forms ofwhich are given on pagei? J2 and 73.Every pulley consists of ~ h r parts.First of all there is a plate or disc orwheel with a groove cut round its cir-cumference. This wheel, which is able

On the left is the simple machine known s the wheel and axle in which two cylinders one large and one small are fixedon one axle. The -0ther three drawings show familiar uses .of this device, which is really an adaptation of the lever7r


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THE M NY KINDS OF PULLEYS TH T RE USEDThe pulley is one of the most useful devices that man has invented to help him do his work and as we see on page 75 itis really only an adaptation of the lever. The simplest form of pulley which is shown in picture A is merely a wheelfixed in a frame turning on an axle and having a groove round its circumference for a rope. But there are many varia-tions as we can see in this drawfng that runs across the two pages. Sometimes the pulley is fixed while at other times it ismovable and the greatest advantage is obtained by using a series of fixed and movable pulleys as shown in many of thesepictures. The amount of power that has to be exerted to balance a certain weight is shown in each case and of course i fa l i t t le more power is exerted the weight is then raised. In A the pulley merely changes the dir:ection of the pull.. Wherethe weight is much greater than the pull exerted the power has to be made through a greater distance. than the weightis raised. In example D where 10 pounds balance.160 pounds it is necessary in order to raise the weight one foot thatthe pull should be exerted through 16 feet. Where mechanical advantage as well as change of direction is wanted the typeof pulley used is that shown in examples to 0 It is by means of a train .of pulleys that a couple of men are able toraise a great steel safe weighing several tons from the pavement to a fifth or sixth floor window. f we watch such anoperation going on we shall notice that though the men pull yards and yards of chain through the pulley the safe goes up

7


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THE PULLEYto move freely on an axis, is called thesheave. Then there is a frame inwhich the wheel is placed and in thesides of which the axle. of the wheel isfixed for support. This is called theblock. Finally there is a cord whichpasses over the groove in the sheaveand this is called the tackle.

There are two .kinds of pulley oneknown as the fixed pulley and theother as the movable pulley. Thesimplest of all pulleys is a single fixedpulley,_ such as that on the clothes-linepost in the garden or that over whichthe sash-cord works in a window.Now the clothes-post in the gardenis an excellent example of the advantage which the pulley gives in doing

on one end is communicated to everyother part so that i f she pulls with aforce equal to say half a hundredweight this pull is communicated rightthrough the rope.Of course, some of the work put intothe pull by the maid is lost in frictionbetween the rope and the pulley wheel..We see this simple form of pulley inuse in many spheres. When a roof isbeing repaired the tiles, mortar andother materials are often pulled up in abasket by means of a rope passing overa single fixed pulley. The work can bedone much more rapidly and easily inthis way than i f a labourer had to climbup a long ladder carrying the basket ofmaterials with him.

by pulling. upward on the other end ofthe rope is able to raise a weight equalto double the force put into the pulley.Half the weight is, of course, supported by that part of the cord which isfixed to the beam while the other halfis supported by that part of the cordwhich the man is holding.Such an arrangement is inconvenientbut i t can be made of much greatervalue i f instead of pulling upward theman passes his end of the cord over afixed pulley attached to the beam asshown in the second example on page 72,and pulls downward.The movable pulley halves the powerwhich has to be exerted to support theweight and the fixed pulley enables the

The capstan on a ship which raises and lowers the anchor or does other work is an excellent example of the wheel and axlework. In order that they may dryquickly the clothes on the line must behauled high up where they will catchthe wind. t would J>e exceedingly inconvenient if the maid had to get aladder or pair of steps climb to thetop of the pole and then pull the lineup with the clothes on it.What does she do ? She takes holdof the end of a rope which passes upand over the pulley wheel, the otherend of which forms the clothes-line.Instead of pulling the clothes-line upshe pulls it down and is able to dothis easily and comfortably.What the pulley has done is to changethe direction of the pull. The rope beingflexible, without being elastic, the forcethich is brought to bear by the maid

But the change in the direction of thepull or work clone is not the onlyadvantage which is obtained by the useof the pulley. When a combination ofpulleys is used a great weight can beraised by the exertion of a little force,exactly in the same way as can be doneby adjusting the arms of a lever.With a single fixed pulley the amountof weight that can be raised is no more than the force put into the downwardpull on the other end of the rope. Ofcourse, in actual practice the weightraised is less. as some of the work is lostin overcoming friction.Now take the of a single movablepulley. One end of the cord or rope isfixed to a beam and passes round thegroove of the movable pulley. A man7

force to be exerted in a downwarddirection instead of an upward.Now if we think carefully about thistype of pulley while looking at thepicture of it we shall realise that forevery two feet that the man pulls downthe rope the weight rises only one foot.The reason is easy to understand. Thelength of two feet pulled down by theman is divided between the two ropeson each side of the movable pulley.When the man lengthens his end of therope by two feet each of those ropes isshortened by one foot, and so the weightis raised only one foot.This is the principle of the pulleywhich makes it such a valuable machinein the performance of work.What we lose in distance we gain in


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The upper pictureshows how the pulley is an adaptationof the lever andthe other drawingsm k e clear theidentity of the wheeland axle with leversof the first, secondnd third orderseach h ving .un-equal arms. Theaxle of the pulleywheel is of coursethe fulcrum of thelever

power and when a number of fixedpulleys and a nuniber of movablepulleys are used together we are ableby the use of very little force exertedthrough a great distance to lift a verygreat weight through a short distanceVarious examples of this are givenon pages 72 and 73. In actual practice where several fixed and movablepulleys are used together, these aregenerally fixed side by sige jn theblocks for by such an arrangement agreat deal of space is saved.

How the Pulley HelpsWe may often see as we go about intown or country examples of thetremendous advantage which the useof the pulley gives in raising heavyweights. Sometimes a very heavy safehas to be pulled up to the top of a tallbuilding. At other times a massivegirder has to be raised. A very large number of men wouldbe needed to do this work if theymerely lifted the great mass. .But byhaving the chain which holds the safeor girder passed round and round thesheaves or wheels of the pulley two orthree men are able to raise the greatweight gradually. f we watch them,however we shall see that though thesafe or girder goes up very slowlyindeed the men are all the time pullinga great length of chain.The principle on which the work isdone can be seen by examining theexample J on page 73. There are twopulleys each with three blocks onebeing fixed and the other movable.There are six ropes three on one sideof the sheaves and three on the other

side and it s clear thatfor every six inches thatthe loose rope is pulleddown by the man, theweight will rise only oneinch. But the power exerted to support the weightwill be only one-sixth ofthat weight. Thus, i f theweight is sixty pounds, apower or weight of onlyten pounds on the looseend of the rope will support that heavy weight,and a little more power

exerted will draw theweight up slowly.The use of the pulley invarious forms can be seenin the different types ofcrane which are used atdocks railway depots andfactories. Some examplesof these are given onpages 76 and 77 and thedrawings there shouldbe studied in conjunctionwith the pictures of thepulleys on pages 72 and73.Of course in actualpractice some of themechanical advantagegained in the use of thepulley is wasted in over-7

THE PULLEYcoming the friction of the rope on thesheaves and of the sheaves as they turnon their axles. Some work also has tobe expended in raising the movablepulleys themselves as the weight goesup.

But all this does not affect theprinciple of the pulley which is one ofthe most useful devices ever inventedby man:Perhaps one of the most interestingexamples of the action of a pulley isthat where a rope thrown over a fixedpulley has one end of it attached to aman s body or a seat in which a manis sitting, and the other end is in thebands of the man himself.

By pulling on the rope with his handswith a force equal to half his weight orless he is able to support himself andwith a little more exertion can easilydraw himself up to the pulley. . Atfirst sight this might seem as thoughthe fixed pulley was a balancer ofdifferent intensities of force but whatactually happens is that the man throwsmore than half his weight by bisstrength on one side of the pulley, andthus the rope which supports him onone side balances the rope which he isholding on the other side. As stated,a little extra exertion enables him topull himself up.A man by a pulleythus arranged can lethimself down into adeep well or from thebrow of a diff, knowingthat he will be ablequite easily to pull himself up again withouthelp. t is a wonderthat the pulley has notbeen more used as afire-escape from theupper stories of loftybuildings. A pulley ofthis kind also offers aconvenient means oftaking a plunge bathfrom the stern windowsof a sailing ship.


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THE M NY KINDS OF CR NE IN WHICH THEIn this drawing we see all sorts of cranes large and small such as do the lifti.ng 11t a dock. In the top left-hand is anelectric transporter crane the cradle of which is moved i \ long the extended girders by gears driven by electric motors in thehanging cabin. This cabin also houses the l i f t ing motors for operating the grab l:Juoket, When a ship is about to comealongside the QU< .Y the cabin is run back and the whole extended front pf the crane is raised by the motors and pulleys on thetop. The ship is thus enabled to come immediately tlnderneath and when she is in position the front is lowered once moreand the cabin and grab bucket run out to begin the unloading rqm the ho ds. In the electric hammer-headed crane show-IIon the left of the drawing pulleys worked by electric motors pull the cradle along the top of the crane while a separate set ofmotors operates the lift ing pulleys. The whole head of this crane can be turned round on its turntable thus giving a verywide range to the apparatus. The small hand-crane shown at the angle of the dock basin is operated as its name impliesby hand-power through a winch and gears. The Scotch derrick crane shown at the bottom of the picture is worl


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PRINCIPLE QF THE PULLEY S ADAPTED TO USElaunched. Powerful electric motors work the l i f t ing gear, and also the t i lt ing gear, to enable the crane to get right.over theship. Two sets of pulleys are shown, one for l ighter material and the lower set for very heavy loads The whole revolveson a turntable. Without the pulley thewhole structure of modern civilisationwould collapse, for it wouid be quite m p o s ~sible to load ships with heavy and bulkymaterial, and set up great modern build-ings constructed of massive iron or steelgirders without the crane.We have only to look at anydock, rai lway goods yard orbig building enterprise to show much the pulley comesinto use The invention ofthe pulley is ascribed to11 rchimedes in the thirdcentury before Christ


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HOW HEAT AFFECTS A.GAS LIQUID AND SOLIDHeat is really movement in the molecules of a body. When a gas is heated as inthe balloon at the top the molecules begin to move violently and spread out. As they move about they knock against the walls of the balloon and so when the gas isheated the balloon expands. The same thing happens when a liquid is heated. Themolecules rush about faster and spread out so that the volume of the liquid expands.This is shown in the middle of the drawing. At the bottom weseethe metals of a railway line with the molecules close together.When a solid is heated the molecules cannot spread but swingto and fro and the metal expands as shown on the right

Moleculesofq s nJa loon


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account of the Physical Pr-rUes of Mauer and some of the great Natural IForces harnessed by man

LIGHT, SOUND, HEAT, MAGNETISM and ELECTRICITY .P H Y S I C S ~ n

WH T HE T RE LLY St used to be thought that heat was a substance which was contained in all hotbodies, but, as we read here, it is now known that heat is really a form of motionI F we put a hot poker into a pail ofcold water the poker will becomecool, but the water will be warmed.The poker has given up heat and thewater has received it. Similarly, i f weput a cold spoon into a cup of hot teathe spoon will get hot, whereas thecup of tea becomes cooler. The spoonreceives heat and the tea gives it up.Many examples of this kind couldbe given, and it is perhaps not surprising that for centuries men ofscience supposed that heat was somekind of fluid contained to a greater orsmaller ~ g r e e in


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THE WONDER OF REFRIGERATOR SHIPOne of the reasons why the great mass of the people in a country l ike Great Britain can beso well fed to-day is that vast quantities of cheap food meat fruit and other products arebrought from distant parts of the world. This can only be done because of the principle ofrefrigeration. When the meat or other food is kept below a certain temperature nodecomposition sets in and the meat will remain fresh and sweet for weeks and evenmonths. In order to maintain the cold that is necessary .to preserve t ~ e meat anelaborate refrigerating plant is installed on the ship and similar arrangements are madefor storage when the meat arrives ori shore. The principle adopted in modern coldstorage is to absorb the heat from refrigerating chambers and keep these insulated so

that heat cannot enter from outside. In doing this ammonia is used. The chemical isinserted into a system of pipes iri the liquid form under pressure. The cold l iquidammonia passes to the refrigerating holds through pipes and the pressure being relievedt changes into a gas absorbing heat as t does so. A substance always absorbs heatwhen changing from a l iquid into a gas. The absorption of heat from the pipes andrefrigerating chambers keeps these cold. The ammonia ga s at low pressure eventually

o


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OWOUR ME T S C RRIED IN COLD STOR GEpasses back to a compressor where it is put under pressure and then passes at highpressure to a condenser where cold water dripping over the pipes in which the ammoniagas is passing changes it into a l iquid taking up its heat. The ammonia then passesthrough a valve. back into the coils of pipes in the refrigerating cfl.ambers the wholeprocess being repeated continuously. The water passing over the condenser pipesfalls into a tank and is pumped back to the top by means of centrifugal pumps.The same ammonia is used again and again. The refrigerating chambers are insulatedby layer.s of cork which prevent the heat from outside getting into the chambers. Thereare of course automatic valves in .the ammonia compr.essor and also hand valves whichcontrol the whole system. The pumps whi.ch keep the cold water in circulation areworked by an electric motor. It was a famous Englishman Sir Francis Bacon who firstdiscovered the value of refrigeration and thus gave the world one of its most valuableideas. He stuffed a chicken with snow and in doing s caught a cold from which he dieda few days later. New discoveries are constantly being rriade in connection with thepreservation o foods at cold temperatures

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WHAT HEAT IStransformed into heat energy and afterallowing for friction and a certainamount of heat imparted t thecalorimeter itself J oub found that theratio of the work done on the calori-meter t the heat produced was aconstant quantity that did not vary insuccessive experiments.

In other words he found that whenwork is transformed into heat, the heatproduced is mechanically equivalent tthe work done. La1er experiments of different kindsproved that the results were the samewhen the change of mechanical into

and material objects are made up ofvery small particles called moleculesfar too small to be seen even with apowerful microscope. In a solid thesemolecules are packed closely together ;in a liquid they are less closely packed,and in a gas they are packed still moreloosely.But, as the discoverer of the Brownianmovement described on page 21 foundthe molecules are not still and fixed. ngases -and liquids they are in rapidmotion, while even in solids they arealso moving.The difference is that in a gas or

when boiling water pours out of thekettle spout, and when the child sexpanded balloon swells out larger andlarger in the sunshine or in front of afire.Each little. molecule possesses massor weight and in a liquid or gas _presses against the sides of the vesselcontaining it. When the balloon iswarmed the pressure of the gas insideincreases and this pressure is due t theimpact of the molecules against thesides as their speed is increased.The number of molecules of gasenclosed in the balloon remains the

A striking example of incomplete combustion. This is a fire at a large rubber works and, as we can see although the tyresand other rubber articles were destroyed the heat was not sufficient to consume the carbon in the rubber and so vast quantitieswent up in myriads of tiny particles as smoke. Intenser heat would have consumed these particles of carbon o m p l ~ t e l yheat energy was made in other ways asfor example, when silk was rubbed oncopper and when lead was hammeredon sandstone.To raise 1 pound of water Fahrenheit, 778 foot-pounds of work arerequired. We see on page 32 what afoot pound is.Joule s work will always remain ofthe greatest importance in physics. twas wonderfully accurate seeing hewas a pioneer but in recent years withmore elaborate apparatus more strictlyaccurate results have been obtained.They only :confirm however, theV


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points, ot a gong, be sounded so thatthe metal is made to vibrate violently,though it appears stationary, and weplace .our finger close to the vibratingmetal or actually upon it, we shall get asensation very much like that oftouching a heated metal object-ourfinger will tingle. In both cases themolecules of the metal are vibrating.In a solid body the molecules ofwhich it is composed are held closetogether by a force called Cohesion.t is because of this power of cohesionthat the molecules of a solid body areunable to move about freely as in a

At this point it is important that weshould know the difference between theterms temperature and heat. Temperature expresses the condition of a body,that is, indicates whether it ishotorcold,or how hot or cold it is. Heat, on theother hand, is a form of energy which ahot object can give out or a cold objectreceive to make it hotter.Suppose we put on the gas-stove twovessels of cold water, one a small kettleand the other a large saucepan. Thenumber of gas-jets in each ring may bethe same. The kettle of water soonboils, but it takes a much longer time

anes fixedto side ovesselater becominqhot as ener q.11is imparted fbil

liquid orgas wherecohesion betweenthe molecules is muchless strong.If however, the heat beincreased greatly so thatmolecules move more and nior.eapart, their swing or vibration al{lastbecomes so great and they separafe;s0far that the power of cohesion is w e ~ ~ened and the solid becomes a liquid,;,, r makef the heat be still further i n c r e a s e ~ the Dig saucethe substance is at last e11hged intif: ; "pan- boi la gas, by the molecules being still _other words a greatfurther separated as they move about, ' deal more heat or energyviolently, has to be put into the bigOn the contrary, if heat be lessened, saucepan than into the small kettle toa gas will become a :quid and bring the temperatures of the twoevenhially a solid. t is belause in a quantities of water to the same figure,gas or liquid the molecules are free to namely, 212 Fahrenheit.go where they like that matter in these Similarly, the furnace of a smallforms adapts its shape to the vessel locomotive and that of a giant linercontaining it. Ice remains a solid may be at the same temperature, butblock, but water fills a jug or vessel of the big furnace of the liner containsany shape, and so does water vapour much more heat or energy than does

83

WHAT HEAT Sthe small furnace of the locomotive.Take another illustration. We puta large vessel of cold water on a gas-ring and it takes a certain time to boil.f we put the vessel on two lighted gasrings we boil it in half the time, notbecause we have raised the temperatureof the gas-flames underneath, butbecause we have doubled the quantityof heat.All boiling water under normal conditions is at the same temperature,namely, 212 Fahrenheit, but two quartsof boiling water contain twice as muchheat as one quart.We may say that temperature is a condition of a bodywhich can be altered by addingor taking away heat, while heatis a form of energy which we mayhave in either large or small

n this drawing we seehow Dr. Joule, a Britishstientist proved that heatis a form of energy andmeasured the amount of energyrequired to produce given quantityof heat. He rotated a paddle in a vesselof water by all.owing two weights to fall adista.nce of sixty,three feet. After doing thistwenty times in succession the amount of heatgenerated by the mechanical action of thep ddles was measured by noting the ri se intemperature of the water

quantities. Temperatu re has also beenaccurately defined as the degree of hotness of a body and the condition whichdetermines the direction in which heatwill flow when two bodit:s are placed incontact with one another. To test temperature a thermometer must be used.Touch is an unreliable guide.


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PHYSIOGRAPHY A description of the Physical Universe with tlie Daily Monthlyand Yearly Happenings in Earth and SkyAStRONOMY GEOLOGY PHYSICAL CEOGRAPHY and METEOROLOGY

THE MARVEL OF THE LIFE-GIVING SUNIf the Suii were blotted out all life would disappear from the Earth, for, aswe read here, it is the Sun s warmth and light that sustain life on the EarthW HEN we look up at the Sun in thesky it seems about the same sizeas the Moon, but this is onlybecause it is much farther away. Whilethe Moon is but 252,970 miles distantat its farthest point, the Sun is

94 ,524 ,000 miles away at its farthest.We are not surprised, therefore, to

learn that the Sun is much greater insize than the Moon, but how muchgreater it is will probably astonish usif we have never thought of the matter.The Moon is 2,163 miles in diameter,or a little more than a quarter that ofthe Earth, but the Sun is 865,000 milesacros , and it would take 64,000,000Moons to make up one Sun in size.I t is important that we should knowsomething about the Sun, for it is, afterour own Earth, the most importantmember of the solar system. Indeed,without the Sun shedding forth its heatand light there could be no life on theEarth at all.The Sun is the parent of all the otherplanets and how it is believed to have

given birth to its family of worlds isshown in the picture-diagram on page 7.While these worlds have cooled downand become solid bodies the Surt re mains a great ball of fire, with a heatthat is quite inconceivable to us.We speak of it as fire, but it is notburning in our usual sense of the word,

On theleft is aphotographof the Suntaken with or -d i n r y li htshowing sunspotsand on the r ightone taken by hydrogenl ight showing the upper layersof the Sun s atmosphere, whichconsists of hydrogen gas85

for there is no chemical combinationgoing on between oxygen and the otherelements in the Sun.Everything on the Sun is too hot toburn. The temperature of the surfacehas been measured and proves to beabout 7,000 centigrade or 12,000Fahrenheit. This is nearly twice thehighest temperature we can obtainartificially on the Earth, namely, thatof the electric arc, which is about4,000 centigrade.But deep down in the body of theSun, according to Sir Arthur Eddington,the distinguished Cambridge astronomer, enormous temperatures exist anda t the Sun s centre the temperature isprobably 55,000,000 centigrade.We cannot, of course, conceive suchtremendous heat, but Sir James Jeanshelps us somewhat by explaining thatto maintain a pinhead at such a temperature would need all the energygenerated by an engine of 3,000 millionmillion horse-power, and then the tinypinhead would generate enough heat to


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THE MARVEL OF THE SUNkill anyone within a thousand milesof it.

Now that marvellous instrument, thespectroscope, which breaks up light intoa colour band and shows lines atvarious places on the band accordingto the particular element which is

our successive photographs o f huge sun-spot taken at MountW i son ObservatoryCali fornia. In the fourthphotograph the sunspothas reached the Sun sedge and is about todisappear s the Sunrotates. These picturesclearly show that sun-spot is a raging andwhirl ing, tempest of fire

givmg out the light, enables us todiscover some of the elements thatmake up the Sun. They are the sameas those that make up the Earth, aconfirmation of the fact that theEarth was once part of the Sun.So sensitive is this spectroscopicanalysis that the instrument will inthe case of some metals disclose thepresence of one-3,000,000,oooth of anounce.

The Sun is surrounded by a glowingatmosphere of fiery gases and it is, ofcourse, these only that we can examine.But already forty elements which existon the Earth have been found in theSun. These include iron, tin, copper,zinc, lead, sodium, silver, nickel, cobalt,hydrogen, calcium, carbon, silicon,aluminium, magnesium, manganese,and helium.Finding a Strange Line

t is a marvellous fact that this lastmentioned element, helium, was discovered in the Sun. by means of thespectroscope before it was known toexist on the Earth. Then a search wasmade and it was found on the Earth.During an eclipse in I868, Sir NormanLockyer, the. English astronomer, tookphotographs, with the aid of the p e c t r ~scope, of the fiery clouds round the Sun'sdarkened disc and found a strange lineon the spectrum which seemed toindicate an unknown element. He.

called it helium, from the Greek namefor the Sun, helios.Then scientists at once began tosearch on the Earth for this unknownelement, and in 1895 Sir WilliamRamsay found it while examining thespectrum of a gas extracted from pitchblende. Now helium gasis used for filling airships.

t is found in large quantities on ly in Nor thAmerica.To return to the manyelements found in the Sun,it must not be supposedthat these exist there inthe same state as they doon the Earth. On the Earththey are mostly in theform of chemical compounds, whereas in the

Sun they are almost en- til'.ely uncombined andexist as gases at enormously high temperatures. ven iron isfound as a gas in theSun.t is curious thatmost of the elementsfound in the Sun aremetals, but some of theheaviest element.s suchas gold and quicksilverhave not yet been discovered. No doubtthey are deep down in the Sun's body.

An ordinary photograph of the Sun'sdisc shows a more or less even disc withperhaps a few dark spots which we callsunspots. But by taking the photograph. in a special way so that only aparticular kind of light such as hydro-86

gen or calcium light comes throughthe camera, we get a photograph thatshows the Sun's face to be mottledor granulated all over. t appears toconsist of smaU luminous masses withdarker openings b.etween. Because oftheir appearance these mottle marksare often called rice grains.Sheet of Luminous Clouds .Why the Sun has this appearance isnot quite certain, but scientists believethat the photosphere, or visible surface,is a sheet of clouds floating in a lessluminous atmosphere, just as clouds ofwater vapour float in the Earth'satmosphere.This photosphere is believed to beintensely bright, for the same reasonthat a gas mantle is bright : it outshinesthe flame which heats it.

The outer layer of gas surroundingthe Sun . like an atmosphere andbelieved to be 5,000 miles or more indepth is called the chromosphere,which means colour sphere. Photosphere means light sphere. Thechromosphere is made up chiefly of thegases hydrogen, helium, and calcium.Then there is what is known as theSun's corona, a beautiful halo of apearly-white colour surrounding theSun and visible only when the Sun'sdisc is hidden by the Moon during atotal eclipse. t is not evenly dis-tributed like the Earth'satmosphere, but hasstreamers reaching outhere and there, sometimes to a distance ofseveral million miles.

The spectroscope tellsus much about thecorona, arid shows thatit is due partly to thepresence of incandescentgases ?nd partly to re.fleeted sunlight. t sbelieved that mixed up

with the gas is some kind of fog ordust of meteoric origin.Unfortunately, the corona can onlybe studied during the few moments ofa total eclipse, so there is much yet tobe learned about it.The mottling of the. Sun's surface is


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not the only marking detected on itsface. There are the dark sunspotswhich can sometimes be detected withthe naked eye when the Sun isexamined through a piece of darkcoloured or smoked glass.Seen through a telescope a sunspotconsists of a darkcentral part surr o u n d e d by alighter fringe. Butthe apparent darkness is due to thecontrast with thegreater brillianceof the other partsof the Sun s surface. Even theblackest portion ofa sunspot is muchbrighter than thedazzling light of anacetylene lamp.

other spots have been forming. Theirregular group stretches out ecist andwest, and generally there is a large spotin front of the group and another a t therear. Sometimes the rear spot is thelarger. After a time the smaller spotsdisappear and frequently the leading

THE MARVEL OF THE SUNthese storms the fiercest tornado on theEarth is mere child s play. A stormon the Sun rages over 150 000 squaremiles or more, and reaches a height ofover half a million miles. In such atitanic storm of fire the Earth and allthe planets would be consumed in amoment of time.When the sunspot

in its p a s s a g eacross the Sun sdisc is seen toreach the edge agreat red flameis thrown up tensof thousands andsometimes h u n -dreds of thousandsof miles high.

Sunspots generally a p p e a r ingroups, and it usedto be supposed thatthey were all cavi-.ties in the surfaceof the Sun. Thesunspots t r av eacross the Sun sdisc and when theynear the edge theyoften appear assaucer-shaped hollows with slopingsides.

sunspot which is a stirring-up of th e Sun s surface, releases heat from the interior in the sameway as heat is released from the inside of the fire when we stir it up with a poker

For some reasonnot yet understoodthe gases in theSun s atmosphereexpand suddenlyand as always happens when a gasexpands suddenly,its temperaturedrops. t is thatdrop in . temperature that causesthe sunspot toappear dark incomparison wi thh e surroundingsurface.The tempera.ture

All the spots, however, do not givethis appearance, and it is now thoughtthat the spots are at various levelssome really forming cavities whileothers are raised up.While some of these sunspots aresmall not exceeding 500 miles inwidth, in others the dark part is50 000 miles across and the less darkpart surrounding brings the total widthup to I 5 0 OOOmiles or nearlytwenty times thediameter of theEarth. Such a spotis big enough toswallow up thewh o le of t h eplanets at a gulp.

big spot goes with them. Then thebiggest spot of all may break up.Now what are these curious spots ?Well it is believed that they are giganticstorms or tornadoes of flaming hydro- .gen. A photograph of the Sun s disctaken in a hydrogen light shows thespots as huge maelstroms, and asuccession of such photographs takenat close intervals makes clear that there

of a sunspot isabout 3 000 centigrade, comparedwith the 6 ooo to rn 000 of otherparts of the Sun s disc. The hotterparts are deep down in the hydrogen atmosphere.t was by the movement of thesunspots across the Sun s disc that it

was first discovered that the Sunrotates on its axis. The fact has beenconfirmed by the spectroscope, for the motion of the Sunas i t goes roundcauses the linesin the spectrum toshift, and the timethe Sun takes toturn on its axiscan be worked outby studying theseshifting lines.here are various stages in thedevelopment of asunspot. First ofall appear brightstreaks and patchescalled faculae aLatin word meaning little torches.Next come a number of small darkpoints which increase in size andjo in toge the r .

This chart clearly shows how the activity of the magnetic needle on the Earth, Auroral displaysn the Arctic and Antarctic, and the height of water in Lake Victoria Nyanza all keep pace withthe appearance from time to time of sunspots on the Sun s surface

One thing themovement of thes u n s p o t s hasshown is that thewhole of the Sun ssurface does notrotate about itsaxis at the samerate. The equatortakes about 25days to completethe circuit, whereasa t latitude 30Then a kind of fringe made up offilament-like structures appears andforms a penumbra round the umbra ordark part.The whole process may take severaldays, but on the other hand it mayhappen in a few hours, and meanwhile

is a vortex motion, and the sunspotappears as a huge whirlpool of flame.These titanic revolving storms onthe Sun are believed to be due to thesudden expansion of solar gaseshydrogen from above being suckeddown into the vortex . Compared with87

which on the Earth would correspondwith the Canary Islands and Florida,the period of rotation is 27 days, and atlatitude 45 corresponding with Franceand Nova Scotia, the period is 9 days.The polar regions take 35 days tocomplete the circuit.


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THE MARVEL OF THE SUNDo these sunspots; these terrificstorms on the Sun's surface, affect theEarth? Well, there seems no doubtat all that they do. Professor Schwabe, .of Dessau, after years of patientstudy, discovered that the number ofspots not only varies from year to year,but that the numbers run in cycles overregular periods of 11 t yyars. n other

words the number of spots that occurvaries from yea:r; to year, but after II years the conditions of the previous11 t years are more or less closelyrepeated.In the first place the sunspotsundoubtedly have something to dowith the displays of the Northern andSouthern Lights, for careful observationhas shown that the variations in theAurora Borealis and the sunspots: overa course of years closely coincide.Then the variations ih the magneticneedle, and the appearance of sunspotsin -greater or lesser numbers also correspond very, closely. When themagnetic needle is much disturbed itmeans 1;hat a magnetic storm is goingon, and if a chart be drawn with twolines representing yariations in thenurriber of sunspots and also in thenumber of magnetic storms the l t ~ ~ :correspond very closely. At periods ofgreat magnetic ac;tivity we find thatour wireless is a good deal interferedwith and that the so-called atmos-pherics are very bad. .What really happens is that the suncspots throw out streams of particleslike, electrons and ions (the electrifiedparticles into which a substance isbroken up by an electric current) andthese travel across the interveningspace and enter our atmosphere.When they collide with the upperatmosphere. they cause the auroraldisplays and set up electric currentswhich disturb the Earth's magneticfield.

The particles take about a day and ahalf to reach the Earth and in theirpassage describe a series of arcs, something like those when water falls froma revolving garden hose.The particles thrown out by the sun-spots interfere with wireless trans-mission because the wireless wavestravel in the same upper layers of theatmosphere as those in which themagnetic variations are produced.There is soine reason to suppose thatsunspots also affect the Earth's weather.The weather seems to run in cycles ofIIi years like. the sunspots, and it hasbeen noticed that when the sunspotschange their latitude or position on theSun's surface .there is a change in thestorm belt on the Earth.To what extent the sunspots affectthe Earth's weather it is difficult, asyet, to say. When the spots are at theirremarkable series of photographs of a flamethrown up by the Sun and photographed duringthe total eclipse of1928. One hour and elevenminutes elapsed between the taking o f thetop photograph and. the bottom one. In that

maxilnum they never cover so much- asa thousanclth part of the visible solardisc, and so it would seem that theycould not affect.very much the amountof heat received by the Earth. The curious thing, however, is thatthough th,e temperature of sunspots isless than that of the other parts ofthe Sun's disc, the Sun gives three orfour ner cent more heat at the period ofsunSP.()t maximum than it does whenthe spots are at their minimum.It is thought that these vast stormson the Sun's surface have the sameeffect as when we stir the domestic firewith a poker. . The storms stir up the

s u r f c ~ of the Sun and release some ofthe intense' interior heat.The' waxing and waning of thenumoer of sunspots, it is found, corr e s p o n ~ with the change from hot drysummers to cold wet seasons. Anotherinteresting fact has been discovered byProfessor Douglass, who says that theconcentric rings seen in the crosssection of a fir or pine tree indicate bytheir thickness whether the year inwhich each one grew was dry or wet.It has also been found that the heightof water in the big African lake Victoria 'lya:ijza keeps pace with theappearance of sunspots. When sun-spots aie frequent the year is wet, andthe lake high. . ..As to the Sun, regarded as a greatglobe of fire, its mass or weight is333,000 times that of the Earth, or inother words it is l,998 million millionmillion million tons. In volume it is1,305.000 times the Earth, or 339,300million million cubic miles. Its surfacearea iil over 12,000 times that of th,eEarth1 or2,:i83,621 millionsquaremile jl.At its. equator the Sun's rotation isat the rate of 4.407 miles an hour, anda complete rotation is made in 25 days,7 hours 48 minutes. The pull of gravityat the Sun's surface is nearly .28 timesthat of the Earth. While at its greatestdistance from the Earth the Sun is94,524,000 miles, at its least distance itis 91,406,000 miles.

The .total attraction between theEarth and Sun is equal to the pull ofmore :than three and a half millionmillion-tons,, The energy radiated fromeach square.foot of the Sun's surface isequal tO 1_5 000 horse-power. The lightgiven out by the Sun is l,575 millionmilli on' million: milli on times as greatas a stan\fard candle would give.About 40 per cent of the Sun's radia-tion is absorbed by the atmospherebefore it reaches the solid earth, that iswhen the atmosphere is clear. Incloudy weather more is absorbed.The energy radiated by every squareyard of the Sun's surface is equal toi4o,ocio horse-power, and to generate somuch. energy a layer of anthracite coal25 feet thick wo.uld have to be con

s u m ~ ey_ery hour. The heat given outby the Sun would melt a layer of ice4,000 feet thick every hour all over itssurfape. . Yet only about a hundred

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