Page:Popular Science Monthly Volume 3.djvu/501

Rh excess, not merely of light and beat, but of gravitating force, of magnetic influence, and of actinic or chemical action, exerted upon the earth as a whole. Accordingly we find that there is a recognizable increase in the activity of the earth's magnetism in December and January as compared with June and July. But, assuredly the effect produced is not of such a character as to suggest that we should find the means of predicting weather, if it were possible for us now to discover any solar law of change resulting in a corresponding variation of solar action upon the earth.

This leads us to consider the first great law of solar change as distinguished from systematic variations like the sun's varying change of distance and his varying daily path on the heavens. This law is that which regulates the increase and decrease of the solar spots within a period of about eleven years. The sun's condition does not, indeed, admit of being certainly predicted by this law, since it not unfrequently happens that the sun shows few spots for several weeks together, in the very height of the time of spot-frequency, while on the other hand it often happens that many and large spots are seen at other times. Nevertheless, this general law holds, that, on the whole, and taking one month with another, there is a variation in spot-frequency, having for its period an interval of rather more than eleven years.

Now, the difference between a year of maximum spot-frequency and one of minimum frequency is very noteworthy, notwithstanding the exceptional features just mentioned, which show themselves but for short periods. This will be manifest on the consideration of a few typical instances. Thus, in the year 1837, the sun was observed on 168 days, during which he was not once seen without spots, while no less than 333 new groups made their appearance. This was a year of maximum spot-frequency. In 1843, the sun was observed on 312 days, and on no less than 149 of these no spots could be seen, while only 34 new groups made their appearance. This was a year of minimum spot-frequency. Passing to the next maximum year, we find that in 1848 the sun was observed on 278 days, during which he was never seen without spots, while 330 new spots made their appearance. In 1855 and 1856 together, he was observed on 634 days, on 239 of which he was without spots, while only 62 new groups made their appearance. The next maximum was not so marked as usual, that is, there was not so definite a summit, if one may so speak, to the wave of increase; but the excess of spot-frequency was none the less decided. Thus, in the four years, 1858-'61, the sun was observed on 335, 343, 333, and 322 days, on not one of which he was spotless, while the numbers of new groups for these four years were, respectively, 202, 205, 211, and 204. The minimum in 1867 was very marked, as 195 days out of 312 were without spots, and only 25 new groups appeared. The increase after 1867 was unusually rapid, since in 1869 there were no spotless days,