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aggregation, which I had discovered when a particle which 
ultimately forms the vertex of the cone starts on its down- 
ward career and encounters other particles ; these adhere to 
its lower face. The mass, therefore, grows in thickness 
downwards, and as some of the particles strike the face 
so close to the edge that they overhang, the lower face con- 
tinually grows broader, and a conical form is given to the 
mass above. 
When a particle first starts it moves slowly, and the 
force with which it meets the other particles is slight, and 
consequently its texture is loose, but as it increases in size 
and velocity it strikes the particles which it overtakes with 
greater force and so drives them into a more compact mass. 
Assuming that the temperature at which hailstones are 
formed is not greatly below 32° the particles must actually 
freeze together. For the effect of squeezing two pieces of 
ice together at or near the temperature of 32° is to cause 
them to thaw at those points where the pressure is greatest, 
at which points they freeze again as soon as the pressure is 
removed. 
In illustration of the force with which the particles strike 
the face of the hailstone, I instanced the action of the 
particles of sand In Mr. Tilgh man’s sand blast used for 
cutting glass and other hard materials. 
I also reverted to the possibility of making 
Artificial Hailstones , 
by blowing a stream of frozen fog against a small object, 
making as it were the cloud to rise up and meet the stone 
instead of the stone falling through the cloud. 
I had not, however, then overcome the difficulty of 
obtaining such a stream of frozen fog, but I gave two 
sketches of plaster stones, which as far as their shape and 
the striated appearance of their surface were concerned, 
closely resembled hailstones, and which plaster stones bad 
