in Fluids. 321 



all the Heat which the earth exhales is employed in 

 dissolving the ice at its under surface, while the sun on 

 the other side acts still more powerfully to produce the 

 same effect. 



Though ice is transparent, yet it is not perfectly so ; 

 and as the light which is stopped in its passage through 

 it cannot fail to generate Heat when and where it is 

 stopped or absorbed, it is by no means surprising that 

 snow should be found to melt when exposed in the sun's 

 rays, even when the temperature of the air in the shade 

 is considerably below the point of freezing. Snow ex- 

 posed to the sun melts long before the even surface of 

 ice begins to be sensibly softened by its beams, and it is 

 not till some time after all the hills are bare that the ice 

 on the lakes and rivers breaks up. 



The rays which penetrate a bank of snow, being often 

 reflected and refracted, descend deep into it, and the 

 Heat is deposited in a place where it is not exposed to 

 be carried off by the cold air of the atmosphere ; but the 

 rays which fall upon the horizontal and smooth surface 

 of ice are mostly reflected upwards into the atmosphere; 

 and if any part of them are stopped at the surface of the 

 ice, the Heat generated by them there is instantaneously 

 carried off by the cold air, and a particle of water is no 

 sooner made fluid than it is again frozen. 



Hence we see that the snow which in cold countries 

 covers the ice that is formed on the surface of fresh wa- 

 ter not only prevents the Heat of the water from being 

 carried off by the air during the winter, but also assists 

 very powerfully in thawing the ice early in the spring. 



Should the waters of a lake be so deep, or so imper- 

 fectly transparent as to intercept a great proportion of 

 rays of the sun before they reach the bottom, in that 

 VOL. I. 21 



