74 CAUSES AFFECTING TEMPERATURE. SECT. X. 



by the whole earth must be actually on the decrease. We have, 

 therefore, an evident real cause to account for the phenomenon." 

 The limits of the variation in the excentricity of the earth's orbit 

 are unknown. But, if its ellipticity has ever been as great as 

 that of the orbit of Mercury or Pallas, the mean temperature of 

 the earth must have been sensibly higher than it is at present. 

 Whether it was great enough to render our northern climates fit 

 for the production of tropical plants, and for the residence of the 

 elephant and other animals now inhabitants of the torrid zone, it 

 is impossible to say. 



Of the decrease in temperature of the northern hemisphere 

 there is abundant evidence in the fossil plants discovered in 

 very high latitudes, which could only have existed in a tropical 

 climate, and which must have grown near the spot where they 

 are found, from the delicacy of their structure and the perfect 

 state of their preservation. This change of temperature has been 

 erroneously ascribed to an excess in the duration of spring and 

 summer in the northern hemisphere, in consequence of the excen- 

 tricity 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 portion, 

 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 varia- 

 tion of the perihelion. Yet Sir John Herschel has shown that 

 by this alternation 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 



