36 On the Electrical Processes in Thunder -clouds. 



potential to a smaller space, and must again act by induction 

 and with increasing power on the newly formed masses of 

 clouds. As soon as the formation of rain begins in these the 

 same process is repeated ; by a further aggregation of the 

 clouds a fresh increase of potential is produced, and so forth. 

 It can easily be conceived that in this way the electrical po- 

 tential of a rain-cloud may go on increasing until discharge 

 takes place. 



In the experiment just described the vessel A represents 

 the lower inducing layer of cloud ; B the upper rainy part of 

 the same. The contraction of surface, although corresponding 

 but little to the actual process in the thunder-cloud, is repro- 

 duced by the introduction of the vessel B into A. 



The question of the origin of the initial charge of the lower 

 layer of cloud will not be so easy to answer. In any case a 

 very small potential will be sufficient, with a heavy rainfall, to 

 be increased to the greatest amount. It is perhaps the ordi- 

 nary atmospheric electricity which, passing to the clouds, 

 produces a greater local charge. Without being kept up 

 by the fall of rain, the strongest charge would not last. To 

 look upon the condensation of aqueous vapour in itself as 

 a source of electricity appears to us inadmissible, as it is not 

 clear in what way the separation of the electricities can take 

 place in this process. Friction can only explain the initial 

 charge, and is quite inadequate to account for the enormous 

 disengagement of electricity which takes place in nature. 

 We might, moreover, think of a friction of fine aqueous vapour 

 against large drops or against hailstones. It is not impro- 

 bable that the difference between the capillary superficial 

 tension of the smallest and largest drops of water may in 

 friction produce a separation of the electricities ; but in any 

 case such an excitation has not been ascertained with certainty. 

 Supposing it, however, to be so, is the vis viva lost in this 

 friction sufficient to produce the mechanical equivalent of even 

 one flash ? 



It might be urged against the above view, that the diminu- 

 tion in volume of a cloud consists essentially in the coalescence 

 of particles of vapour to form larger drops ; that therefore 

 the electricity must accumulate on the latter, and ultimately 

 be carried away with it as it falls. This is undoubtedly par- 

 tially the case. We must, however, remember that for a 

 certain time the drops which form in the lower layer of cloud 

 must be supported by the ascending current of air, the force 

 of which must decrease with the height ; while the drops 

 which come from the upper layers must reach the lower layers 

 with greater velocity, and consequently must more easily 



