The small dials show what time it is in those states and countries of America and Europe that use Standard Greenwich time, when it is 12 o'clock in New York. France and Portugal have just adopted Greenwich time. Copyright 1910 by Munn & Co., Inc. WEGENER'S "PROFILE OF THE ATMOSPHERE." The lowest dotted line (about 7 miles above the ground) is "where the air stops growing colder." It is the upper limit of ordinary clouds, of storms, and of balloon ascensions by human beings. Nearly all the moisture of the atmosphere lies below this level. Above this line comes the second layer of the atmosphere, the stratosphere (also called the "isothermal" layer," because a thermometer carried up through it would show little change of temperature with change of elevation). This layer has been penetrated by sounding-balloons, carrying meteorological apparatus but no human aeronaut, as far as 20 miles above the earth. At about 50 miles-the upper limit of twilight-begins a region in which the atmosphere consists chiefly of hydrogen. Near the lower border of this region clouds of fine dust have sometimes been observed, shining by reflected sunlight on summer nights. These "noctilucent clouds" are commonly explained as the product of volcanic eruptions on the earth (they were frequently seen after the eruption of Krakatoa), but may be of cosmical origin. Concerning the uppermost regions of the atmosphere we have little positive knowledge. Above about 130 miles from the earth. Dr. Alfred Wegener, the author of this diagram, believes that a gas ("geocoronium"), much lighter than hydrogen prevails, to which he attributes the characteristic green line in the spectrum of the higher auroras. This is hardly more than a guess at present. CHAPTER III. METEOROLOGY.* BY C. FITZHUGH TALMAN. COMPOSITION, PRESSURE AND HEIGHT OF THE ATMOSPHERE. At Up to a height of eight or ten miles above the earth the composition of the atmosphere is remarkably uniform, as to its principal constituents. Pure dry air is a mixture (not a chemical compound) of gases in the following proportions, by volume: Nitrogen, 78.03%; oxygen, 20.99%; argon, 0.94%; carbon dioxide, 0.03%; hydrogen, 0.01%; together with minute quantities of neon, krypton, xenon, helium, and possibly other gases. the levels habitable by man the air always contains invisible water vapor (from a small trace to about 5%), and usually small and variable amounts of ozone, ammonia, nitric acid, and other gases, which, on account of their irregular occurrence, are not classed among the normal constituents of the atmosphere. Lastly, the lower air always contains solid impurities, in endless variety, generically known as dust. The pressure of the air at sealevel averages about 14.7 pounds to the square inch, which corresponds to a reading of 29.92 inches of the barometer. The density and the pressure of the air decrease rapidly as we ascend. At an altitude of 3.6 miles above sea-level they are reduced onehalf; i. e., half the whole mass of the atmosphere lies below this elevation; yet the atmosphere extends at least 300 miles above the earth. At great altitudes the tenuity of the atmosphere is comparable to that of the best “vacuums" attainable in the laboratory. THE UPPER ATMOSPHERE. The investigation of the upper atmosphere, which has been prosecuted most actively since the beginning of the twentieth century, constitutes a special branch of research known as *Copyright, 1912, by Munn & Co., Inc. aerology. It has made meteorology a "science of three dimensions." The atmosphere is "sounded" by means of meteorological instruments attached to kites and balloons. The greatest height ever attained by a kite was 4.51 miles above sea-level, at Mount Weather, Va., May 5, 1910; by a balloon, 20.14 miles, at Uccle, Belgium, June 9, 1911. Above the levels attainable by these means, the atmosphere is studied by observations of the aurora, meteor trains, and optical phenomena, and by computation of the distribution of the atmospheric gases, as determined by their atomic weights. If Since the year 1902 it has been known that the atmosphere is divided into at least two layers, or shells, having quite different properties. we could travel in a balloon to the top of the atmosphere we should find the air rapidly growing colder as we ascended, until, at a height of about 7 miles, this fall in temperature suddenly ceased, as we entered the isothermal layer, or stratosphere. air below this level-the tropospherecontains practically all the moisture of the atmosphere; hence all clouds (except possibly dust clouds of volcanic or cosmical origin). All storms, also, are confined to the troposphere. The a During our ascent through the stratosphere we should find ourselves in a region of comparatively gentle winds and of uniform temperature in vertical direction. We should find the atmosphere gradually ceasing to be "air," and becoming mainly nitrogen. Later we should reach a region in which nitrogen was replaced by the lighter gas hydrogen. Possibly a gas even lighter than hydrogen exists in the atmosphere, and if so it must be most abundant at the highest levels. Its existence is conjectured on the evidence of the spectrum of a certain type of aurora, |