Page:Popular Science Monthly Volume 29.djvu/369

Rh latitude regions, during the three middle months of the arctic day, is greater than is received in the same time at the equator.

In Dr. Kane's "Arctic Exploration," we read that the difference between maximum and minimum (summer and winter) temperature, in latitude 78° 37', was 120° Fahr. At St. Michael's, latitude 63°, the thermometer ranged from +76° to-55°, a variation of 131°. It would, I think, be a moderate estimate, should we attribute at Spitzbergen a variation of 100° to the changes in the sun's declination, or, in other words, to the obliquity of the earth's axis.

The cold undoubtedly was greatly modified by the latent heat of the surrounding ocean, and by the inflow of ocean-currents. But the same capacity for giving off heat exists now, and the same currents continue to flow; yet in Spitzbergen—a not large island, surrounded by a broad expanse of water—the cold is very intense. The specific heat of water has undergone no change; so far as that is concerned, the surrounding ocean does as much now, as then, to make the Spitzbergen winters mild.

Did the Gulf Stream, or the Japan Current, in those remote times, have a greater flow than now? Their effective cause is the difference between polar and tropical temperature. If this was nothing, the flow would be nothing. In geological times the difference of the temperatures must have been small, since the same species of plants and animals extended from the tropics to as near the poles as has been explored. Hence the flow of these streams, to say the least, could not, in those times, have been greater than it is at present.

Whether 100° is an overestimate of the difference between the summer and winter temperature at Spitzbergen, due to the long days and nights, it is certain that the sun produces a great effect upon the temperature in high latitudes. Whatever other thermal influences may have existed in the Miocene, or in other and earlier periods of geology, their effect was no greater in winter than in summer. Admitting it to have been the same—a matter of great doubt the temperature, as the nights grew longer, must have fallen until it reached a point at which the loss of heat, by radiation into space, was just equal to that brought in by the ocean-streams, and by such aerial currents as might blow from warmer regions. In summer there were the same sources of heat plus a sun shining not twelve nor fifteen hours, but for months. Calling the winter heat A, and the increment from the sun B, the heat during summer equaled A plus B. The difference at the present day between the temperature of arctic seasons is enormous. It is difficult to see how it could have been so reduced as to render life possible for plants whose fellows of the same species were, at that very time, growing in regions thousands of miles nearer the equator.