Page:Popular Science Monthly Volume 66.djvu/511

Rh as rainfall or barometric pressure, which it has been customary to record. We find periods of a day and a year, but the differences of the averages from the actual observations are very considerable.

The next step consists in trying to find out whether cycles can be discovered in these differences, and for this purpose the same method is followed. But the periods to be examined are by no means obvious. The time of revolution of the moon, under the supposition that the moon has any influence, is not found in meteorological observations, or at any rate the effect is so small as to be negligible in comparison with what we want to find, and no other astronomical period seems very likely to find a counterpart in the records. There is, however, one period which at first sight appears to be a possible one in meteorological observations, namely, that known as the sunspot period. Before going into the evidence for its existence as a climate factor, it is necessary to say a word or two about the effects of periodic changes.

These changes may be roughly divided into two classes, those of short period and those of long period. From the former we should expect changes from day to day or every few days, constituting what we usually call the 'weather.' Nothing at all is known of the existence of any such period, all attempts to trace one having ended in absolute failure. The long period changes are in general those which affect the character of a season and which show the difference between one season and the next, or the variations which a given season of the year will show in several successive years. In countries where the most important interests are manufacturing and ocean transportation, the 'weather' is the first consideration; in lands where the agricultural interests are predominant, although the weather is important, it is more essential to have an idea of the climatic or seasonal changes. Thus if the sunspot period could be shown to have a large influence on the climate of any region, we should be able to forecast to a corresponding extent the general characteristics of future seasons, and lay plans accordingly. The practical value of such an investigation is obvious, but its scientific interest is not less if it leads to a more accurate knowledge of the laws of nature. The case therefore deserves examination.

In order to establish any connection between sunspots and meteorological changes, two things are necessary. First, the periods in which the sunspots can be grouped must be discovered and their relative importance determined. Second, the meteorological records over long intervals of time must be examined in order to see whether they exhibit similar cycles of change and whether those cycles, supposing that they exist, are sufficiently well-marked to be considered important factors in the variations of the seasons at any place from year to year.

Although the existence of sunspots has probably been known from