SECT. X. CAUSES AFFECTING TEMPERATURE. 75 



cases the quantity of heat and the angular velocity vary exactly 

 in the same proportion, a perfect compensation takes place 

 (N. 144). So that the excentricity of the earth's orbit has little 

 or no effect on the temperature corresponding to the difference of 

 the seasons. 



Sir Charles Lyell, in his excellent works on Geology, refers the 

 increased cold of the northern hemisphere to the operation of 

 existing causes with more probability than most theories that 

 have been advanced in solution of this difficult subject. The 

 loftiest mountains would be represented by a grain of sand on 

 a globe six feet in diameter, and the depth of the ocean by a 

 scratch on its surface. Consequently the gradual elevation of a 

 continent or chain of mountains above the surface of the ocean, or 

 their depression below it, is no very great event compared with 

 the magnitude of the earth, and the energy of its subterranean 

 fires, if the same periods of time be admitted in the progress of 

 geological as in astronomical phenomena, which the successive 

 and various races of extinct beings show to have been immense. 

 Climate is always more intense in the interior of continents than 

 in islands or sea-coasts. An increase of land within the tropics 

 would therefore augment the general heat, and an increase in the 

 temperate and frigid zones would render the cold more severe. 

 Now it appears that most of the European, North Asiatic, and 

 North American continents and islands were raised from the 

 deep after the coal-measures were formed in which the fossil 

 tropical plants are found ; and a variety of geological facts in- 

 dicate the existence of an ancient and extensive archipelago 

 throughout the greater part of the northern hemisphere. Sir 

 Charles Lyell is therefore of opinion that the climate of these 

 islands must have been sufficiently mild, in consequence of the sur- 

 rounding ccean, to clothe them with tropical plants, and render 

 them a fit abode for the huge animals whose fossil remains are 

 so often found; that the arborescent ferns and the palms of 

 these regions, carried by streams to the bottom of the ocean, were 

 imbedded in the strata which were by degrees heaved up by 

 the subterranean fires during a long succession of ages, till the 

 greater part of the northern hemisphere became dry land as it 

 now is, and that the consequence has been a continual decrease 

 of temperature. 



It is evident, from the marine shells found on the tops of the 



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