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Secr. II] INTER-GLACIAL PERIODS. 29 
the heat at the equator great. The equator would be the furnace 
where evaporation would take place, and the snow and ice of 
temperate regions would act as a condenser. 
_ “The foregoing considerations, as well as many others which 
might be stated, lead to the conclusion that, in order to raise the 
mean temperature of the globe, water should be placed along the 
equator, and not Jand, as was contended by Sir Charles Lyell and 
others. For if land be placed at the equator, the possibility of 
conveying the sun’s heat from the equatorial regions by means of 
ocean currents is prevented.”* 
Inter-Glacial Periods.—Allusion has already been made to 
the accumulating evidence that changes of climate have been re- 
eurrent, and to the deduction from this alternation or periodicity that 
they have probably been due to some general or cosmical cause. Dr. 
Croll has ingeniously shown that every Jong cold period arising in 
each hemisphere from the circumstances sketched in the preceding 
pages, must have been interrupted by several shorter warm periods. 
“When the one hemisphere,’ he says, “is under glaciation, the 
other is enjoying a warm and equable climate. But, owing to the 
precession of the equinoxes, the condition of things on the two 
hemispheres must be reversed every 10,000 years or so. When the 
solstice passes the aphelion, a contrary process commences; the snow 
and ice gradually begin to diminish on the cold hemisphere and to 
make their appearance on the other hemisphere. The glaciated 
hemisphere turns by degrees warmer, and the warm hemisphere 
colder, and this continues to go on for a period of ten cr twelve 
thousand years, until the winter solstice reaches the perihelion. By 
this time the conditions of the two hemispheres have been reversed ; 
the formerly glaciated hemisphere has now become the warm one, 
and the warm hemisphere the glaciated. The transference of the ice 
from the one hemisphere to the other continues as long as the 
eccentricity remains at a high value. It is probable that, during the 
warm inter-glacial periods, Greenland and the Arctic regions would 
be comparatively free from snow and ice, and enjoying a temperate 
and equable climate.” 
1 That climate, however, may be considerably affected by changes, such as are known 
to have taken place in the distribution of land and sea, must be frankly conceded. 
‘This has been recently cogently argued by Mr. Wallace in his “Island Life,” 1880. 
