HEAT. 715 



the perihelion less, but not varying the mean distance or the amount 

 of heat received annually by the earth from the sun. Maxima in the 

 eccentricity occur once in 100,000 to 200,000 years. With the sun's 

 mean distance 92,400,000 miles, the present aphelion distance is 

 about 93,950,000 miles, and the perihelion, 90,850,000 miles, the ec- 

 centricity being 0-0168. But at a time of extreme maximum eccentric- 

 ity (= 0*075, nearly) the aphelion distance would be about 99,300,000 

 miles, and the perihelion 85,500,000, making the sun 13J millions of 

 miles nearer the earth in summer than in winter. 



The eccentricity passed one of its maxima, according to Stockwell's calculations 

 (Am. J. Sci., II., xlvi., 1868), about 110,000 years since (E = 0-0460); another, higher, 

 210,000 years (00575); another, not so high, 300,000 years (00424); a rather low min- 

 imum, 410,000 years (00170); a low maximum, 475,000 years; a very low minimum, 

 520,000 years (0-0166) ; a maximum, 570,000 years (0417) ; two maxima, the second 

 750,000 years (0 0575); a very low minimum, 800,000 years (0-0132) ; an extreme max.- 

 iinum, 850,000 years (0-0747); another very low minimum, 900,000 years (00102); a 

 maximum, 950,000 years (00517); and so on. For the past 50,000 years the eccentric- 

 ity has been between 00109 and 0-0188, and, in 1850, was 0-0168. In future time, 

 there will be a very low minimum, 24,000 years on; a low maximum, 150,000 years; 

 auother low maximum, 250,000 years; a very low minimum, 300,000 years; a low max- 

 imum, 400,000 years; a very high maximum, 515,000 years. 



At the maximum which occurred 110,000 years since, the aphelion 

 and perihelion distances were 96,650,000 and 88,150,000, the differ- 

 ence, 8,500,000 miles. Owing to the increasing eccentricity there is 

 an increasing difference in the length of the winter half of the year as 

 compared with the summer half; and at the extreme maximum, it is 

 36 days longer than the summer half. 



As the amount of heat which the earth receives varies inversely as 

 the square of the sun's distance, increasing eccentricity diminishes the 

 amount on the aphelion side and increases it on the other ; and if 

 aphelion comes in winter, the winter cold is greatly augmented, besides 

 continuing longer. The summers, on the contrary, would be propor- 

 tionally hotter, but, in the same proportion, shorter. With aphelion 

 in summer the winters would be relatively mild and the summers cool. 

 Mr. James Croll, from whom the above on the effects of change in 

 eccentricity is cited, holds that the cold which would be a consequence 

 of the aphelion coming in winter during a period of maximum eccen- 

 tricity would be sufficient to bring on a Glacial era. He argues that 

 in the north hemisphere, under the supposed condition, " the southeast 

 trade winds would be stronger than the northeast and would blow 

 over upon the northern hemisphere as far probably as the Tropic of 

 Cancer ; " so that " all the great equatorial waters of the ocean would 

 be impelled into the northern hemisphere, which would in consequence 

 have its temperature raised, and snow and ice would to a great extent 

 disappear from the Arctic regions." When the earth, on the contrary, 



