38 TRANSLATIONS AND SELECTED ARTICLES. 
364646 and at 20° 116448. Hence during the condensation of the 
vapour, an enormous diminution of volume must take place. The 
vacuum so formed can only be filled from the sides and from above, 
the colder upper air rushing into the partial vacuum causing still 
further condensation of water and diminution of volume, and is thus 
in its turn the cause of the attraction of higher ard colder masses of 
air. In proportion to the rapidity of the condensation of the vapour 
by the descending current, the superior vertical layers must rush in 
to fill up the void, and the surrounding horizontal layer will have 
less time to enter. The less dense masses of air in their descent 
alter their volume according to Mariotte’s law, and this is a second 
cause of the disturbance of equilibrium, as every mass of air un- 
dergoes a great change of volume by a simple change of vertical 
position. 
Hence the mass of air set in motion over the place of condensation, 
will be in the form of a funnel, and will be larger than the vacuum it 
is destined to fil! in the lower strata. A certain amount of heat will 
be given out by the compression of the descending air and by the 
deposition of water, but this will be small in comparison with the 
cold of the upper strata, the hail will not be quite as cold as the air 
which forms it. 
The formation of hail must commence with that of water, cold air 
is drawn in from above and the drops of water become frozen, if air 
from a great height be brought down into the partial vacuum, the 
masses of ice may be cooled many degrees below 0°, and hence in their 
descent will cause the general deposition and solidification of water 
on their surfaces. When an irruption of very cold air takes place, 
not only may single drops freeze into solid ice but many may join 
together and form larger masses. The vapour of water is cooled 
down to the freezing temperature by the descending current, while 
a solid body (the hail-stone) is present to induce a deposition on its 
surface. The same cause which produces the hail-stone, is active in 
forming those depositions of ice on the branches of trees, which are 
not unfrequently seen during the winter season. It is possible also 
that the noise which is occasionally heard preceding a hail-storm, may 
arise from the particles of ice being driven against each other by the 
rapidly descending masses of air. 
Hail can only ire formed when so considerable a diminution of 
volume takes place, that the heavier lateral masses of air have not 
