21 
“ There is no relation, at the periods in question, between the intensity of 
the sun’s heat and the temperature of the summer. One is apt to suppose, 
without due consideration, that the summers ought then to be as much 
warmer than at present as the winters are colder. Sir C. Lyell in his 
1 Principles ’ has given a column of summer temperature calculated from 
my table on this principle. Astronomically this is correct, but physically, 
as shown in ch. iv., it is wholly erroneous, and would convey a wrong impres- 
sion on the whole subject of geological climate. The summers of that period, 
instead of being much warmer than at present, would in reality be much 
colder, notwithstanding the great increase in the sun’s heat from her 
diminished distance.” 
36. I think there is not the least solid ground for the con- 
trast here affirmed, and that the want of due consideration is 
on the other side. 
First, let us inquire what will be the summer temperature, if 
the principle in the previous calculations of midwinter heat is 
maintained. The contrast will then be between the present 
heat, when the sun is near the aphelion, and the perihelion 
heat with the increased excentricity. Adopting the three cor- 
rections already introduced, first, of the value of the excentricity, 
secondly, of the law of radiation, and thirdly, of the maximum 
heat or cold a month after the solstice, the inci’ease of summer 
heat would be 34 0, 88 and 28 0, 5 at the two eras proposed. 
Thus, instead of 39° and 64°, the present midwinter and mid- 
summer heat in our island, the temperatures would be, by the 
corrected rule, 5 0, 8 and 98 0, 88 for the earlier, and 10 o, 3 and 
92 0, 5 for the later date. 
37. The reasons assigned, why glaciation should have resulted 
indirectly from the increased excentricity about 200,000 years 
ago are these : First, the midwinter temperature would be 
lowered to an enormous extent. I have just shown that this is 
not correct. The decrease would be only 28°*7 instead of 3 7°' 7, 
and the resulting tempei’ature 10 o, 3. This is nearly the same 
as that of Canada, near Quebec, while the summer tempera- 
ture, by the previous estimate, would be almost 30° higher. 
This is wholly different from the conditions of a glacial 
period. 
The winters, it is said, would be longer as well as colder. 
Instead of being 8 days shorter than the summer, as now, the 
excess would be 36 days. But for the period mainly in question 
the difference is 26 days, or 13 days is the excess of the winter 
over half a year. The mean rainfall of our island is 32 inches. 
Without some unproved change in the physical conditions, tho 
rainfall of the winter months would be less than 20 inches, or 
if snow be reckoned six times lighter than water, this would 
amount to a depth of 10 feet only. But the latent cold of ice 
is 140°, and water has four or five times the specific heat of 
