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it is that hailstones sometimes acquire magnitudes never 
approached by rain drops. 
A drop retains its form simply by the surface tension of 
the 'water, and as this is the same whatever may be the size 
of the drop, its power to hold the drop together diminishes 
as the size of the drop increases, whereas the velocity and 
consequent tendency of the air to disturb the shape of the 
drop increase with its size. Hence it must eventually arrive 
at such a size that it can no longer hold together, but will 
be blown to pieces by the rush of air past it. This action 
may be seen in a waterfall or a fountain, where, in passing 
through the air, a solid column of water is separated into 
drops not larger than large raindrops. 
The same reasoning does not hold for hailstones^ which 
are held together by the adhesion of the particles through- 
out their entire mass, and whose compactness and strength 
increases with their size. It is, however, the case that the 
smaller end of the stone, where the texture is looser, appears 
to be blown off in its subsequent descent, especially when 
the stones acquire a large size. 
It seems, therefore, that, so far as the growth of a drop or a 
stone is concerned, the particles it overtakes in its downward 
path are a necessary and sufficient cause ; but the origin of 
the drops and stones requires further explanation. Why 
should some of the particles in a cloud be larger than the 
others, as it is necessary for them to be in order that they 
may commence a more rapid descent ? 
A cloud does not always rain ; and hence it would seem 
that in their normal condition the particles of a cloud are all 
of the same size and have no internal motion, and that the 
variation of size is due to some irregularity or disturbance 
in the cloud. 
Such irregularity would result when a cloud is cooling by 
radiation from its upper surface. The particles on the top 
of the cloud being more exposed would radiate faster than 
those below them, and hence they Would condense more 
