662 THE POPULAR SCIENCE MONTHLY. 



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 un- 

 changing 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 varia- 

 bility 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 ap- 

 proaching, 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 varia- 

 tions 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 observa- 

 tions have gone, it appears that the temperature upon the earth 

 is slightly lower when sun-spots are most numerous. This is ex- 

 actly 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 tempera- 



