WHICH RAINDROPS AND HAILSTONES ARE FORMED. 53 



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

 the stone has increased. 



It appears as though it might be possible to make arti- 

 ficial hailstones. 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 my way to obtain such a stream of frozen fog, 

 I thought it might be worth while to try the effect of 

 blowing very finely powdered plaster of Paris. I therefore 

 introduced a stream of this material into a jet of steam is- 

 suing freely into the air (which I hoped would moisten the 

 powdered plaster sufficiently to cause it to set firmly in 

 whatever form it collected into). The jet was directed 

 against a splinter of wood. 



In this way I obtained masses of plaster very closely re- 

 sembling hailstones. They were all more or less conical, 

 with their bases facing the jet. But as might be expected, 

 the angles of the cones were all smaller than those of the 

 hailstones. Two of their figures are shown in the sketches 

 annexed (p. 54). 



The striae were strongly marked, and exactly resembled 

 those of the hailstone. The bases also were rounded. 

 They were somewhat steeper than those of the hailstone ; 

 but this was clearly due to the want of sufficient cohesive 

 power on the part of the plaster : it was not sufficiently 

 wet. Owing to this cause also it was not possible to pre- 

 serve the lumps when they were formed, as the least shake 

 caused them to tumble in pieces. 



I also tried a jet of the vapour of naphthaline, which at 

 ordinary temperatures is solid, driven by means of a cross 

 blast of air against a small object; and in this way I obtained 

 masses closely resembling hailstones : but these also were 

 too fragile to bear moving. At ordinary temperatures the 

 powdered naphthaline does not adhere like ice when pressed 



