26 
it grows in size and its velocity increases it will strike the 
particles it overtakes with greater force, and so drives them 
into a more compact mass. If the velocity were sufficient 
the particles would strike with sufficient force to adhere as 
solid ice, and this appears to be the case when the stones 
become large, as large^as a walnut, for instance. 
An idea of the effect of the suspended particles on being 
overtaken by the stone, may be formed from the action of 
the particles of sand in Mr. Tilghman’s sand-blast, used for 
cutting glass. The two cases are essentially the same, the 
only difference being that the hailstone is moving through 
the air, whereas in the case of the sand-blast, the object 
which corresponds to the stone is fixed, and the sand is 
blown against it. 
By this sand-blast the. finest particles of sand are made to 
indent the hardest material, such as quartz or hard-steel; so 
that the actual intensity of the pressure between the surface of 
the particles of sand and that of the object they strike, must 
be enormous. And yet the velocity of the blast is not so much 
greater than that at which a good-sized hailstone descends. 
It is easy to conceive, therefore, that the force of the impact 
of the suspended particles of ice, if not much below the 
temperature of freezing, on a large hailstone, would drive 
them together so as to form solid ice. For the effect of 
squeezing two particles of ice together is to cause them to 
thaw at the surface of contact, and as soon as the pressure 
is relieved they freeze again, and hence their adhesion. 
Nor does there appear to be any other way in which these 
ordinary hailstones can be formed. They are clearly not 
rain- drops frozen, or they would be somewhat transparent; 
neither are they aggregations of snow crystals. Nor can 
they be formed by the condensation and refrigeration of 
vapour on a nucleus of ice. For there is no way of get- 
ting rid of the heat which must be developed by such a 
process. The heat developed by the condensation of vapour 
one-seventh of the weight of the stone, would be sufficient 
to thaw the entire stone. 
