CHANGES IN THE EARTIl's ORBIT. 271 



by degrees, would produce a decrease of temperature, and that 

 the glacial epoch would be the result. Thus, while one hemi- 

 sphere would pass through a glacial period, the other, on the 

 contrary, would enjoy a warmer and more uniform climate, 

 more especially as the marine currents which convey to high 

 latitudes much heat proceeding from the torrid zone would 

 take a different direction in consequence of the change of ex- 

 centricity. Mr. Croll has determined the excentricity of the 

 orbit for the last 3,000,000 years, and has thus found three 

 periods of greatest excentricity. The first of these commenced 

 2,630,000 years ago, and terminated 2,460,000 years ago ; the 

 second commenced 980,000 years ago, and lasted 260,000 years ; 

 the third dates back 240,000 years, and would close 80,000 years 

 ago. During these three periods of greatest excentricity the 

 northern hemisphere would always be in one of its glacial 

 periods when it had the longest winters (in its aphelion). 

 During these periods the southern hemisphere would enjoy a 

 milder climate. This state of things would change every 10,000 

 years. At each of these changes a warm period would suc- 

 ceed a colder one. According to this hypothesis, we should 

 have not only a whole series of glacial periods interrupted 

 by warmer periods, but also long periods in the most distant 

 ages during which the climate would have been nearly like that 

 of the present day. We might even say that these periods, 

 taken on the whole, ought to be regarded as normal, whilst the 

 others, corresponding to the great excentricities, would form the 

 exceptions. 



It may be objected to all these speculations that we do not 

 sufficiently know the influence exerted upon the intensity 01 

 efficacy of the solar rays by the length of the course these rays 

 have to traverse in order to arrive at the earth. Lyell has justly 

 called attention to the fact that, according to Dove's calculations, 

 the earth is warmer in the month of June (that is to say, in the 

 season during which it is most distant from the sun) than in the 

 month of December (when it approaches the sun most closely) . 

 This phenomenon is due to the distribution of land and sea, 

 which is not the same in the northern and southern hemispheres, 

 and hence the northern half of the globe has warmer summers 

 than the southern hemisphere. This proves that the distribu- 



