138 J. Croll—Cause of Mild Polar Climates. 
Art. XXI.— On the Cause of Mild Polar Climates ; by 
JAMES CROLL, LL.D., F.R.S. 
[Continued from page 29.] 
Influences of Eccentricity during the Tertiary Period.—This be- 
ing the state of things on the southern hemisphere, the glacial 
condition of the hemisphere, when its winter solstice was In 
aphelion, would tend in a powerful manner to impel the warm 
water of the south over on the northern hemisphere, and thus 
raise its temperature. This, again, is a view which has also 
been urged by Mr. Wallace. “Though high eccentricity 
would,” he remarks, “not directly modify the mild climates 
produced by the state of the northern hemisphere which pre- 
vailed during Cretaceous, Eocene, and Miocene times,* It 
might indirectly affect it by increasing the mass of antarcti¢ 
ice, and thus increasing the force of the trade-winds and the 
resulting northward-flowing warm currents. .... And as wé 
have seen that during the last three million years the eccen- 
tricity has been almost always much higher than it is now, wé 
should expect that the quantity of ice in the southern hemt- 
sphere will usually have been greater, and will thus have tet 
ded to increase the force of those oceanic currents which pro 
duce the mild climates of the northern hemisphere” (p. 192 
There is litthe doubt but that the climate of the Tertiary 
period was greatly affected by eccentricity ; but, owing to the 
difference in the geographical conditions of the two hemr 
spheres, eccentricity would exercise a much greater influence 
powerfully in impelling the equatorial waters toward the are 
when the solstice was in aphelion. In this case the northerm 
hemisphere would be heated to a greater extent when its winter 
solstice was in perihelion than it would be cooled when the 
solstice was in aphelion. It is this circumstance which, - 
think, has misled geologists, and induced them to conclude 
that because the physical agents brought into operation whe? 
the winter solstice was in aphelion, during a high state of e¢ — 
centricity, failed to produce a well-marked glacial epoch 
* High eccentricity might not directly modify the mild climates, but certainly — 
the physical agents brought into operation by the high eccentricity would do 80. L 
