70 CAUSES AFFECTING THE TEMPERATURE. SECT. X. 



from the secular variation of the perihelion. Yet Sir 

 John Herschel has shown that by this alteration neither 

 hemisphere acquires any excess of light or heat above 

 the other ; for although the earth is nearer to the sun 

 while moving through that part of its orbit in which the 

 perihelion lies than in the other part, and consequently 

 receives a greater quantity of light and heat, yet as it 

 moves faster it is exposed to the heat for a shorter 

 time. In the other part of the orbit, on the contrary, 

 the earth being farther from the sun receives fewer of 

 his rays, but because its motion is slower it is exposed 

 to them for a longer time. And as in both cases the 

 quantity of heat and the angular velocity vary exactly in 

 the same proportion, a perfect compensation takes place 

 (N. 141). So that the eccentricity of the earth's orbit 

 has little or no effect on the temperature corresponding 

 to the difference of the seasons. 



Mr. Lyell, in his excellent work 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 scratcl^ 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 va- 

 rious races of extinct beings show to have been immense. 

 Climate is always more intense in the interior of con- 

 tinents 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 indicate the existence of an ancient 



