THE FORCES THAT PRODUCE STORMS. 



65 



falling below 32°. For it is well known that when a body of 

 water is frozen by a moderate degree of cold, the process of 

 freezing is slow, as the conversion of a part of the liquid into 

 ice liberates heat enough to preserve the remainder in the 

 form of water ; and it requires time for the liberated heat to 

 pass away before a fresh portion of the water can be frozen 

 by the existing degree of cold in the locality. In this way a 

 mixed mass of water and ice may remain a considerable time 

 at the temperature of 32°, in a part that is below that tem- 

 perature, the heat given out to the water by freezing being 

 nearly equal to that which is passing away ; and this compa- 

 ratively slow operation continues until all the water is frozen. 

 The same process must take place in the atmosphere, when 

 the particles of water produced by condensation of vapour 

 are frozen into snow or hail, that is into ice. As the ice is 

 formed the heat of liquidity of the water is set free, and the 

 temperature of the locality and of the substances that are in 

 it, is prevented sinking below 32° until all the water in the 

 part is frozen. It follows from this, that when an ascending 

 atmospheric column takes newly formed water that is within 

 it to a height sufficient to freeze the water, the column for 

 some time retains the temperature of 32°, while it is ascending 

 successively into colder regions. Tho respective temperatures 

 of the undisturbed cold air in the vicinity, and of the warmed 

 ascending column that is passing through it, may, under 

 these circumstances, be as shewn in the following table, com- 

 mencing from the temperature of zero at the surface ; whilst 

 the differences between the temperatures of the two airs 

 would be those which are inserted in the tabular column of 

 the differences: — 



