SECT, x.] DECREASE OF TEMPERATURE. 



perature. That accomplished author, in pointing out the 

 mutual dependencies of phenomena, says, " It is evident that 

 the mean temperature of the whole surface of the globe, in 

 so far as it is maintained by the action of the sun at a higher 

 degree than it would have were the sun extinguished, must 

 depend on the mean quantity of the sun's rays which it re- 

 ceives, or which comes to the same thing on the total 

 quantity received in a given invariable time ; and, the length 

 of the year being unchangeable in all the fluctuations of the 

 planetary system, it follows that the total amount of solar 

 radiation will determine, cceteris paribus, the general climate 

 of the earth. Now, it is not difficult to show that this 

 amount is inversely proportional to the minor axis of the 

 ellipse described by the earth about the sun (N. 140), re- 

 garded as slowly variable ; and that, therefore, the major 

 axis remaining, as we know it to be, constant, and the orbit 

 being actually in a state of approach to a circle, and conse- 

 quently 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 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 tem- 

 perature of the earth must have been sensibly higher than 

 it is at present. Whether it was great enough to render 

 our nothern 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 disco veredf 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 



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