186 M. Samia-Solaro on the Artificial Production of Hail, 



are heard. These are due to the effort produced by the dilata- 

 tion of the enclosed water, which more or less violently distends 

 and ruptures the envelope formed by each successive congelation. 

 If the icy capsule be not sufficiently thick and compact, these 

 noises are but slightly heard, or not perceived at all by the ob- 

 server. This is why this circumstance does not always accom- 

 pany the meteoric product ; for to render it appreciable, the icy 

 crusts of the hailstones must be compact, and consequently also 

 produced by a very intense degree of cold. We are of opinion 

 that the cold required for this object must at times be from 30° 

 to 40° C. below zero. No causes of cold hitherto recognized 

 are capable of producing so low a temperature as that just 

 spoken of; and further, supposing that they could produce it, 

 the refrigeration could not be instantaneous. In the theory 

 proposed, we have to account for two circumstances : the first, 

 how the liquid masses to be transformed into hailstones are 

 formed in the atmosphere; the second, how the cold is produced 

 that attacks these collections of fluid and more or less rapidly 

 freezes their surface to a certain depth. We explain the former 

 circumstance by the reaction of electricity on a cloud at the 

 moment of its discharge from it, and the latter by the sudden 

 expansion which ensues on the reaction. 



To make this explanation clear, we assume a storm-cloud to 

 be charged with electricity, then at the instant that this fluid has 

 reached its maximum tension it must escape or be discharged. 

 In the act of escape it exercises a violent reaction on the cloud, 

 which compels a portion of its vapour to assume a liquid form. 

 But, on the other hand, as this reaction has produced a conden- 

 sation in the cloud in which that vapour floated, this air, in order 

 to regain its previous volume, rushes instantaneously into the 

 void caused by the electric discharge, and consequently undergoes 

 expansion to an equal degree with the preceding condensation. 

 Thus the liquid masses are subjected to rapid evaporation, and 

 thereby to a loss of heat, more or less considerable in amount, 

 and hence the congelation of the surface to a greater or less 

 depth. When the cold is not sufficiently intense to freeze the 

 masses of water, these fall in a liquid state ; and we have in this 

 fact an explanation why the first rain-drops of a thunder-storm 

 are commonly the largest in size, and why such a prodigious 

 rainfall often ensues immediately after a thunder-clap. Let us 

 see how far these statements are borne out by recorded facts. 



M. Beudant, speaking of a hail-storm observed by him in 

 1838, says, " There was a clap of thunder, and almost immedi- 

 ately the number of hailstones was greatly augmented" (Comptes 

 Rendus, vol. vi.). M. Elie de Beaumont, speaking of hail upon 

 which he made observations in 1837, says, " Three thunder-claps 



