4 
Proceedings of the Royal Irish Academy. 
for those times in wliich the distribution of land and water did not 
materially differ from the present. At two periods within the last 
1,000,000 years the calcnlated temperatures would be sufficient to 
account for the phenomena of the Glacial Period. The nearest of these 
occurred 200,000 to 210,000 years ago ; and the more remote, 750,000 
to 850,000 years ago — an interval even more temperate than the pre- 
sent intervening between the two latter periods, 800,000 years ago. 
The excentricity, 210,000 years ago, was 0*0575, or 3^ times the 
amount of the present ; so that winter in aphelion would exceed sum- 
mer in perihelion by 27*8 days. The mean temperatures of the hottest 
and coldest months, calculated for a place in the position of London, 
would, upon the hypothesis above stated, be 113° Eahr., and 0°-7 Fahr. 
The excentricity 750,000 years ago was the same as 210,000 years 
ago; then came a period, 800,000 years ago, at which the excentricity 
was only 0*0132, or less than at present, being now 0*0168. This 
excentricity gives 6*4 days more in winter in aphelion than in sum- 
mer in perihelion, and mean temperatures for the hottest and coldest 
months at London of 82° and 22° Tahr. 
One hundred thousand years previously, that is, 850,000 years ago, 
the excentricity amounted to 0*0747 ; this should have given a winter 
in aphelion 36*4 days longer than summer in perihelion, and a mean 
temperature for the hottest and coldest months at London of 126° Pahr. 
and -7° Fahr., a true glacial cold. 
These extreme temperatures, produced in opposite phases of the 
revolution of the apsides, are necessarily accompanied by extreme 
winter and summer temperatures in the same year, at any given place. 
If the average temperature of the coldest winter months in London was 
so low as - 7° Fahr., the summer temperature must have been corre- 
spondingly high ; because, as I have said before, the actual amount of 
heat falling upon the northern hemisphere during the shorter summer 
in perihelion must have been equal to that falling during the longer 
summer in aphelion. Professor Tyndall has well observed, that the 
aim of writers on the subj ect of the formation of glaciers is the attain- 
ment of cold, whereas heat as well as cold is required. The true con- 
ditions are, in fact, extreme temperatures combined with certain 
geographical conditions — namely, a suitable expanse of sea to act as an 
evaporating surface to produce vapour, and of high lands and moun- 
tains to act as condensers of the vapours. In Siberia and Central Asia 
excellent condensers are to be found ; but there are no evaporating 
surfaces, for the vapours of the Indian Ocean are intercepted by the 
Himalaya. Hence no glaciers are found on the southern declivities 
of the Altai and Sayan mountains. More vapour appears to come 
from the ITorthern Ocean than from the high central table land ; for 
it is worthy of remark, that the only glaciers on the Sayan range are 
on the northern declivity, as, for instance, the curious glacier of the 
Mungau Xardik. The extreme temperatures which the revolutions 
of the seasons, under the influence of a high obliquity of the earth's 
orbit, should produce, appear then to suggest a possible cause of a 
