LAWS OF HEAT. THE EARTH. 77 



fluous heat from its surface into the surrounding 

 space. These processes are called conduction 

 and radiation, and have each their ascertained 

 mathematical laws. 



By the laws of conduction, the daily impres- 

 sions of heat which the earth receives, follow each 

 other into the interior of the mass, like the waves 

 which start from the edge of a canal ;* and like 

 them, become more and more faint as they pro- 

 ceed, till they melt into the general level of the 

 internal temperature. The heat thus transmitted 

 is accumulated in the interior of the earth, as in 

 a reservoir, and flows from one part to another of 

 this reservoir. The parts of the earth near the 

 equator are more heated by the sun than other 

 parts, and on this account there is a perpetual 

 internal conduction of heat from the equatorial to 

 other parts of the sphere. And as all parts of the 

 surface throw off heat by radiation, in the polar 

 regions, where the surface receives little in re- 

 turn from the sun, a constant waste is produced. 

 There is thus from the polar parts a perpetual 

 dispersion of heat in the surrounding space, 

 which is supplied by a perpetual internal flow 

 from the equator towards each pole. 



* The resemblance consists in this ; that we have a strip of 

 greater temperature accompanied by a strip of smaller temperature, 

 these strips arising from the diurnal and nocturnal impressions 

 respectively, and being in motion; as in the waves of a canal, 

 we have a moving strip of greater elevation accompanied by a 

 strip of smaller elevation. We do not here refer to any hypo- 

 thetical undulations in the fluid matter of heat. 



