474 



NA TURE 



[September 17, 



the rain-makers were roused by a crashing peal of 

 thunder, and the rain was soon beating on the roof. At 

 sunrise a magnificent double bow arched the heavens, 

 and the downfall of rain did not cease till 8 o'clock a m. 

 A number of heavy charges of dynamite were then made, 

 and after every one the drops again poured down, till at 

 last the clouds were entirely expended." 



In these quotations is given all that is essential in the 

 newspaper reports now before us. Although deficient in 

 many details that it would be desirable to know, they are 

 written by one who witnessed what he described, and 

 there seems no reason whatever to doubt their genuine- 

 ness and good faith ; we may therefore, discuss the 

 information they afford, without misgivings of its sub- 

 stantial trustworthiness. 



It is not antecedently improbable that, in certain states 

 of the atmosphere, the liberation of a large amount of 

 heated gas consisting wholly or in great part of 

 water vapour, at an elevation where aerial move- 

 ments are but little retarded by terrestrial friction, 

 may suffice to generate an ascending current ; and 

 elementary physical considerations teach us that a 

 mass of air that would be called relatively dry at 

 the lower level, will in ascending speedily become 

 saturated and condense its surplus vapour, first as 

 cloud, and eventually as rain, not indeed by acquiring 

 mere vapour, but in virtue of dynamic cooling as it 

 progressively expands under the diminished pressure 

 of greater altitudes. But unless the atmospheric strata 

 thus immediately affected be already in a condition 

 of unstable equilibrium, unless the vertical decrease of 

 temperature in these strata is already somewhat greater 

 than the adiabatic rate of decrement, so that the ascending 

 movement once started can be maintained by the store of 

 energy already present in the form of sensible temperature 

 and the latent heat of the included vapour, the effect must 

 of necessity be temporary and local — more of the nature of 

 a small thunder-storm, or cloud-burst, than of the widely 

 extended or sporadic rainfall that accompanies a baro- 

 metric depression. 



In fact, the possibility of rainfall production depends 

 on the possibility of producing and maintaining an up- 

 ward movement in the atmosphere. There is always 

 some vapour present in the air, generally sufficient to 

 form clouds when dynamically cooled by an ascent 

 through two or three thousand feet ; although such air, 

 while resting on the ground and warmed by its contact, 

 may be very dry as judged by our feelings and by the 

 evidence of the hygrometer. The amount of energy 

 yielded by any moderate provision of oxyhydrogen bal- 

 loons and dynamite is but infinitesimal in comparison 

 with that already locked up in the atmosphere and its 

 vapour, and which, under the circumstances above spe- 

 cified, viz. a vertical decrease of temperature exceeding a 

 certain fixed rate, is available for maintaining a move- 

 ment once set up ; and the part played by the heated 

 gases of such experiments as those now described can 

 be little more than that of a trigger that releases a 

 detent. 



It seems highly probable that on August i8 the atmo- 

 sphere was in this unstable condition. Even in the 

 warm stratum resting on the ground, the humidity was 

 60 per cent, of saturation, clouds (indicating saturation) 

 existed at some height, and rain began to fall almost im- 

 mediately on the conclusion of the explosions. It maybe 

 noticed, too, that the time of day was that at which the 

 barometer is lowest and the humidity highest in the cloud- 

 forming stratum, although, in fine weather, lowest at the 

 ground surface. In the absence, then, of any observa- 

 tions of the temperature and humidity of the strata pri- 

 marily stirred up by the exploding balloons and dynamite, 

 it seems likely that they were in a condition to maintain 

 ascending currents once started, and even to communicate 

 the disturbance to regions around. 



NO. I 142, VOL. 44] 



On the 26th, the atmosphere was evidently in a much 

 more inert condition, and four hours elapsed before rain 

 fell, the disturbance being then apparently more local, and 

 of the nature of a thunder-storm. However, with the 

 meagre data as yet before us, it would be premature to 

 attempt any critical discussion of the processes in opera- 

 tion. 



It is needless to say that popular theorizing, on this as 

 on most other physical phenomena, concerns itself chiefly 

 with the things that are most obvious to the senses, but 

 often have little or nothing to do with the process. Thus 

 we find that attention has been fixed on the explosion ; 

 and we are told that the idea of breaking clouds by pro- 

 ducing a motion in the air, and so destroying the equi- 

 librium of the suspended globules of moisture, which in 

 coalescence form rain, is not a new one ; that it was the 

 custom to keep a cannon in French villages, with which 

 to fire at passing clouds and thus hasten the downpour ; 

 that at the battles of Dresden and Waterloo the con- 

 cussion of the air by the cannonade led to the descent of 

 torrential showers ; and we are reminded that " in the 

 same way" rain follows a peal of thunder caused by the 

 passage of a lightning- flash through a moisture-laden 

 atmosphere, &c. Now, all this noise and disturbance 

 have no more to do with the production of rainfall than 

 has the thrashing which the village rain-maker of Central 

 India receives from his fellow villagers to stimulate him 

 to fresh exertions when he is thought to have neglected 

 the performance of his official duties, or the London 

 street-boy's whistle, with which Sir Samuel Baker startled 

 a rain-making king in the Southern Soudan, and which 

 wa= followed by such a deluge that even the rain- 

 making potentate implored him to arrest the working of 

 the spell. ^ The effect of a concussion, as such, is to pro- 

 duce an instantaneous compression of the air, and a 

 momentary heating in a wave which travels away at the 

 rate of about 1000 feet per second, and is incapable of 

 generating any translational movement of the atmosphere, 

 and certainly of promoting condensation. Nor do we 

 know of any recorded observations in support of the idea 

 that it can cause the coalescence of cloud corpuscles into 

 raindrops. Neither does the concussion of the air by a 

 thunder-clap stand to the downpour that follows it in the 

 physical relation of cause to effect. In this case Sir John 

 Herschel adopts the opinion originally put forward by 

 Eeles, that the order of succession is the reverse of that 

 here assumed, that the formation of the rain-drop is the 

 antecedent phenomenon, and the lightning-flash (and 

 ergo the thunder) the consequent ; the electrical discharge 

 being determined by the sudden concentration of the 

 electricity of (say) one thousand corpuscles on the 

 surface of the single resulting rain- drop, in which 

 case its intensity would be increased ten-fold. What 

 causes the coalescence is still a matter of much ob- 

 scurity, though some light has been thrown upon 

 it by the ingenious experiment exhibited by Mr. Shel 

 ford Bidwell at the Royal Society's conversazione on 

 May 14, 1890, and described in vol. xhi. (p. 91) of this 

 journal. When the shadow of a small (condensing) 

 steam jet was thrown upon a white screen, under ordinary 

 conditions, it was of feeble intensity and of a neutral tint ; 

 but when the jet was electrified, the density of the shadow 

 was at once greatly increased, and it assumed a peculiar 

 orange-brown tint. It appeared that electrification pro- 

 moted the coalescence of the exceedingly minute particles 

 of water contained in the jet, thus forming drops large 

 enough to obstruct the more refrangible rays of light. On 

 this view, then, electrification would appear to be the 

 cause of coalescence, and the electrical discharge the 

 ulterior result ; but as yet we know too little of the 



' This story has probably been told by Sir Samuel in one of his well-known 

 works on Africa, and is too good to be spoilt by condensation. It is. at all 

 events, authentic, the present writer having heard it from his own lips at a 

 Simla dinner-table. 



