Page:Popular Science Monthly Volume 12.djvu/379

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phenomena of sun-spots are now familiar: multitudes of people have seen them, and everybody has read about them. It is well known that the surface of the sun is not that uniform disk of light that it was formerly supposed to be, but abounds in gulfs, dark chasms, up-rushing streams of flaming gases, and lurid prominences, sometimes 100,000 miles high. But these striking effects are not uniform: the sea of solar fire, like our own oceans, is sometimes violently agitated and sometimes quiet. The spots are variable, being now many and enormous in size, and again few and small. This periodicity, moreover, is proved to be regular. Prof. Schwabe, of Dessau, discovered that, instead of being uniform in number and intensity from year to year, spots increase and decline at definite rates for a term of years. As a result of 9,000 observations, during which he discovered 4,700 groups, he traced three complete oscillations from maximum to minimum, which he estimated to take place in about ten years. Prof. Wolf, of Zurich, went into an exhaustive history of the subject, and, by collating a vast number of observations and records from 1750 to 1860, he verified Schwabe's general results, but showed that the period of oscillation is about eleven years. His data, scattered through a course of 140 years, comprehended observations in the seventeethseventeenth [sic] century made on 2,113 days; in the eighteenth century, on 5,490 days; and in the nineteenth century, on 14,860 days, or a total of 22,463 days. On this broad basis of observation, made with no reference to any hypothesis of variation, it is established that the solar energy changes in intensity by a regular law of rise and fall from a maximum to a minimum of effect; and that the maximum, or greatest activity, coincides with the period of violent perturbation when there is the greatest number of eruptions of heated matter from below, and the most conspicuous display of sun-spots and prominences; while at the minimum periods these manifestations are greatly reduced, or almost entirely wanting.

It is now an admitted fact of science that the earth is dependent upon the sun for the chief portion of the energy by which terrestrial effects are produced. With the exception of the ebb and flow of the tides, all the forms of earthly power are recognized as having, directly or indirectly, a solar origin. Wind-power, water-power, steam-power, the activities of organic growth, all animal energy, and the great phenomena of changes in the crust of the globe, due to the circulation of waters through atmospheric agency, are caused by the forces of solar radiation. But if the solar energy is variable, the question naturally arises, "Is that variation manifested in terrestrial effects, and, if so, in what manner, and to what extent?" The subject is vast and new, but the indomitable energy of modern scientific inquiry has rapidly accumulated evidence which answers the first question in the affirmative, and gives instructive replies to the others. The sun-spots, for thousands of years unknown, and for centuries after they were known regarded as mere matters of curious and idle speculation, are now linked indissolubly to the whole scheme of activity which we observe upon earth, and of which we are ourselves a part. Even the famines by which nations are periodically desolated seem to be connected with this intermittence of solar energy. The evidence upon the subject has been summed up in an able and impressive paper contributed by Messrs. Lockyer and Hunter to The Nineteenth Century, and which will be found in full in No. VIII. of We can here do little more than indicate the remarkable