Page:Popular Science Monthly Volume 21.djvu/435

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ing of the sun's fiery matter. None of these theories is fully satisfactory. The hypothesis of clouds of smoke hardly agrees with the immensely high temperature we have to ascribe to every part of the sun's surface, and the supposition of a dross would require a greater degree of cooling than could possibly take place there. Herr Edmund von Ludwighausen Wolff has advanced the theory in "Kosmos" that the spots, instead of being cooler, represent parts of the sun that are vastly hotter than the rest of its body; that is, that they are regions in which all the heat-movements have reached the intensity of the ultra-violet and invisible rays; consequently, they appear dark. Herr Wolff remarks that Secchi's observation that the spots gave out not less but more heat than the rest of the sun's surface, and Fraunhofer's that the forces that produce the spectrum-lines appear to be more active in the spots, and the fact that flashes of light are frequently seen to spring up from the midst of them, are contradictory to all previously received theories, but agree fully with that which he proposes, and are clearly explainable by it.

The Laws of Rain-fall.—Professor E. Loomis has prepared, in aid of his studies of the laws affecting the amount of rain-fall at different places, a graduated table of the average annual rain-fall at more than seven hundred points. Of two hundred and four stations at which the mean exceeds seventy-five inches (rising from this amount to 492·45 inches at Cherapunji, Assam), some are elevated more than two thousand feet above the sea, and nearly all are within one or two hundred miles of elevated mountains. Rain chiefly occurs when the wind from the ocean is blowing toward the mountains, and the extraordinary rain-fall at most of them is probably due to the influence of the mountains, by which the wind is deflected upward to such height that a considerable part of the contained vapor is condensed by the cold of elevation. The cases in which the rain-fall is excessively deficient are, on the other hand, those of places in which nothing exists that may cause an upward current of air. Another cause of deficient rain-fall, frequently exemplified, is the descent of a current of air which has been forced up to a great height and suffered condensation of its vapor, after it has crossed the mountain, by the influence of which it has been raised, when its temperature rises and it becomes dry. Such effects are produced by the Rocky Mountains on the plains east of them and by the Himalayas on the Desert of Gobi; and the operation of these two causes will assist in explaining most of the rainless districts of the globe. Other influences modifying the amount of rain-fall are, the meeting of the northeast and southeast trade-winds, which results in a great rain-belt surrounding the globe; the irregular barometric depressions of the middle latitudes, indicating frequent storms; proximity to the ocean, especially when the prevalent wind comes from the sea; and the projection of capes and headlands into the ocean, which contribute to frequent rains. Uniformity in the direction of the winds throughout the year, such as prevails in the trade-wind regions, obstruction of the free movement of surface-winds by mountains, remoteness from the ocean measured in the direction from which the prevalent wind proceeds, and high latitude, tend to produce a dry climate. These principles do not seem to be fully borne out by the phenomena of rain on either side of the Alleghany Mountains, but we have not yet systematic enough or careful enough observations to enable us to determine what is their real influence. Mount Washington, in New Hampshire, exerts a marked influence. The mean annual precipitation there is seventy-seven inches, while in the surrounding districts it is only forty inches.

Improved Sanitary Condition of London.—The report of the English Registrar-General for 1880 completes the fourth decade of reports since the weekly return of that officer was first published. It shows that the death-rate in London for the year (taking the population of the metropolis as given by the last census) was not more than 21·5 per thousand inhabitants, than which a lower death-rate has been returned in only three of the last forty years. The decade closing with 1880 was one of lower mortality in London than any of the three decennial periods for which trustworthy statistics are available, the rate having been