SECT. XXV, SUPERFICIAL HEAT. 237 



periods. In consequence of this variation, and also in conse- 

 quence 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 volcanic eruptions, elevate continents, 

 and, possibly, raise mountain chains." 



The numerous vents for the internal heat formed by volcanoes, 

 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 con- 

 cluded that the heat of the earth, above the zone of uniform 

 temperature, is entirely owing to the sun. 



The power of the solar rays depends much upon the manner 

 in which they fall, as we readily perceive from the different 

 climates on our globe. Although the sun is about three millions 

 of miles nearer to the earth in winter than in summer, his rays 

 strike the atmosphere in the northern hemisphere so obliquely 

 that it absorbs a much greater quantity of heat than when they 

 are more direct (N. 217). Indeed it is so great that, when the 

 sun has an altitude of 30, one half of his heat is absorbed by the 

 atmosphere, and it increases very rapidly as he sinks towards the 

 horizon. However, that heat is not lost : it is most beneficial to 

 the earth, being really the heat which supplies the greatest part 

 of that which is radiated into space during the absence of the sun. 

 Professor Dove has shown, by taking at all seasons the mean of 

 the temperatures of points on the earth's surface diametrically 

 opposite to each other, that the average temperature of the whole 

 earth's surface in June, when we are farthest from the sun, con- 

 siderably exceeds that in December, when we are nearest to him, 

 owing to the excess of water in the southern hemisphere, and 

 that of land in the northern, which gives a general insular 

 climate to the former, and a continental climate to the latter. 



The observations of the north polar navigators, and those of 



