3 20 Of the Propagation of Heat 



off their sensible Heat to the under surface of the ice, 

 never return to the bottom, this communication of the 

 Heat which exhales from the earth produces very little 

 motion in the mass of the water ; and this circumstance 

 is, no doubt, very favourable to the preservation of the 

 Heat of the water. 



When a strong wind prevails, and the surface of the 

 water is much agitated, ice is not formed, even though 

 the whole mass of water should, by a long continuance 

 of cold weather, have been previously cooled down to 

 that point to which it is necessary that it should be 

 brought, in order that its internal motions may cease, and 

 it may be disposed to congeal ; for though the particles 

 at and near the surface may no longer have any tendency 

 to descend, on being farther cooled, yet, as they have so 

 considerable a quantity of sensible Heat (eight or ten 

 degrees) to dispose of, after their condensation with cold 

 ceases, and as the agitation into which the water is thrown 

 by the wind does not permit any particle to remain long 

 enough in contact with the cold air to give off all its 

 Heat at once, there is a continual succession of fresh 

 particles at the surface, all of which give off Heat to 

 the air; but none of them have time to be cooled suf- 

 ficiently to be disposed to form ice. The water will lose 

 a vast quantity of Heat, and as soon as the wind ceases, 

 if the cold should continue, ice will be formed very rap- 

 idly. 



But it is not merely the agitation of the water which 

 renders the communication of the Heat very rapid, the 

 agitation of the wind also tends to produce the same ef- 

 fect. 



On the return of spring, the snow melting before the 

 sun as he advances and his rays become more powerful. 



