CORRESPONDENCE AND NOTES 20 1
dynamics. In horizontal motion it is not the earth's tangent plane which osculates the curve, but the small circle plane (or in the special case of great circle motion, the great circle plane). If R is the radius of the earth and a the semi-angle of the osculating cone with vertex a t the earth's centre, then
which is zero for a great circle. The curvature of the circle in which COSeC 31 . The corresponding centrifugal this cone cuts the sphere is - R
1,'* cosec C( R
is, however, inclined a t an angle C( to the hori- acceleration
lontal and its horizontal component is equal to ~ or V 2 ~ as defined. T,'2 cot a R C. H. B. PRIE~TLEY
Council for 5cientific and Industrial Research,
April 16 1948 Melbourne.
5 j1.506.1(42) Meteorological observations at the Royal Observatory, Greenwich,
during the year 1947. (Communicated by the Astronomer Royal) The details given below, extracted from the meteorological register,
indicate the principal features of the weather a t Greenwirh during a year which, in several aspects, was among the most remarkable of all the 107 years covered by routine meteorological observations at the Royal Observatory.
The maximum temperature of the air was 9.7,' F. and occurred on June 3 . The reading in the open (or "Glaisher") screen on June 3 was 9j .7" F., which would make this day the aarmes t June day ever recorded a t Greenwich. ( I t should be noted, here, that until the end of 1937 all Greenwich temperatures \\ere obtained from "open icreen" readings.) The temperature rose to 80" and above on 32 days, and to 90' and above, on 3 days. If , hoaever, open screen temperatures are considered, these occasions increase to 49 and 9 respectively-in each case a rccord number for any year of the Greena ich series--and include two examples of four consecutive days mith maximum temperature above oo" (May 31- June 3 ; .\ugust I j-iS)-also a record without precedent in the Greenwich 3eries.
Temperature reached 70' on every day in August, which again is a record a t Greenwich.
Compared with the previous warmest August (191 I) mean maximum temperature in the open screen, namely 81.9", mas 0.8" higher in 1947, although the average daily mean temperature (67.3") mas 0 . 2 ~ lower.
The minimum temperature occurred on February 24 and was 9' (Glaisher 9.9'). On February 2 5 the Glaisher minimum reading was 9.3', but the Stevenson minimum was higher (10'). There were ; I days on \bhich temperature fell to 32O or beloa., 18 being in January, 26 in February and 13 in March. In February the thermometer reriiained continuously below 32' from 11th to z j t h , nhile the maximum tempcrature (41.3') \\.as the lowest maximum for any month in the Greenwich wries. Snow persisted on the ground practically for the whole month.
202 CORRESPONDENCE AND NOTES
The niean temperature for the year mas 50.6", which is oaly O-CJ' abovc the sverngr for roo years 1841-1c)40. The mean teillperature fol the first three months was much below aveiage; January 3 . 7 " , February 10.5' and M;irch 1.8" below; but in each of the following nine months the incan R :IS notably above average, outstanding examples being May (+3.9'), .iugust (+ 5 . ~ ~ ) and September (+ 3.9'). With regald to Febi uai y, coinparison may be made with February IYc);, when itlthoLr~I1 loher temperatures were sometinics reached at night, on account of clear skies, the mean daily maximum ma5 higher than in 1947.
'l'hc maximum reading of thc solar radiation thermometer was 150.3' , recorded on June 2 ; the ~niilimum telnperattlrc on thc giound \\as rerorded (surprisingly) on Ikcember I and ua5 4'. There here , however, t\\ o occasions (Januai y 28 and 29) when lower readings would ])robably have been reached, but on \chich the thermometer was found deeply buried in snox~. .\ ground temperature of 4.2' was rrcorded on February 25.
The tcadings of the barometer :it 152 feet above sea level ranged from 28.738 inches on December 5 to 30.524 inches on April 10.
The total number of hours of bright sunshine registered \\as 1400.6 \\hich is 33 hours below the average for 50 w a r s 1897-1046 and represents 31 per cent of the amount theoretically possible. The sunniest month was i\ugust with 25o.h hours, that is 34 per cent above average. The sunniest clay was May 28 with 92 pel cent of the possible total. Sunshine rcgistered in February and March 'i\as, in both cases, the Ionest amount on record; there were only 16.5 hours in February, and 22 entirely sunless days. I n the &hole year there \\ere 88 entirely itinless days, 10 o f these being in the period April to Septembrr.
The night-sky camera recorded the trare of 6 1Jrcl.r A4lnortc during 37 per cent of the total time of exposure.
The number of days noted as overcast throughout \ \ a s 9 1 , 13 of these being in February. Only two days were noted as cloudless throughout.
The total horizontal movement of the air, recorded by thc Robinson anemometer, was 89,047 miles ; the greatest daily movement was 675 miles recorded on April 23 ; the greatest hourly movement was 52 miles, recorded on March 16, on which day, also, the greatest pressure, 35 Ib. to the square foot, occurred.
The total rainfall during the w a r was 20.296 inches, which is 4.03 inches less than the average for ioo year\ 1841-1940. The year wa5 notable for the aettest Marrh on iecord during 132 years. The total amount recorded was 5 . 2 16 inches, and surpassed the previous greatest March total (1916) by 1.09 inches. On the other hand the second half of the year was the driest on record, the total of 6.26 inches being below the previous lowest total (1921) by 0.33 inch. The months chiefly responsible were Augus t (0.09 inch) and October (0.14 inch--in each case the driest of its name in the =hole Greenwich series. The year was further notable for the longest period of absolute drought ever recorded at Greenwirh, namely 33 days August 6 to September 7 without measurable rain. The number of "rain days" (days on which a t least o.oo5 inch was registered) in the year was 157. The wettest day in the year was June 27 when a total of 1.668 inches \vas recorded.