SECT, xxvi.] VOLCANIC THEOEIES. 287 



time coasts. Many break out even in the bottom of the 

 ocean, probably owing to some of the supports of the super- 

 ficial crust giving way, so that the steam and lava are forced 

 up through the fissures. 



Finally, Mr. Babbage observes that, " in consequence of 

 changes continually going on, by the destruction of forests, 

 the filling up of seas, the wearing down of elevated lands, 

 the heat rediated 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 tem- 

 perature within the earth are continually changing their 

 form, and exposing thick beds near the exterior to altera- 

 tions of temperature. The expansion and contraction of 

 these strata may form rents and veins, produce earthquakes, 

 determine volcanic eruptions, elevate continents, and, pos- 

 sibly, raise mountain chains." 



The numerous vents for the internal heat formed by vol- 

 canoes, hot springs, and the emission of steam, so frequent 

 in volcanic regions, no doubt maintain the tranquillity of the 

 interior fluid mass, which seems to be perfectly inert unless 

 when put in motion by unequal pressure. 



But, to whatever cause the increasing heat of the earth and 

 the subterranean fires may ultimately be referred, it is certain 

 that, except in some local instances, 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 uni- 

 form temperature, is entirely owing to the sun. 



The power of the solar rays depends much upon the man- 

 ner 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 sum- 

 mer; but the rays strike the northern hemisphere more 

 obliquely in winter than in the other half of the year. 



The observations of the north polar navigators, and those 

 of Sir John Herschel, at the Cape of Good Hope, show that 



