There is one theory, however, to which his remarks justly apply, viz., the theory that the great changes of climate during geological ages resulted from the passage of our globe through different temperatures of space. What Professor Tyndall says shows plainly that the glacial epoch was not brought about by our earth passing through a cold part of space. A general reduction of temperature over the whole globe certainly would not produce a glacial epoch. Suppose the sun were extinguished and our globe exposed to the temperature of stellar space (−239° F.), this would certainly freeze the ocean solid from its surface to its bottom, but it would not cover the land with ice.
Professor Tyndall’s conclusions are, of course, equally conclusive against Professor Balfour Stewart’s theory, that the glacial epoch may have resulted from a general diminution in the intensity of the sun’s heat.
Nevertheless it would be in direct opposition to the well-established facts of geology to assume that the ice periods of the glacial epoch were warm periods. We are as certain from palæontological evidence that the cold was then much greater than now, as we are from physical evidence that the accumulation of ice was greater than now. Our glacial shell-beds and remains of the mammoth, the reindeer, and musk-ox, tell of cold as truly as the markings on the rocks do of ice.
Objection from the Present Condition of the Planet Mars.—It has been urged as an objection by Professor Charles Martins[46] and others, that if a high state of eccentricity could produce a glacial epoch, the planet Mars ought to be at present under a glacial condition. The eccentricity of its orbit amounts to 0·09322, and one of its southern winter solstices is, according to Dr. Oudemans, of Batavia,[47] within 17° 41′ 8″ of aphelion. Consequently, it is supposed that one of the hemispheres should be in a glacial state and the other free from snow and ice. But it is believed that the snow accumulates around each pole during its winter and disappears to a great extent during its summer.
There would be force in this objection were it maintained that eccentricity alone can produce a glacial condition of climate, but such is not the case, and there is no good ground for concluding that those physical agencies which led to the glacial epoch of our globe exist in the planet Mars. It is perfectly certain that either water must be different in constitution in that planet from what it is in our earth, or else its atmospheric envelope must be totally different from ours. For it is evident from what has been stated in [Chapter II.], that were our globe to be removed to the distance of Mars from the sun, the lowering of the temperature resulting from the decrease in the sun’s heat would not only destroy every living thing, but would convert the ocean into solid ice.
But it must be observed that the eccentricity of Mars’ orbit is at present far from its superior limit of 0·14224, and it may so happen in the economy of nature that when it approaches to that limit a glacial condition of things may supervene.
The truth is, however, that very little seems to be known with certainty regarding the climatic condition of Mars. This is obvious from the fact that some astronomers believe that the planet possesses a dense atmosphere which protects it from cold; while others maintain that its atmosphere is so exceedingly thin that its mean temperature is below the freezing-point. Some assert that the climatic condition of Mars resembles very much that of our earth, while others affirm that its seas are actually frozen solid to the bottom, and the poles covered with ice thirty or forty miles in thickness. For reasons which will be explained in the Appendix, Mars, notwithstanding its greater distance from the sun, may enjoy a climate as warm as that of our earth.
CHAPTER V.
REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE NORTHERN.
Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical Mistake in regard to Radiation.—Professor J. D. Forbes on Underground Temperature.—Generally accepted Explanation.—Low Temperature of Southern Hemisphere attributed to Preponderance of Sea.—Heat transferred from Southern to Northern Hemisphere by Ocean-current the true Explanation.—A large Portion of the Heat of the Gulf-stream derived from the Southern Hemisphere.