Page:Physical Geography of the Sea and its Meteorology.djvu/374

348 the south pole—the antarctic winter—is annually a week longer (§ 366) than the arctic. Thus, during the period of 10,500;years, the antarctic regions will experience 142 years of night, or winter, in the aggregate, more than the arctic. Therefore it is manifest, say the cataclysmatists, that though the two hemispheres do receive annually the same amount of solar heat, yet the amount dispensed by radiation is very much greater on one side of the equator than the other. The total effect of the alternate cooling down on each side of the equator causes an accumulation of ice at the pole—when the nights are longest—sufficient, say they, to disturb the centre of gravity of the earth, causing it to take up its position on the icy side of the equator. As the ice accumulates, so is the water drawn over from the opposite hemisphere. Such, brie% stated, is the theory which has found very ingenious and able advocates in the persons of MM. Julien and Adhemar.

646. Are the climates of the earth changing?—This theory is alluded to here, not for the purpose of discussion, but for the purpose of directing attention to certain parts of this work in connection with it, as Chapters VII. and XXI., for example, and of remarking upon the stability of terrestrial climates. Though the temperate regions be cooler in the southern than in the northern hemisphere, it does not appear certain that the climates of the earth are now changing. Observations upon the subject, however, are lacking. The question is one of widespread and exceeding interest; and it may be asked if we have not in the strength of the trade-winds a gauge, or in their barometric weight an index, or in the equatorial calm belt a thermometer—each one of the most delicate construction and sensitive character—which would, within the compass of human life, afford unerring indications of a change of climates, if any such change were going on? If the temperature of the S.E. trade-winds, or the barometric pressure upon the N.E. (§ 641), were to be diminished, the S.E. trades would force this calm belt still farther to the north, and we might have a regular rainy season in what is now the great desert of Sahara; for where this