23 
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 np 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 very 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 difference 
in the density of the clouds and the spray. I shall now 
endeavour to put the apparatus in operation, and exhibit 
the production of some of these artificial stones. 
Snow Crystals. 
I have not yet had an opportunity of examining the tex- 
ture of these artificial stones under the microscope, but to 
all appearance they consist of an aggregation of small 
spherical particles of ice ; and it seems worthy of notice, 
that while nothing like a grown crystal appears ever to be 
produced in the ether spray, the moment the blast is stopped 
the end of the ether tube becomes covered with ice, which 
often assumes the form of snow crystals* 
