Page:Popular Science Monthly Volume 7.djvu/740

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mate. Nor is this a phenomenon of the present age only. Geology proves from its records that, while a torrid zone may have existed since climates began, yet polar and temperate regions have witnessed changes both of climate and of life in wonderful succession, and in periods of immense duration. The causes which have contributed to these results have been sought and studied by some of the most eminent scientists of our time. Humboldt, Sir John Herschel, Sir William Thomson, Lieutenant Maury, Sir Charles Lyell, and more recently Dr. Carpenter and others, have investigated the subject in many of its aspects, and their conclusions are before the world; but no one, we believe, has presented it from so many points of view, or attacked its complex problems with greater vigor, than Mr. James Croll, whose volume is now before us. His conclusion is, that not only great secular changes of climate, but the distribution of temperatures upon the earth's surface at the present time, are due to causes which alter the volume, intensity, and direction, of the trade-winds and other prevailing winds of the globe. For the question at issue is not the amount of heat received upon the earth's surface, but the means by which it is distributed. It is not claimed that the great heat of the equatorial regions is carried directly to the polar regions by winds. That such a result is impossible is shown by the fact that the heated air rises at the equator, and moves toward the poles at an elevation where the temperature is at freezing, and its heat is lost or radiated into space.

But the prevailing winds, and mainly the trades, give rise to great surface-movements of the ocean, chief of which is the Gulf Stream. Here, however, the author enters on disputed ground. We have not space to follow him in his criticisms on Lieutenant Maury and Dr. Carpenter, each of whom attributes oceanic circulation to difference of specific gravity of the water rather than to winds.

It is evident, however, that the value of this question depends upon a previous one, What influence have ocean-currents on climate, and what is their capacity for the transference of heat? For clearness and brevity we will confine our inquiry to the North Atlantic and its great current, the Gulf Stream. By one of the lowest estimates made of the volume and velocity of this stream, there is conveyed by it not less than 2,787,840,000,000 cubic feet of water every hour. This is about 1,200 times as much as the average hourly discharge of the Mississippi. "No droughts affect it;" the flow is incessant, and the volume continues unchanged, save by causes which we will presently mention.

The mean temperature of this water as it emerges from the Gulf is about 65° Fahr. As the stream spreads over the North Atlantic its heat is imparted to the atmosphere, and diffused by winds. It is certain that the entire volume of Gulf-Stream water loses in this way 25° of its heat; and this represents its warming capacity. To this we refer the mild climate of England, of Norway, and Iceland. The heat thus