312 The History of our Earth. [July, 
well known that at present at the equinoxes, when day and 
night are equal, snow, and not rain, prevails in the arctic 
regions, and can we suppose it would be otherwise in the 
case under consideration ? How, we may well ask, could 
these regions, deprived of their summer, get rid of their 
snow and ice?” The circumstances which Mr. Belt con- 
siders that physical astronomers have left out of account in 
calculating the variability of the ecliptic, Mr. Croll does not 
think capable of materially affecting the question at issue. 
But in the chapter in which he criticises the views of 
Mr. Belt, Mr. Croll makes certain admissions which very 
materially qualify his declaration which we have just 
quoted. He tells us (p. 398) that the ‘‘ conclusion generally 
come to by geologists and physicists ”’ is that no great effect 
can be ascribed to a change in the obliquity of the ecliptic, 
but that, after giving special attention to the matter, he has 
been ‘‘ led to the very opposite conclusion!” ‘‘It is quite 
true,” he proceeds, “that the changes in the obliquity of 
the Equator cannot sensibly affect the climate of temperate 
regions, but it will produce a slight change in the climate of 
tropical latitudes, and a very considerable effect on that of polar 
regions, especially at the poles themselves.” An increase of ob- 
liquity from 23° 28’ to the known maximum 24° 50’ 34” would 
cause the poles to receive 1-18th more solar heat than they 
do at present, and, all other things being equal, would pro- 
duce “‘a rise in the mean annuai temperature equal to 14° 
or 15°, raising the temperature of the poles to that now 
prevailing at latitude 76°.” But as the polar regions are 
covered with snow and ice, this extra heat would have little 
effect in raising the temperature, and would mainly serve to 
increase by 1-18th the total yearly amount of ice melted at 
the poles. Thus the maximum of obliquity, if coinciding 
with other phenomena to which we shall refer in expounding 
Mr. Croll’s own theory, would have a decided effect in pro- 
ducing the warm interglacial periods. 
Let us now examine the theory of Sir Charles Lyell. 
This great geologist contended that the extraordinary cli- 
matic changes of which the “stone book” bears record 
were due to differences in the distribution of land and 
water. Were the land all accumulated in high latitudes, 
and were the intertropical regions occupied exclusively by 
water, the general temperature of the earth would be 
lowered sufficiently to account for the glacial period. Con- 
versely, were the land collected within or near the tropics, 
and were the polar regions occupied by sea, the temperature 
of the globe would be as strikingly raised. This doctrine 
