changes, which greatly affect the distribution of heat over the 

 seasons of the year. We are at present, and have been for 

 80,000 or 100,000 years past, in a period of small excentricity. 

 At present the earth's aphelion, or maximum distance from the 

 sun, occurs near the midsummer of the northern hemisphere. 

 When the position of the aphelion in the year was the same as 

 at present, while the excentricity was at the greatest, our planet 

 received at midsummer of the northern hemisphere nearly a 

 tenth less heat than it does at present, so that the snow of 

 winter would not be melted in summer to anything like the 

 extent that it is now, and many parts of the lands of high 

 northern latitudes, which are now covered with vegetation, 

 were then buried under continental ice like Greenland. At 

 the same time, the great heat of the summer in the non-glaciated 

 hemisphere, where the perihelion would fall at midsummer, 

 would raise a great quantity of vapour, a part of which would 

 be condensed and fall as snow in the glaciated hemisphere, thus 

 increasing its glaciation ; and this action would be promoted 

 by the active circulation of the winds between the summer and 

 the winter hemispheres, which obviously must be greatest at 

 the perihelion of maximum excentricity. 



After about 10,500 years, the aphelion would have moved 

 round to the midsummer of the opposite hemisphere, which 

 then became the glaciated one ; so that, while a period of great 

 excentricity lasts, the two hemispheres are glaciated alternately. 

 The periods of great excentricity are of irregular duration, but 

 very much longer than 10,500 years; — we ought consequently 

 to expect to find geological evidence that the glacial climate 

 has not been continuous but intermittent and recurrent, with 

 warm, or at least not glacial, periods between ; and there 

 appears to be satisfactory evidence of this.* 



Mr. Croll has the great merit of being the first to call atten- 

 tion to the effect on climate of changes in the excentricity of 

 the earth's orbit, and has stated his views at length in his work 

 on Climate and Time. He has, however, made what I think 

 the mistake of attributing glaciation to winter cold, and con- 

 * See Croll's Climate and Time, ch. 15, "Warm Interglacial Periods." 



