HAILSTONES, RAINDROPSj AND SNOWFLAKES. 163 



formed is uot 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 great- 

 est, 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. Tilghman'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 ob- 

 taining such a stream of frozen fog ; but I gave two sketches 

 of plaster stones, which, as far as their shape and the stri- 

 ated appearance of their surface were concerned, closely 

 resembled hailstones, and which plaster stones had been 

 obtained by blowing some finely-divided plaster of Paris 

 against small splinters of wood by means of a jet of steam. 



In the discussion which followed my paper Dr. Cromp- 

 ton suggested 



The Ether Spray, 



such as is used in surgery, as a means of obtaining a frozen 

 fog. And shortly after the Meeting I tried this ether spray, 

 using an instrument such as surgeons use. But although I 

 found that the spray would freeze any thing such as a small 

 tube of water, I could get no deposit of ice particles on the 

 outside of any object. I varied the form of the apparatus, 



m2 



