in Fluids. 319 



with snow (which commonly happens soon after the ice 

 is formed), this is an additional and very powerful ob- 

 stacle to prevent the escape of the Heat out of the wa- 

 ter ; and though the most intense cold may 1-eign in the 

 atmosphere, the increase of the thickness of the ice will 

 be very slow. 



During this time the mass of water which remains un- 

 frozen will lose no part of its Heat ; on the contrary, it 

 will continually be receiving Heat from the ground. 

 This Heat, which is accumulated in the earth during the 

 summer, will not only serve, in some measure, to re- 

 place that which is communicated to the atm.osphere 

 through the ice, and prevent its being furnished at the 

 expence of the latent Heat of the water in contact with 

 its surface, but when the temperature of the air is not 

 much below that of freezing, this supply of Heat 

 from below will be quite sufficient to replace that which 

 the air carries off; and the thickness of the ice will not 

 increase. 



Whenever the temperature of the air is not actually 

 colder than freezing water, the Heat which rises from the 

 bottom of the lake will be all employed in melting the 

 ice at its under surface, and diminishing its thickness. 



It will indeed frequently happen, when the ice is very 

 thick, and especially when its upper surface is covered 

 with deep snow, that the melting of the ice at its under 

 surface will be going on, when the temperature of the at- 

 mosphere is considerably below the freezing point. 



As the particles of water which, receiving Heat from 

 the ground at the bottom of the lake, acquire a higher 

 temperature than that of 40°, and being expanded^ and 

 becoming specifically lighter by this additional Heat, 

 rise up to the upper surface of the fluid water, and give 



