SECT. X. DECREASE OF TEMPERATURE. 69 



not difficult to show that this amount is inversely pro- 

 portional to the minor axis of the ellipse described by 

 the earth about the sun (N. 140), regarded as slowly 

 variable ; and that, therefore, the major axis remaining, 

 as we know it to be constant, and the orbit being actu- 

 ally in a state of approach to a circle, and consequently 

 the minor axis being on the increase, the mean annual 

 amount of solar radiation received 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 eccentricity 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 Jaave 

 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 resi- 

 dence 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 eccentricity 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 eccentricity, small as it is, 

 any line passing through the center 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 



