Page:Popular Science Monthly Volume 39.djvu/681

Rh, while primarily brought about by the condition of the sun, is directly due to the action of local causes, and that in other parts of the earth a simultaneous increase of heat may be experienced. A very great increase of solar radiation, however, could not be thus masked in its effects upon the earth.

Although during the historical period there has probably been no sufficient variation in the activity of the sun to produce very serious terrestrial results, yet it is known that the sun-spot cycle is subject to considerable variations, both as regards the length of the periods and the intensity of the forces concerned in the disturbance. The latest maximum of sun-spots in 1883-'84 was a couple of years overdue. What peculiarities may mark the maximum now approaching time alone can reveal. But, at any rate, the known irregularities of the sun suggest a striking resemblance to what we see in some of the variable stars; and it is highly probable that the changes of the latter, except in certain cases where other more satisfactory causes have been inferred, are due to phenomena resembling sun-spots, if not in fact directly analogous to them. Is sun-spottedness, then, a progressive condition; and will our sun in time become, through this cause, variable to the extent shown by many of its compeers in the heavens?

It is true that on account of the remoteness of any calamitous effects resulting from such gradual changes in the sun's condition we can afford to regard them with indifference, so far as the welfare of our race for many thousands of years is concerned; but when we rise to a higher point of view, and put aside merely human measures of time, the question becomes one of deep interest, since it involves the probable ultimate fate of our planet as the scene of the development and achievements of intelligent creatures. Will the earth become a desert like its companion the moon through the exhaustion of its vital forces and the disappearance of its air and water, while the sun yet shines upon it with unfailing splendor; or will the end of terrestrial life be brought about by the agency of the sun itself, either through the failure of the solar energies, or through an overwhelming outburst of them? These questions are not the less interesting, and not the less certain to obtrude themselves, because it is at present impossible for us to answer them. They have also a bearing upon the geological life record of the globe. Already, under the enormous demands for time made by the evolutionary doctrine, geology is asking for far longer periods of stability in the light and heat supplies of the sun than astronomy, also supporting itself upon the principles of evolution, is able to grant. But if the sun has emerged from the stage of a third or fourth type star, and by the gradual elimination of its obstructive envelope has arrived at that point of comparative regularity of radiation in which we behold it, the time