52 PROF. OSBORNE REYNOLDS ON THE MANNER IN 



through the air^ whereas iu 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 tliat 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 toge- 

 ther 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 

 raindrops 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. 



The hailstones are clearly aggregations of small frozen 

 particles such as those which form a cloud. Nor is it 

 possible that they can have been drawn together by some 

 electrical attraction ; for whatever such attraction we can 

 conceive, it will not explain the conical shape of the stones 

 or their increase in density towards their thicker sides. 

 These clearly show that the particles have aggregated from 



