HAILSTONES, RAINDROPS, AND SNOWFLAKES. 1G9 



lowing estimate tends to show that the artificial are probably 

 formed more quickly than the actual stones. 



The speed of the jet of air at the point at which the 

 stones are formed is nearly equal to that at which the. 

 larger stones would fall through the air. This is shown by 

 the fact that if a large stone becomes accidentally detached 

 from its splinter of wood it rather falls than rises, but when 

 this happens with smaller stones they are driven up by the 

 force of the blast. 



I find that the speed of the blast varies from 150 to 200 

 feet per second, i. e. from one to two miles a minute. The 

 larger stones, therefore, traverse from one to three miles 

 of frozen spray. So that if we imagine a cloud as dense 

 as the spray, it would have to be from one to three miles 

 thick in order that the stones might, in falling through it, 

 attain the size of the artificial stones ; and considering that 

 the stones would only gradually acquire a speed equal to 

 that of the blast, the time occupied in falling through the 

 cloud would, in all probability, be veiy considerable, at 

 least from five to ten minutes, after the stone had acquired 

 a sensible size. 



As regards the proportion which the density of spray 

 bears to that of a cloud, a comparison may be made from 

 the fact that when working in saturated air at a tempera- 

 ture of 60° or 70° F., the condensation of vapour supplied 

 sufficient ice to form the spray ; and since it is probable 

 that the dense summer clouds, from which hail is formed, 

 result from the cooling of air from temperatures nearly, if 

 not quite, equal to this, there is probably no great diff'erence 

 in the density of the clouds and the spray. 



Snow Crystals. 



I have not yet had an opportunity of examining the tex- 

 ture of these artificial stones under the microscope ; but to 



