Page:Popular Science Monthly Volume 7.djvu/744

724 their relations to the atmosphere and ocean in respect to their movements. The trade-winds owe their existence to the difference of temperature between the equator and the poles. Whatever increases this difference increases the strength and volume of the winds, whether in the Northern or Southern Hemisphere. The coming on of a period of northern glacial cold would be concurrent with increasing violence of the northeast trade-wind. It would sweep at its maximum far beyond the equator, for the Southern Hemisphere would be heated and the line of greatest equatorial heat would be southward from its present position. A deflection of the great equatorial current of the ocean would occur corresponding with this, and its vast volume of heated waters would pour into the southern instead of into the northern ocean. The Gulf Stream would cease to flow, or flow only with greatly diminished volume. "In the severest droughts it never fails," said Prof. Maury, but it may fail from other causes, and leave half a hemisphere rigid from the austerity of cold. Depleted of the Gulf Stream, the surface-waters of the North Atlantic would be warmed only by the direct rays of the sun, and would rapidly approximate to the low temperatures which now prevail only a few fathoms beneath the surface. The mean temperature of Scotland for January is 28° higher than its normal, and 15° above its normal for the year. The loss of the Gulf Stream would change all this.

Theories which have made the Ice period depend wholly on cold are shown to be untenable by Prof. Tyndall, who calls attention to the fact that great heat is as necessary to the production and growth of glaciers as intense cold, the one being needed to produce vapor, the other to condense and freeze it. Any accumulation of snow and glacial ice is impossible without this combination of circumstances; but these constitute an integral part of Mr. Croll's theory, which assigns a mild climate to one hemisphere while the other is wrapped with ice.

Alternation of climates in geological time is as certain as diversity of climate at the present day. Evidence of it is found in the geological record, and eras of glaciation have succeeded each other, but each one has buried or erased many traces of preceding ones, and only the last one is before us, the monumental history of which reveals its startling and wonderful features.

It is held indeed by Mr. Croll that eras of cold and glaciers alternating with those of temperate climate are fully accounted for by the causes stated. His conclusions, however, are not accepted by many eminent geologists. Prof. Dana says that climatic changes effected by the Gulf Stream have been brought about, "not by diversions of the current from the ocean, and its restoration to it again, but by variations in the amount and height of arctic lands, in one case closing and the other opening the arctic regions to the tropical stream, and the same for the Pacific current." While this view may not call in question the warming influences of the stream, it assigns