Prof. Tyndall on Foam and Hail. 353 



the air enclosed by the waves wheu they fell over, aud in the 

 compression and heating of which a portion of the vis viva of 

 the falling mass was expended. Tlie air thus enti'apped has, on 

 account of its warmth, an increased capacity for aqueous vapour : 

 this, and the evaporation from the extended sui'faces in contact 

 with the free atmosphere, concentrate the solution of which the 

 films are composed, and is probably the cause of the increased 

 bitterness. 



I am tempted to make a remark here upon another subject. 

 While reading, a short time since, an able and elaborate sum- 

 mary of the opinions at present entertained regarding the forma- 

 tion, shape and constitution of hail, I found one circumstance 

 omitted which must have some influence, and may have a very 

 sensible one. While crossing the Hoch Joch in the Tyrol last year, 

 when Dr. Frankland and myself were assailed by a fierce hail- 

 shower, the thought occurred to me that the conditions in front 

 and behind a hailstone, during its motion, must in some cases afi'ect 

 its shape and constitution. The theory of meteorolites generally 

 received at the present day is, that they are opake bodies raised 

 to a state of incandescence by their motion through the atmo- 

 sphere. Whether this theory be correct or not, the resistance of 

 the atmosphere is a true cause, and a generation of heat equiva- 

 lent to the vis viva lost through this resistance is inevitable. 

 ^Vhen a hailstone moves through the air, it has a condensation 

 in front and a rarefaction at its back ; a possible melting tempe- 

 rature before it, with a possible freezing temperature behind it, 

 which may sensibly influence both the shape and constitution of 

 the hailstone. In the shower to which I have referred, the pre- 

 dominant shape of the hailstones was conical, and this shape is 

 also noticed by Dr. Hooker in his journal. It seems a fair 

 matter for inquiry how far this shape, and the stratified appear- 

 ance sometimes exhibited by hailstones, may be dependent on 

 the condensation and rarefaction to which I have alluded. 



Both the circumstances referred to may have been already 

 noticed ; but I am not aware that they have, and this will per- 

 haps excuse me if I should be calling attention to things already 

 sufficiently known and appreciated. As regards the foam, the 

 facts recorded appear to have some bearing upon the determina- 

 tion of the mechanical equivalent of heat by the churning up 

 of water. But the skilful experimenter to whose industry we 

 owe this determination has, I doubt not, excluded every such 

 remote possibility of error. 



Royal Institution, April 1857. 



Phil. Mag. S. 4. Vol. 13. No. 87. Maij 1857. 2 B 



