8o 



NATURE 



[November 17, 19 10 



interest if it had been written by an unknown per- 

 sonage. But in its permanent form the relatively 

 trivial press errors and slips of the pen should be 

 corrected and all extraneous matter not connected with 

 natural history, cut out. 



The illustrations — drawings as well as photographs 

 • — are admirable. Mr. Roosevelt deserves praise for 

 having carefully photographed the small mammals as 

 well as the big. 



Special triumphs of the expedition were the shooting 

 bv Mr. Theodore Roosevelt of the rare Somali reticu- 

 lated giraffe, and by Mr. Kcrmit Roosevelt, of the 

 East African sable antelope. In regard to this 

 achievement, the writer of this review has enjoyed 

 some satisfaction. In describing his own journey to 

 Kilimanjaro in 1884, he stated that he had seen the 

 sable antelope on the way thither. This statement 

 was somewhat rudely derided bv a succeeding traveller, 

 who declared that the sable antelope was never found 

 north of the region opposite Zanzibar Island. 



H. H. Johnston. 



ATMOSPHERIC ELECTRICITY AND RAIN. 



I'^HE fact that raindrops often bring down a 

 measurable charge of electricity has been known 

 for twenty years, but numerical measurements have 

 been comparatively few, and data of even moderate 

 trustworthiness are scarce. A recent memoir of the 

 Indian Meteorological Department ^ contains an 

 account of the important work done on this subject 

 in 1908 and 1909 by Dr. G. C. Simpson. This work 

 is partly observational, partly experimental, and partly 

 theoretical. To see its true bearing, reference is 

 necessary to some other aspects of atmospheric elec- 

 tricity. 



If we denote by v the electric potential at a height 

 s above the ground, and if dvjdz represents the rate 

 of increase of v with height just above ground level, 

 then treating the conductivity of the air as negligible 

 the earth must have a charge the surface density a 

 of which is —{dvjdz)! AfT. ^" ordinary fine weather v 

 increases as we go upwards, and so a- is negative. 

 In practice one usually derives dvjdz from the differ- 

 ence of potential between two points in the same 

 vertical one metre apart. This quantity, termed the 

 potential gradient, varies much from day to day, or 

 even hour to hour, and the average value seems to 

 vary considerably at different parts of the earth. If, 

 for example, we suppose it to be 150 volts, then re- 

 membering that the centimetre is the unit of length, 

 and that the electrostatic unit equals 300 volts, we 

 deduce <r = — (i/47r)(i*5/3oo)=: — 4*0 x 10-* E.U. (or 

 electrostatic units). 



Atmospheric air is in reality not a perfect non- 

 conductor. If one gives a body in air on a perfectly 

 insulating support a charge, whether positive or nega- 

 tive, this is gradually lost. Of the numerous observa- 

 tions on the rate of loss of charge those made by Mr. 

 C. T. R. Wilson, with an apparatus which he devised 

 a few years ago, appear least open to criticism. In 

 a paper published in 1908, Wilson^ gives the result of 

 a considerable number of observations on the loss of 

 negative electricitv under fine weather conditions. 

 His mean rate of loss exceeded 8 per cent, of the 

 charge per minute of time. In other words, a charge 

 equal to the earth's charge at any instant was lost 

 every twelve minutes. During these observations the 

 mean value of the potential gradient was 187. This 

 answers to a surface density of — io-*X4'97 E.U., or 



^ Vol. XX., part 8, "On the Electricitv of Rain and its Orie'n in Thunder- 

 storms.'' Rv Dr. George C. Simpson, Imperial Meteoro'ogist (also in 

 Trans, and Pro-. R.S.). 



2 Roy. Soc. Proc, A, vol. Ixxx., p. e,-^y. 



NO. 2142, VOL. 85] 



— i6"6xio-'* coulombs. Taking an 8 per cent, loss 

 per minute, the loss per second — i.e. the value of an 

 upwardly directed negative, or downwardly directed 

 positive current — is (8/60)10-^ x i6'6x lo-'* or 

 2'2xio-** in amperes. If this represented average 

 conditions, we should have in the course of a year 

 from each sq. cm. of the earth's surface a loss of 

 7x10-* coulombs, or 21 E.U. of negative electricity. 

 During rain the potential gradient is often negative, 

 but the total duration of negative gradient in the 

 course of a year is not large. We are thus led to the 

 conclusion that whilst 21 E.U. is probably an over- 

 estimate of the charge lost annually per sq. cm. of 

 surface by conduction through the air, it is unlikely 

 to be much in excess of the truth unless the con- 

 ductivity of the air is exceptionally high at times 

 when the gradient is negative. The question thus 

 arises : How is the earth's charge maintained? 



Of the hypotheses advanced of late years, the one 

 that has met with most approval is due to C. T. R. 

 Wilson, who suggested that while districts enjoying 

 fine weather are losing negative charge, other districts 

 are deriving a corresponding amount of negative elec- 

 tricitv from falling rain, the circuit being completed 

 below by earth currents, and overhead by horizontal 

 currents at a considerable height. Our knowledge of 

 earth currents at the present moment does not enable 

 us either to afiirm or to deny a systematic transfer of 

 electricity between wet and dry areas. 



When Wilson's suggestion was made, it was be- 

 lieved that while the electricity brought down by rain 

 was sometimes positive, still negative largely pre- 

 dominated, that being the result arrived at by Elster 

 and Geitel, who were the chief of the early observers. 

 Dr. Simpson's first contribution to the subject was the 

 invention of an ingenious apparatus giving a con- 

 tinuous record of the amount and sign of rainfall . 

 electricity. This apparatus has been in operation at J 

 Simla during the monsoon or rainy seasons of 1908 I 

 and 1909, and the results are of an unexpected char- 

 acter. What the apparatus really does is to collect 

 and record rainfall electricity for two-minute intervals. 

 The data represent the total charges received for each 

 successive interval and the corresponding rainfall. 

 During the two monsoons 172" i cm. of rain were re- 

 corded, with 44'o E.U. of positive and i3'8 E.U. of 

 negative electricity, or a balance of 30*2 E.U. of posi- 

 tive. The two-minute intervals during which a posi- 

 tive charge was measured amounted in all to 4*16 

 days, as against i"7o days of negative. During about 

 37 per cent, of the total duration of rainfall no sensible 

 charge was measured. Snow is rare at Simla, but 

 for such snow as fell there was much the same rela- 

 tive excess of positive electricity as in the case of rain, 

 the chief difference being that snow brought down 

 more electricity than an equal weight of rain. An 

 annual rainfall of 86 cm. is norrnal enough, and if 

 the corresponding balance, 15 E.U., of electricity had 

 been negative, it would have fitted Wilson's theory 

 well so far; but being positive, it adds to the mystery 

 respecting the source of supply of the fine weather 

 current. 



There are some features which raise doubts as to 

 whether Simla phenomena are fairly representative. 

 Rain there seems to be accompanied by much thunder 

 and lightning, and the excess of positive electricity 

 was especially prominent during the very heavy rain 

 accompanying thunderstorms. In 1908, when rain 

 was falling at a less rate than 0*17 inch per hour, the 

 time during which negative electricity was recorded 

 was about 90 per cent, of that during which 

 positive was recorded, and the mean charge 

 per c.c. was 2*2 E.U. for negative, as against 

 i"7 E.U. for positive, so that in the lightest 



