Page:Popular Science Monthly Volume 39.djvu/680

662 what is to be said of the condition of a planet subjected to the terrific mutations of Eta Argus, a star that in 1843 rivaled Sirius itself in brilliancy, and that since 1868 has been invisible to the naked eye, and has sunk as low as the eighth magnitude? Some of the comets undergo far less severe alternations than such a world must endure. In either direction, then, the prospect of solar evolution seems unfavorable, considered from the planetary standpoint. What the planet most wants is an unchanging and unchangeable sun. But this is impossible. In the presence of eternity a sun, whether it grows hot or grows cold, white or red, with age, is a thing as essentially evanescent as a zephyr.

But we can not rest with the assumption that, since the sun is evidently no Mira and no Sirius, therefore it is practically an unchanging radiator which for an indefinite period will continue to cause the earth to bloom in the beneficent effulgence of its life-inspiring rays. A sun may affect the welfare of its planets either through the gradual mutations which it undergoes in the course of its evolution, or through the more rapid and violent changes that characterize the stars that are ranked as variable. We have seen that most of these latter belong to the third and fourth classes, but there is reason to suspect that the majority of all the stars are variable to a slight degree, and evidence of variability in the case of the sun is furnished by the phenomena of sun-spots. A spectator, viewing the sun from a distant point in space, would perceive that its brilliancy was slightly increased once in about every eleven years. These accessions of light should correspond, not with the periods of fewest spots, but with those of most spots, because the energy of the sun's radiation is greatest during the spot maxima. At present a sun-spot maximum is approaching, and since last winter the face of the sun has frequently exhibited startling indications of the tremendous disturbances now affecting the solar globe. Our imaginary observer in space would probably behold at the present time a very slight increase in the sun's brilliancy, and this increase may go on for three or four years to come. While we, dwelling upon a globe that is bathed in the sun's rays, may be unable to perceive these variations directly, yet their effects have long been recognized by the changes that they produce in terrestrial magnetism. It is also highly probable that a perceptible influence upon the weather is exercised by variations in solar radiation corresponding with the presence or absence of sun-spots. So far as trustworthy observations have gone, it appears that the temperature upon the earth is slightly lower when sun-spots are most numerous. This is exactly the opposite to the effect that might have been anticipated; but as the observations from which the inference is derived are confined to India, it seems probable that the lowering of