SKCT. XXVI. SUPERFICIAL HEAT. 251 



of changes continually going on, by the destruction of 

 forests, the filling up of seas, the wearing down of ele- 

 vated lands, the heat radiated from the earth's surface 

 varies considerably at different periods. In consequence 

 of this variation, and also in consequence of the covering 

 up of the bottom of the sea by the detritus of the land, 

 the surfaces of equal temperature within the earth are 

 continually changing their form, and exposing thick 

 beds near the exterior to alterations of temperature. 

 The expansion and contraction of these strata may form 

 rents and veins, produce earthquakes, determine vol- 

 canic eruptions, elevate continents, and possibly raise 

 mountain chains." 



The numerous vents for the internal heat formed by 

 volcanos, hot springs, and the emission of steam so 

 frequent in volcanic regions, no doubt maintain the tran- 

 quillity of the interior fluid mass, which seems to be 

 perfectly inert unless when put in motion by unequal 

 pressure. 



But to whatever cause tha increasing heat of the 

 earth and the subterranean fires may ultimately be 

 referred, it is certain that, except in some local in- 

 stances, they have no sensible effect on the temperature 

 of its surface. It may therefore be concluded that the 

 heat of the earth above the zone of uniform temperature 

 is entirely owing to the sun. 



The powe*of the solar rays depends much upon the 

 manner in which they fall, as we readily perceive from 

 the different climates on our globe. The earth is about 

 three millions of miles nearer to the sun in winter than 

 in summer, but the rays strike the northern hemi- 

 sphere more obliquely hi winter than in the other half 

 of the year. 



The observations of the north polar navigators, and 

 those of Sir John Herscbel at the Cape of Good Hope, 

 show that the direct heating influence of the solar rays 

 is greatest at the equator, and that it diminishes gradu- 

 ally as the latitude increases. At the equator the 

 maximum is 48|, while in Europe it has never ex- 

 ceeded 29i. 



M. Pouillet has estimated with singular ingenuity, 

 from a series of observations made by himself, that the 



