April, 192I.] CUP, RENT TOPICS. 577

the total weight of 336 kg., leaving 255 kg. to be lifted by the screws which were driven by a 25-horsepower motor of old type. In the month of January, I92I , 5~r. Oehmichen made several flights on board this aircraft. He went up only to a height of 3 metres above the ground, having a certain distrust of parts of the apparatus, and carry- mg no parachute. He reports an absence of pitching in the vertical plane containing the axes of the two screws, while there was consid- erable rolling. There was a remarkable stability in altitude. Land- ings without rebound were made when there was no wind.

The author lays stress on the profile of the screws which he planned to resemble the outlines of the wings of moths observed in motion by a stroboscopic method.

The fast series of flights were made in the presence of an officer formalh- designated to be a witness.

G. F. S.

The Problem of Soaring Flight. E. H. HANKIN, Chemical Ex- aminer to Government, Agra, India. (Proc. Cambridge Phil. Sot.. Michaehnas Term, I92O.)--Two kinds of soaring flight are shown 1)v birds, flying-fishes and dragon-flies. In slow flight speeds of from I I to 22 miles per hour prevail, while in rapid flight the speeds range from 33 miles per hour in the case of dragon-flies to 45 miles per hour for vultures. In the slow soaring flight of birds and fishes the tips of the wings are at a higher level than the body, while they are either flat or below the body in rapid soaring flight. As a rule, sun- shine must be present for slow soaring flight to be possible, while wind is indispensable for rapid soaring flight.

F. Handley Page remarks: " Doctor Hankin's discovery of the soaring flight of dragon-flies and flying-fishes affords a means of more closely investigating the phenomena of soaring. Observation of birds soaring at 2ooo or 3ooo feet is difficult compared with the observation of insect flight at a few yards' distance."

Cranes in a flock while in soaring flight keep their relative dis- tances with great exactitude. " They resemble a number of dead birds pinned on a blue wall." This " furnishes a clear proof that the energy involved does not come from any chance or irregular cur- rents of air." There remain two possible causes of such regular flight, c@., unobserved wing motions or some condition that is uni- form throllgh wide stretches of the air. Both dragon-flies and fishes retard their flight by adjusting parts of their bodies so as to act as brakes. This seems to rule out undiscovered wing motions as the cause, for, were flight due to such motions, it would appear probable that the speed of flight would be regulated by change in the wing motion and not by a braking adjustment. Moreover, an albatross cannot soar in calm weather near sea level. This it staould be able to do, were its soaring dependent on imperceptible wing motions.

l~ateral gusts of wind are not the cause of soaring as is shown by the position of the wings which is such that in some cases the thrust

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578 CURRENT TOPICS. [J. F. I.

of the gust would force the flying organism downward and not up- ward. Ascending air currents are ruled out as a cause since inland soaring birds avoid them so long as the sun shines. There is some reason to hold that the turbulence of the air may be a factor in soaring flight, but the burden of the evidence is against it. A soaring dragon- fly may be seen gliding in the midst of a group of flapping flies. " What form of pre~xisting air movement can be imagined that pro- pels the soaring dragon-fly and yet has no effect on the flapping dragon-fly, though the latter is of lighter weight and loading ? "

Having demolished all suggested explanations the author con- cludes " that soaring flight is inexplicable in the light of existing knowledge," and suggests that experimental investigations be under- taken, since observation alone has failed to furnish the solution of the enigma.

In India the boys play with a puttung, a kite made of paper and bamboo. In a proper wind this flies stably at the top of a vertical string. In this particular, as well as in others, it shows itself akin to the soaring of birds and suggests itself as a promising beginning for an experimental attack upon the problem.

G. F. S.

F. W. ASTON and G. P. THOMSON announce, in Nature, Feb- ruary 24, I921 , that their exper imen t s wi th posi t ive rays show lithium to be composed of two isotopes having the atomic weights 6 arid 7, respectively. The former preponderates. The relative pro- portions of the two correspond to the atomic weight of lithium, 6.96.

G. F. S.

Efficiency in Indus t ry . (Nature, February 24, I 9 2 I . ) ~ T h e Daily Mail of London recently organized an exhibition for the pur- pose of emphasizing the advantages of applying scientific principles and results to industry. On the historical side the University of Lon- don presented the development of the thermionic valve from the lamps of 1887 made with internal plates by J. A. Fleming to study the Edison effect up to the most recent type of valve. There was also on exhibit apparatus used by Sir William Ramsay in his study of the rare gases as well as electrical apparatus employed by Clerk Maxwell.

A wireless set was shown, weighing 20 po.unds only, yet capable of receiving messages from continental stations. Chance Brothers exhibited " Crookes " glass for protecting the eyes. This absorbs the ultra-violet part of the spectrum while allowing the visible spectrum to pass. Various devices were shown by which colors can be matched in artificial light.

The range of the exhibition was much wider than would he in- ferred from the exhibits just cited. The interest manifested is indic- ative of the growing appreciation in Great Britain of the importance of science to national welfare in peace as well as in war.

G. F. S.