I04 



NATURE 



[June 4, 1903 



with increasing- distance from the region of precipita- 

 tion. 



We may, on the other hand, suppose that there are 

 everywhere other influences opposing or neutraHsing 

 the ion of electricity in the direction of the electric 

 field; so that no earth-air current rrsults. Geitel has 

 offered an explanation of the maintenance of the electric 

 field in fine weather based on a difference between 

 positive and negative ions which was discovered by 

 Zeleny. Negative ions are more mobile than positive, 

 they travel with greater velocity in an electric field 

 and diffuse more rapidly. In consequence a body ex- 

 posed to a current of ionised air becomes negatively 

 charged; Geitel suggests that the surface of the earth 

 may acquire its negative charge in a similar way. 

 The difference in the velocities of diffusion of the posi- 

 tive and negative ions could not, however, maintain 

 an electric field except close to the ground, unless air 

 currents were present to carry up the positively charged 

 layers produced at the earth's surface. 

 . It is quite conceivable that we may be driven to 

 seek an extra-terrestrial source for the negative charge 

 Oi the earth's surface. The study of the aurora 

 borealis has led several observers to the conclusion 

 that the sun emits kathode rays, which are deflected 

 by the earth's magnetic field, and travel in helical 

 paths round the magnetic lines of force towards the 

 poles. It is conceivable that very penetrating rays 

 Of this type {i.e. negatively charged electrons) may 

 traverse our atmosphere unabsorbed, and be stopped 

 in the solid mass of the earth, giving to it their nega- 

 tive charge. 



We have now to consider the electrical phenomena 

 accompanying precipitation. As already Indicated, pre- 

 cipitation is nearly always associated with the occur- 

 rence of negative values of the potential gradient. 

 Heavy showers of rain, snow, or hail are accompanied 

 by rapid alternations of high positive and high nega- 

 tive values of the electric field, generally too high to 

 be_ measured by electrograph apparatus arranged to 

 suit fine weather conditions. In extreme cases we 

 have thunderstorms. There are cases of rain not 

 associated with negative potential gradients ; these are 

 practically all cases of slight rain, generally mere wet 

 mist or drizzle. Clouds from which rain is not falling 

 rarely show marked electrical effects. To find by 

 direct observation whether rain is charged with elec- 

 tricity is a matter of extreme difficulty. Elster and 

 Geitel 's observations appear to show that raindrops 

 are charged, and that the sign of the charge frequently 

 changes during a shower, negative values, however, 

 on the whole prevailing. 



The following are possible factors in the production 

 of the intense electrical fields which accompany heavy 

 showers. 



A less degree of supersaturation Is required to make 

 water condense on the negative than on the positive 

 ions (C. T. R. Wilson, Phil. Trans., vol. cxclil. p. 

 289). Thus, if condensation takes place from the 

 supersaturated condition, the drops formed are likely 

 to be negatively charged; that the drops, formed in 

 ionised air by expansions slightly exceeding that re- 

 quired to cause condensation on negative ions, are 

 actually negatively charged has been proved by H. A. 

 W'ilson (Phil. Mag., April). Since, however, each drop 

 will only carry the very small ionic charge, the elec- 

 trical effect will be small If only a few large drops are 

 formed; If a large number of negative Ions serve as 

 nuclei of condensation, the drops will be small, and 

 will only fall slowly relatively to the air; the resulting 

 electric field cannot exceed that which drives positive 

 ions downwards as fast as the negatlvelv charged 

 drops fall under the action of gravity. 'The field 

 initially produced may, however, be strong enough 



NO. 1753. VOL. 68] 



to induce coalescence of drops which come in contact 

 (Lord Rayleigh, Roy. Soc. Proc. xxvill. p. 406), and we 

 may thus get drops carrying many times the charge of 

 one Ion, and large enough to fall rapidly. Strong fields 

 may then result. 



Again, we should expect (Nature, vol. Ixil. p. 149) 

 drops falling through Ionised air to become negatively 

 charged as a result of the difference In the mobility of 

 the positive and negative Ions. This effect has, in 

 (act, been experimentally demonstrated by Schmauss 

 (Ann. d. Physik, vol. Ix. p. 224). 



If collisions resulting In splashing occur between 

 raindrops (and they are likely to be frequent In the up- 

 rush of air in thunderstorms), positively charged rain 

 may be formed. For, as Lenard has shown, when 

 splashing of pure water occurs, as, for example, in 

 waterfalls, the air In tlie neighbourhood acquires a 

 negative, the water a positive, charge. 



Apart from the Lenard effect, the splashing result- 

 ing from the collision of drops in an electric field may 

 have large effects, either in Intensifying or diminish- 

 ing the electric field already existing, the action being 

 like that of an electrostatic influence machine. The 

 result would be to increase the intensity 01 the field it 

 the splashes were thrown out from the lower portion 

 of the combined drop. If, for example, the field were 

 such as to produce positive electrification on the lower 

 surface of a neutral drop, a droplet leaving the lower 

 surface would be positively charged, and being carried 

 upwards by the air relatively to the large drop, would 

 add to the intensity of the primarv field. 



C. T. R. Wilson. 



RAINFALL AND RIVER FLOW IN THE 

 THAMES BASIN.' 

 'T'HE Water Committee of the London County 

 ■*- Council in December, 1902, called upon their 

 chief engineer for a report on the diminution of the 

 volume of water in the Thames and Lea, and his re- 

 port was submitted to the Council in February. 

 It deals briefly with the geology of the Thames and 

 Lea basins so far as geology affects waterworks 

 engineering, and in greater detail with the rainfall 

 and the flow of the streams. The general result of 

 the Inquiry is thus stated :— 



" For the past twenty years there has been a decline 

 over the Thames watershed of an annual average of 

 nearly 2i Inches below the mean rainfall of 28.50 

 Inches, as computed by the late Mr. Symons for the 

 forty years 1850-89; and I may add that this diminu- 

 tion has become more accentuated during the last five 

 years. This decline is reflected In the diminished 

 flow of the river as gauged at Teddington Weir, the 

 natural flow having fallen to an average of 11105 

 million gallons daily at the Intakes for the 20 years 

 compared with 1350 million gallons over the i85o-»9 

 period, showing a loss to the river of 239^ million 

 gallons per day. As the diminished rainfall of 2^ 

 inches equals 105 million gallons per day (after making 

 an allowance for evaporation, &c., of roughly 70 per 

 cent.), and the above diminished flow of 2392 million 

 gallons shows a difference from this of 134^ million 

 gallons daily, it would appear as though the condition 

 of the river was becoming more acute, inasmuch as 

 more rainfall would be required year by year to pro- 

 duce the long-period average rate of flow ; in fact, what 

 this means is that the percentage of total rainfall 

 which reaches the river is diminishing as well as the 

 total rainfall itself. Of course, against these facts we 

 have the possibility of a long series of wet years, which 



1 London County Council. Shrinkage of the Thames and Lea Report 

 by Maurice Fitzmaurice, C.M.G-, Chief Engineer. Pp. 18; plates. 

 (London : P. S. King and Co., 1903.) 



