FROM THE HIGHER ATMOSPHERE. 469 



quent buoyancy, and spread over the surface. In a short while, 

 therefore, the heat is distributed through the whole liquid mass, 

 in successive strata, from the bottom to the top. But liquids 

 also conduct heat by a slow process in every direction, like 

 solids, This protracted diffusion of influence, which is exert- 

 ed in restoring the equality of temperature through the body 

 of the liquid, succeeds to the operation of the principle of 

 buoyancy. While the surface becomes again colder, the bottom 

 grows gradually warmer ; the heat working downwards, by a 

 continual transfer, from stratum to stratum, of the stagnant 

 fluid. 



The diffusion of heat, then, depends chiefly on the expan- 

 sion and internal mobility of the liquid medium. Thus, alco- 

 hol rapidly diffuses heat, while the viscid oils, especially at low 

 temperatures, clog its motions. The circulation is in general 

 quicker when the liquid is very warm, its expansibility, and its 

 aptitude for internal migrations being now increased. In ap- 

 proaching the boiling point, water expands largely by equal ac- 

 cessions of heat ; but near congelation, its expansions or contrac- 

 tions are extremely small. The conducting power of water at 

 a very low temperature, is hence nearly the same as if it were 

 a solid, or remained motionless ; the same, in short, as if by 

 the addition of isinglass, it were changed into a thin jelly. 



Such are the conducting powers which a liquid substance 

 combines when at rest, or left merely to the play of its internal 

 motions. But if made to flow in a stream, it will evidently, in 

 consequence of the frequent renewal of eontact, abstract heat 

 morequickly from every warm solid body which is immersed in it. 

 Thus, I find, that, in ordinary circumstances, water, advancing 

 at the slow rate of a mile in three hours, will yet conduct away 

 heat twice as fast as when quite stagnant. Consequently a cur- 

 rent 



