THE atmosphere is not homogeneous. It appears from analysis that, of 100 parts, 99·5 consist of nitrogen and oxygen gases mixed in the proportions of 79 to 21 of volume, the remainder consists of 0·05 parts of carbonic acid and on an average 0·45 of aqueous vapour. These proportions are found to be the same at all heights hitherto attained by man. The air is an elastic fluid, resisting pressure in every direction, and is subject to the law of gravitation. As the space in the top of the tube of a barometer is a vacuum, the column of mercury suspended by the pressure of the atmosphere on the surface of that in the cistern is a measure of its weight. Consequently every variation in the density occasions a corresponding rise or fall in the barometrical column. At the level of the sea in latitude 42°, and at the temperature of melting ice, the mean height of the barometer is 29·922 or 30 inches nearly. The pressure of the atmosphere is about fifteen pounds on every square inch; so that the surface of the whole globe sustains a weight of 11,671,000,000 hundreds of millions of pounds. Shell-fish, which have the power of producing a vacuum, adhere to the rocks by a pressure of fifteen pounds upon every square inch of contact. The atmosphere when in equilibrio is an ellipsoid flattened at the poles from its rotation with the earth. In that state its strata are of uniform density at equal heights above the level of the sea; but since the air is both heavy and elastic, its density necessarily diminishes in ascending above the surface of the earth; for each stratum of air is compressed only by the weight above it. Therefore the upper strata are less dense because they are less compressed than those below them. Whence it is easy to show, supposing the temperature to be constant, that if the heights above the earth be taken in increasing arithmetical progression, that is, if they increase by equal quantities, as by a foot or a mile, the densities of the strata of air, or the heights of the barometer which are proportionate to them, will decrease in geometrical progression. For example, at the level of the sea if the mean height of the barometer be 29·922 inches, at the height of 18,000 feet it will be 14·961 inches, or one half as great; at the height of 36,000 feet it will be one-fourth as great; at 54,000 feet it will be one-eighth, and so on. Sir John Herschel has shown that the actual decrease is much more rapid, and that, in any hypothesis that has been formed with regard to the divisibility of the aërial atoms, a vacuum exists at the height of 80 or 90 miles above the earth’s surface, inconceivably more perfect than any that can be produced in the best air-pumps. Indeed the decrease in density is so rapid that three-fourths of all the air contained in the atmosphere is within four miles of the earth; and, as its superficial extent is 200 millions of square miles, its relative thickness is less than that of a sheet of paper when compared with its