314 The History of our Earth. [July, 
winds, blowing along the surface of the land, and not de- 
pending upon oceanic currents, are not unknown. On the 
south coast of Australia, a hot north wind, blowing from 
the heated interior of that continent, is common. In the 
South of Europe and in South-western Asia warm winds are 
experienced which cannot be fairly ascribed to the Gulf- 
stream. On the other hand, the greatest climatic scourge 
of Western Europe is the north-easterly | wind blowing from 
Siberia and Russia. In America, the ‘‘ norther ”’—which 
sweeps ‘‘sword in hand” from the polar regions down to 
Texas and Mexico—is likewise a land- wind. These faéts 
certainly lend some countenance to the view that the climate 
of any locality in the temperate zone is lowered by land 
lying between such place and the Pole, and raised by land 
lying between such place and the Equator. 
Without, however, going further into this question, we 
can scarcely assume that the land preponderated alternately 
in the frigid and in the torrid zone for every alternation of 
hot and glacial periods that the world has experienced. Part 
of the evidence of a former milder epoch is the discovery of 
large trees, 2m situ, in Greenland, Spitzbergen, and similar 
regions. The comparative warmth of the interglacial 
periods cannot, therefore, be due to the absence of land in 
high latitudes. 
It has been ingeniously suggested that a warm epoch 
might be produced by an alteration in the composition of 
the atmosphere. Carbonic acid gas, whilst perfe¢tly per- 
vious to heat-rays emanating from a body of high tempera- 
ture, such as the sun, is almost impassable for heat-rays of 
low tension given off by non-luminous bodies. Did our at- 
mosphere contain a few per cents of this gas, the earth 
would be warmed by the sun’s rays, during the day, precisely 
as at present; but at night the loss of heat by radiation 
would be practically annulled, and the earth consequently 
would lose little or nothing of the heat absorbed during the 
day. But though this theory might account for a single 
warm epoch, it cannot explain a succession of intensely cold 
periods, separated by a series of warm intervals. 
We must now consider Mr. Croll’s views on the cause of 
the Glacial epochs,—a theory which has a certain resem- 
blance to that of Adhémar, if we strip the latter of certain 
of its exaggerations. 
There are two circumstances which influence the relative 
position of the sun and the earth, and which, to a very great 
extent, affect the climate of the latter. These are the pre- 
cession of the equinoxes and the periodical changes in the 
