36 TRANSLATIONS AND SELECTED ARTICLES. 
pound in weight, in hot climates and in the warmest seasons, natur- 
ally excited attention from a very early period, and manifold have 
been the endeavours to account for the phenomenon. ‘The theories 
proposed however have all proved insufficient, being generally based 
on a fallacy, and at the present time physicists are generally content 
to rank the cause of hail as one of the things unknown. Whether 
Mohr’s ingenious explanations may be admitted as satisfactory, is 
open to question. The writer cannot but believe that the whole 
theory, beautiful as it is many respects, and offering a satisfactory 
explanation of many collateral phenomena, is yet open to the same 
objection as the others, viz., that it is based on a fallacy. 
The first theory proposed was that of Volta. He imagined that 
the sun’s rays were eutirely absorbed by the upper surface of a 
dense cloud, thereby causing a rapid evaporation, particularly if the 
upper stratum of air were dry; by the evaporation so much heat 
became absorbed or removed that the water contained in the cloud 
became ice. Here the heat of the sun is supposed to cause evapora- 
tion, and simultaneously to remove heat from the aqueous vapour in 
the cloud, which is obviously absurd. The increase in the size of 
the hail-stones, was explained by Volta on the supposition that the 
gtones were projected upwards and downwards between two oppo- 
sitely electrified masses of clouds, like the pith figures in a common 
electrical experiment, (the noise preceding a fall of hail has also been 
explained by the rubbing together of the hail-stones during this sup- 
posed attraction and repulsion,) but it is difficult to imagine how 
such heavy masses could be so kept in motion. Whether it be pos- 
sible or not, if the first part of the explanation is wrong, the second 
falls to the ground. 
In the theory of F. Vogel, it is assumed that the aqueous vapour 
which is supposed to exist in the clouds, in a vesicular form, can be 
cooled far below the freezing point without becoming ice, and that 
when particles of sleet (which are usually observed in the interior 
of hail-stones) fall from the higher regions of the atmosphere, the 
aqueous vapour is deposited on them and becomes ice. In this 
theory the increase of the hail-stone is accounted for, but not the 
original formation. 
Leopold von Buch assumes that a mass of damp air being carried 
by the ascending current into the upper regions, deposits drops of 
water which in their deseent through the lower and warmer strata 
