the Theory of Hail. 173 



sufficient for our present purpose to give an outline of his 

 theory. 



This theory has first to account for a reduction of tem- 

 perature sufficient to freeze water on a hot day, and then to 

 explain how the hailstones, so formed, can be held suspended 

 in the air so as to attain a volume often of several inches 

 in circumference. The cold is supposed to be produced by a 

 powerful evaporation, due to the action of the sun on the 

 upper surface of the cloud ; and the evaporation is the more 

 rapid in proportion as the air above the cloud is rarefied and 

 electric, for it is admitted that electricity greatly favours eva- 

 poration. Hence one portion of the cloud, in evaporating, 

 lowers the temperature of the other portion sufficiently to 

 produce congelation. 



The nuclei of the hailstones being thus formed, they cannot 

 attain fresh coatings of clear transparent ice, however low in 

 temperature they may be, in the short time they are falling to 

 the earth ; but it is in the power of electricity so to sustain 

 them, while they are thus being developed in volume and 

 weight. 



Let us suppose that a cloud, strongly electrified, is suddenly 

 congealed at its upper surface, in consequence of an energetic 

 evaporation. The result will be a multitude of small frozen 

 particles which form the nuclei of the hailstones. These 

 particles, repelled upwards by the strong electric action of the 

 cloud, are held suspended at a certain distance, just as a 

 feather is when an excited glass tube is held under it. In 

 like manner, if these frozen particles are placed on an insulated 

 horizontal plane, and this be strongly electrified, they will rise 

 up into the air and remain there so long as the plane retains 

 its electricity, or until they lose their electric charge, when 

 they will fall back by their weight upon the plane, and take a 

 fresh charge and be again repelled. During these motions 

 the hailstones increase in volume by condensing the vapour 

 of water upon their surfaces, and this immediately becomes 

 solid. A few of the stones increase in size more rapidly than 

 the others, and these are the first to fall — the avant couriers 

 of the general shower of hail when the weight of the individual 

 stones is too great for the electric force to maintain them 

 suspended. 



But the action above described is more complete by sup- 

 posing the existence of two or more clouds, one above the 

 other, in opposite electrical states. In such case the motion of 

 the frozen particles is much more rapid ; it is like the pith 

 figures oscillating between two metal plates in opposite elec- 



