57° F-f'^! of (^"I'^O" Frc/h-U'ater Lakf, ilfc. 



dcncy to c'efcend, on being farther cooled, yet, as they have fo confiderablc a quantity of 

 ("■-•nfible heat (eight or ten degrees) to difpofc of, after their condeiifalion by cold ccafcs ; 

 and as the agitation into which the water is thrown by the wind does not permit any par- 

 ticle to rcm.iin long enough in contaft with the cold air to give off all its heat at once; there 

 i-i a continual fucceflion of frefh particles at ilie fuiface, all of which give oil" heat to the 

 sir, but none of them have time to be cooled fufTiciently to form ice. The water will there- 

 fore lofe a vaft quantity of heat; and as foon as the wind ccafcs, if the cold ihould continue, 

 ice will be formed very rapidly. 



But it is not merely the agitation of the water which renders the communic.ition of tJie 

 heat very rapid ; the agitation of the wind alfo tends to produce the fame effecl. 



0;i the return of fpring, the fnow melting before the fun, as he advances and his rays be- 

 come more powerful, all the heat which the earth exhales is employed in diflblving the 

 ice at its under furface, while the fun on the other fide ads ftill more powerfully to pro- 

 duce the fame effect. 



Though ice is tranfparent, yet it is net perfeiflly fo ; and as the light which Is flopped, 

 in its paffage through it, cannot fail to generate heat wken and ivhire it is (lopped or ab- 

 forbed, it is by no means furprifmg that fnow fliould be found to melt when expofed in 

 the fun's rays, even when the temperature of the air in the fliade is confiderably below 

 the point of freezing. Snow expofed to the fun melts long before the even furface of ice 

 begins to be fenfibly fofiened by its beams ; and it is not till fome time after all the hills are 

 bare that the ice on the lakes and rivers breaks up. 



The rays which penetrate a bank of fnow, being often refledled and refraclcd, defcend 

 deep into it, and the heat is depofited in a place where it is not expofed to be carried off by 

 the cold air of the atmofphere ; but the rays which fall upon the horizontal and fmooth fur- 

 face of the ice are moftly reflected upwards into the atmofphere : and if any part of tliem 

 are flopped at the furface of the ice, the heat generated by them ihere is inftantancoufly 

 carried off by the cold air, and a particle of water is no fooner made fluid than it is again 

 frozen. 



Hence we fee, that the fnow which in cold countries covers the ice that is formed on the 

 furface of frefh water, not only prevents the heat of the water from being carried off by the 

 air during the winter, but alfo affifts very powerfully in thawing the ice early in the fpring. 



Let us now fee what the confequences would have been, had the condenfation of water 

 with cold followed the law which obtains in regard to all other fluids. 



As the internal motion of the water could not have failed to continue as long as its fpe- 

 cific gravity continued to be increafed by parting with heat, ice would not have begun to be 

 formed till the whole mafs of water had arrived at the temperature of 32° of Fahrenheit's 

 thermometer. 



To fee what an enormous quantity of heat would be loft when the water is deep, in 

 confequence of its whole mafs being cooled in this m mncr, we have only to compute how 

 much ice this heat would melt, or how much water it would heat, from the point of freezing 

 to that of boiling. 



It has been fliewn by experiments, tliat any given quantity of ice requires as much heat 

 to melt it as an equal quantity of fluid water lofes in cooling 140" ; confequently the quan- 

 tity of ice which might be melted by the heat given off by any given quantity of water in 

 6 cooling 



