SOURCE OF HIGH POTENTIAL 315 



First of all let us think of some of the more obvious physical consequences of 

 a fall of a mere tenth of an inch of rain. Suppose it to fall from the lowest mile 

 of the atmosphere. An inch of rain is 5 Ib. of water per square foot, and gives out 

 on being condensed from vapour approximately 3,000 units of heat on the centigrade 

 scale. The mass of the mile-high column of air a square foot in section is about 

 360 Ib., and its specific heat about a quarter. Thus its temperature throughout 

 would be raised by about 33 C., or 60 F. For one-tenth inch of rain, therefore, 

 we should have a rise of temperature of the lowest mile of the atmosphere amounting 

 to 3'3C, quite enough to produce a very powerful ascending current. As the air 

 ascends and expands it cools, and more vapour is precipitated, so that the ascending 

 current is further accelerated. The heat developed over one square foot of the earth's 

 surface under these conditions is equivalent to work at the rate of a horse-power for 

 twelve minutes. Over a square mile this would be ten million horse-power for half 

 an hour. A fall of one-tenth of an inch of rain over the whole of Britain gives 

 heat equivalent to the work of a million millions of horses for half an hour ! Numbers 

 like these are altogether beyond the limits of our understanding. They enable us, 

 however, to see the full explanation of the energy of the most violent hurricanes 

 in the simplest physical concomitants of the mere condensation of aqueous vapour. 



I have already told you that the source of atmospheric electricity is as yet 

 very uncertain. Yet it is so common and so prominent a phenomenon in many of 

 its manifestations that there can be little doubt that innumerable attempts have 

 been made to account for it. But when we consult the best treatises on meteor- 

 ology we find it either evaded altogether or passed over with exceedingly scant 

 references to evaporation or to vegetation. Not finding anything satisfactory in 

 books, I have consulted able physicists, and some of the ablest of meteorologists, 

 in all cases but one with the same negative result. I had, in fact, the feeling which 

 every one must experience who attempts to lecture on a somewhat unfamiliar subject, 

 that there might be much known about it which I had not been fortunate enough 

 to meet with. Some years ago I was experimentally led to infer that mere contact 

 of the particles of aqueous vapour with those of air, as they fly about and impinge 

 according to the modern kinetic theory of gases, produced a separation of the two 

 electricities, just as when zinc and copper are brought into contact the zinc becomes 

 positively electrified and the copper negatively. Thus the electrification was supposed 

 to be the result of chemical affinity. Let us suppose, then, that a particle of vapour, 

 after impact on a particle of air, becomes electrified positively (I shall presently 

 mention experiments in support of this supposition), and see what further consequences 

 will ensue when the vapour condenses. We do not know the mechanism of the pre- 

 cipitation of vapour as cloud, and we know only partially that of the agglomeration 

 of cloud-particles into rain-drops ; but of this we can be sure that, if the vapour- 

 particles were originally electrified to any finite potential the cloud-particles would 

 be each at a potential enormously higher, and the rain-drops considerably higher 

 still. For, as I have already told you, the potential of a free charged sphere is 

 proportional directly to the quantity of electricity on it and inversely to its radius ; 

 so when eight equal and equally charged spheres unite into one sphere of double 

 the radius, its potential is four times that of each of the separate spheres. The 



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