THUNDER AND LIGHTNING. 



609 



are conductors. Clouds consist of small hollow 

 bladders of vapour, charged each with the same 

 kind of electricity. It is this electric charge which 

 prevents the vesicles from uniting together, and 

 falling down in the form of rain. Even the vesi- 

 cular form which the vapour assumes is probably 

 owing to the particles being charged with electri- 

 city. The mutual repulsion of the electric particles 

 may be considered as sufficient (since they are pre- 

 vented from leaving the vesicle by the action of 

 the surrounding air, and of the surrounding vesi- 

 cles) to give the vapour the vesicular form. In 

 what way these clouds come to be charged with 

 electricity, it is not easy to say. But, as electri- 

 city is evolved during the act of evaporation,* the 

 probability is, that clouds are always charged with 

 electricity, and that they owe their existence, or 

 at least their form, to that fluid. It is very pro- 

 bable that when two currents of dry air are moving 

 different ways, the friction of the two surfaces may 

 evolve electricity. Should these currents be of 

 different temperatures, a portion of the vapour 

 which they always contain will be deposited ; the 

 electricity evolved will be taken up by that vapour, 

 and will cause it to assume the vesicular state, consti- 

 tuting a cloud. Thus we can see, in general, how 

 clouds come to be formed, and how they contain 

 electricity. This electricity may be either vitreous 

 or resinous, according to circumstances. And it is 

 conceivable, that by long continued opposite currents 

 of air, the charge accumulated in a cloud may be con- 

 siderable. Now, when two clouds, charged, the one 

 with vitreous and the other with resinous electricity, 

 happen to approach within a certain distance, the 

 thickness of the coating of electricity increases on the 

 two sides of the clouds which are nearest each other. 

 This accumulation of thickness soon becomes so 

 great as to overcome the pressure of the atmosphere, 

 and a discharge takes place, which occasions the 

 flash of lightning. The noise accompanying the 

 discharge constitutes the thunder-clap, the long 

 continuance of which partly depends on the rever- 

 berations from neighbouring objects. It is, there- 

 fore, loudest and largest, and most tremendous, in 

 hilly countries. These electrical discharges obvi- 

 ously dissipate the electricity ; the cloud condenses 

 into water, and occasions the sudden and heavy 

 rain which always terminates a thunder-storm. 

 The previous motions of the clouds, which act 

 like electrometers, indicate the electrical state of 

 different parts of the atmosphere. Thunder, then, 

 only takes place when the different strata of air are 

 in different electrical states. The clouds interposed 

 between these strata are also electrical, and owe 

 their vesicular nature to that electricity. They 

 are also conductors. Hence they interpose them- 

 selves between strata in different states, and arrange 

 themselves in such a manner as to occasion the 

 mutual discharge of the strata in opposite states. 

 The equilibrium is restored; the clouds, deprived 

 of their electricity, collapse into rain ; and the 

 thunder terminates. In thunder-storms, the dis- 



* M. Pouillpt has lately published a sPt of pxpprimpnts which 



spi'in to overturn Volta'l theory of the evolution of electricity 

 by evaporation. He lias shown that no plectricity is evolved 

 by evaporation, unless somo chemical combination takes place 

 a' the same timp. But it follows from his experiments, that 

 elei tricit y is evolved abundantly during combustion ; the liurn- 

 insf body giving out resinous, aiiil the oxygen vitrpous electri- 

 city. In like manner, the carlionic, arid emitted by vegetable* 

 is charged u ith resinous electricity, and the oxygen probably 

 harmed with vitreous electricity. Thpse two sources are suffi- 

 ciently abundant to account for the vast quantity of elect ri< ity 

 so often accumulated in the clouds. 

 VI. 



charges usually take place between two strata of 

 air, very seldom between the air and the earth. 

 But that they are sometimes also between clouds 

 and the earth cannot be doubted. These discharges 

 sometimes take place without any noise. In that 

 case, the flashes are very bright ; but they are single 

 flashes, passing visibly from one cloud to another, 

 and confined usually to a single quarter of the 

 heavens. When they are accompanied by the noise 

 which we call thunder, a number of simultaneous 

 flashes of different colours, and constituting an 

 interrupted zigzag line, may generally be observed 

 stretching to an extent of several miles. These 

 seem to be occasioned by a number of successive, 

 or almost simultaneous discharges from one cloud 

 to another, these intermediate clouds serving as 

 intermediate conductors, or stepping-stones, for the 

 electrical fluid. It is these simultaneous discharges 

 which occasion the rattling noise which we call 

 thunder. Though they are all made at the same 

 time, yet, as their distances are different, they only 

 reach our ear in succession, and thus occasion the 

 lengthened rumbling noise, so different from the 

 snap which accompanies the discharge of a Leyden 

 jar. If the electricity were confined to the clouds, 

 a single discharge, or a single flash of lightning, 

 would restore the equilibrium. The cloud would 

 collapse, and discharge itself in rain, and the 

 serenity of the heavens would be restored; but 

 this is seldom the case. I have witnessed the 

 most vivid discharges of lightning from one cloud 

 to another, which enlightened the whole horizon, 

 continue for several hours, and amounting to a very 

 considerable number, not fewer certainly than fifty, 

 and terminating at last in a violent thunder-storm. 

 We see that these discharges, though the quantity 

 of electricity must have been immense, did not 

 restore the equilibrium. It is obvious from this, 

 that not only the clouds, but the strata of air 

 themselves, must have been strongly charged with 

 electricity. The clouds, being conductors, served 

 the purpose of discharging the electricity with 

 which they were loaded, when they came within 

 the striking distance. But the electric stratum of 

 air, with which the cloud was in contact, being a 

 non-conductor, would not lose its electricity by 

 the discharge of the cloud. It would immediately 

 supply the cloud, with which it was in contact, 

 with a new charge. And this repeated charging 

 and discharging process would continue to go on 

 till the different strata of excited air were brought 

 to their natural state. From the atmospherical 

 electric journal, kept by Mr Reed, at Knightsbridge, 

 during two whole years, from May 9, 1789, to May 

 9, 1791, it appears that clouds, and rain, and hail, 

 and snow, are always charged with electricity ; 

 sometimes with negative, but more frequently with 

 positive electricity. When the sky is serene and 

 cloudless, the strata of air are generally charged 

 with positive electricity. In such cases, the 

 thunder rod is charged by induction ; the highest 

 end acquiring the opposite state of electricity from 

 the air, and the lowest end the same kind of elec- 

 tricity, while a portion of the rod towards the 

 middle is neutral. During the first year, there 

 occurred only seven days in which no electricity 

 could be perceived ; and during the second year, 

 when the apparatus was much more complete, not 

 a single day occurred which did not give indications 

 of electricity in the atmosphere. During the first 

 year, the electricity was vitreous or positive 241 

 times, and, during the second year, 423 times. 



