Page:Popular Science Monthly Volume 32.djvu/732

712 which the Arctic flatlands are so well known. This marshy condition is produced every Bummer by the deeply-frozen ground t-hawing but a foot or two under the never-setting sun of that season, the deeper strata of ice remaining to act as an impervious shield to prevent the water from draining through, and on the prairies it remains as ice-water, surrounding the hardy mosses that thrive in it, and into which a traveler will sink up to his knees in the worst parts. A few stunted willows grow along the sluggish streams, but otherwise everything is bare of all except the moss. On this moss graze many herds of reindeer that roam with the wind and migrate backward and forward with the seasons." These reindeer furnish the Eskimos of the north and the Indians of the Yukon River with their meat and warm clothing.

Iridescent Clouds.—Mr. J. C. McConnel has studied the phenomena of iridescent clouds at Saint Moritz, Switzerland, where they are very common in winter, occurring usually whenever there are scattered clouds near the sun. Within a circle of about 2° radius, he says, the clouds are white, faintly tinged with blue. This space is surrounded by a ring of yellow or orange. The region of most vivid hues is comprised between 3° and 7°, and the most striking tints are purple, orange, green, and red. These colors are not arranged in rings, but are distributed over the thinner parts of a cloud in irregular patches. Beyond this region the only colors visible are green and red, which become fainter as the distance from the sun is increased. The author has detected them in a few cases at a distance of 21°. At some distance from the sun the greens and reds are frequently arranged in bands parallel to the edge of the cloud. The author supposes these colors to be the result of diffraction of light by fine particles of ice. The particular color assumed by any part of the cloud is determined by the distance from the sun and the average size of the particles. The particles are supposed to be in the form of thin hexagonal prisms, that being the shape among the known forms of ice-crystals best adapted to produce diffraction. Mr. McConnel calculated the probable diameter of the filaments, and found it to be between ·017 and ·009 millimetre for the purple, ·021 to ·010 millimetre for the orange, and 014 to ·009 millimetre for the blue. The absence of the colors from clouds composed of water particles is accounted for by the want of uniformity of size in the water-drops.

Accumulations of Atmospheric Dust.—Ruins of ancient cities and buildings are nearly always found wholly or partly buried. The material with which they are covered has been supposed to originate in the débris of buildings that have been erected and human works that have been going on upon their sites, but this can not always, or seldom wholly, be the case; for the same fact appears in desert and wilderness sites. Much is possibly due to superficial disintegration and the work of vegetation; but still another factor, more effective than has heretofore been supposed, may be sought in the deposition of atmospheric dust. In a note on this subject, read by him before the Geographical Society of Paris, M. Violet d'Aouest referred to Richthofen's account of a vast aërial formation of loess in China, and described his own observations in Mexico, where he found on the flanks of the highest mountains argillaceous strata not deposited by waters nor by the decomposition of the rocks; but investigation showed that they were produced by dust raised by the winds from the plains and deposited on the hills. These deposits varied from one hundred feet to—in some places—more than three hundred feet in thickness. They grow finer and finer as the height increases, and cease at the limit of vegetation.

Results of Nerve-Shock.—Many persons who experienced the earthquake in the Riviera have since suffered seriously from nervous shock, although they did not at the time appear to be greatly disturbed. This indicates that more injury may be done to the nerves by an undue excitement than is perceived at the time. The nerve-centers may, as an English medical journal suggests, be likened to batteries, and regarded as apt to be discharged suddenly and sometimes unconsciously; and when once their residual stock of energy is consumed, it can be restored only after a long time and by the