84 CAUSES AFFECTING THE TEMPERATURE. [SECT. x. 



the duration of spring and summer in the northern hemi- 

 sphere, in consequence of the exeentricity of the solar ellipse. 

 The length of the seasons varies with the position of the 

 perihelion (N. 64) of the earth's orbit for two reasons. 

 On account of the excentricity, small as it is, any line passing 

 through the centre of the sun divides the terrestrial ellipse 

 into two unequal parts, and by the laws of elliptical motion 

 the earth moves through these two portions with unequal 

 velocities. The perihelion always lies in the smaller por- 

 tion, and there the earth's motion is the most rapid. In the 

 present position of the perihelion, spring and summer north 

 of the equator exceed by about eight days the duration of 

 the same seasons south of it. And 10,492 years ago the 

 southern hemisphere enjoyed the advantage we now possess 

 from the secular variation of the perihelion. Yet Sir John 

 Herschel has shown that by this alteration neither hemi- 

 sphere 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 ex- 

 posed 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 excentricity 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 



