FORMATION OF RAINDROPS AND HAILSTONES. 525 



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 be- 

 low 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. 



It is then shown that hailstones, such as those described, can 

 neither be formed by the freezing of raindrops, nor by the condensa- 

 tion of vapor on a nucleus of ice; and that it is impossible that the 

 particles of ice can have been drawn together by electrical attraction 



Fig. 4. Imitation in Plaster of Paris. 



their conical shape, and the increase in their density toward iheir 

 thicker sides, clearly showing that the particles have aggregated from 

 one direction, and with an increasing force as the size of the stone has 

 increased. 



The possibility of making artificial stones is thus considered : If a 

 stream of frozen fog were driven against any small object, then the 

 frozen particles should accumulate on the object in a mass resembling 

 a hailstone. Not seeing his way to obtain such a stream of frozen 

 fog, the author thought it might be worth while to try the effect of 

 blowing very finely-powdered plaster of Paris. He therefore intro- 

 duced a stream of this material into a jet of steam, issuing freely into 



