443 Mr. J. Croll on the Change in the Obliquity of the Ecliptic, 



feet of ice would be melted, independently of the effects of ex- 

 centricity, in about 320 years. And supposing that only one- 

 fourth part of the extra heat reached the ground, 470 feet of ice 

 would be removed in about 640 years. 



As to the exact time that the obliquity was at a maximum 

 previous to that of 11,700 years ago our uncertainty is still 

 greater. If we are permitted to assume that the ecliptic passes 

 from its maximum to its minimum state and back to its 

 maximum again with anything like uniformity, at the rate as- 

 signed by Leverrier and others, the obliquity would not be far 

 from a maximum about 60,000 years ago. Taking the rate of 

 precession at 50"'21129, and assuming it to be uniform (which 

 it probably is not), the winter solstice would have been in aphelion 

 about 61,300 years ago*. In short, it seems not at all impro- 

 bable that at the time the solstice-point was in the aphelion, the 

 obliquity of the ecliptic would be not far from its maximum 

 state. But at that time the value of the excentricity was 0*023, 

 instead of 0*0187, its value at the last period. Consequently 

 the rise of the sea would probably be somewhat greater than 

 it was 11,700 years ago. Might not this be the period of the 

 40-feet beach? In this case 11,000 or 12,000 years would be 

 the age of the 25-feet beach, and 60,000 years the age of the 

 40-feet beach. 



About 22,000 years ago the winter solstice was in perihe- 

 lion ; and as the excentricity was then somewhat greater than it 

 is at present, the winters would be a little warmer and the climate 

 more equable than it is at the present day. This perhaps 

 might be the period of the submarine forests and lower peat- 

 beds which underlie the Carse clays, Scrobicularia mud, and 

 other deposits belonging to the age of the 25-feet beach. At 

 any rate, it is perfectly certain that a condition of climate at 

 this period prevailed exceedingly favourable to the growth of 

 peatf It follows also that at this time, owing to a greater accu- 

 mulation of ice on the southern hemisphere, the sea-level would 

 be a few feet lower than at present, and that forests and peat 

 may have then grown on places which are now under the sea- 

 level. 



* In order to determine the position of the solstice-point in relation to 

 the aphelion, it will not do to assume, as is commonly done, that the point 

 makes a revolution from aphelion to aphelion in any regular given period, 

 such as 21,000 years ; for it is perfectly evident that, owing to the great ir- 

 regularity in the motion of the aphelion, no two revolutions will probably 

 be performed in the same length of period. For example, the winter sol- 

 stice was in aphelion about the following dates : 11,700, 33,300, and 

 61,300 years ago. Here are two consecutive revolutions, the one performed 

 in 21,600 years, and the other in 28,000 years, the difference in the length 

 of the two periods amounting to no fewer than 6400 years. 



