ATMOSPHERE, GASES AND OF. The invisible fluid surrounding the earth is termed the atmosphere. It is composed of oxygen 20.84 and nitrogen 79.16, in 100 parts water, in the form of vapor, and carbonic acid gas is also held mechanically, the former aver aging one per cent,, 'but varying with tempera ture, etc., and other meteorological conditions; and the carbonic acid gas forming 0.1 per cent., or by weight as compared with a volume of an one-thousandth part. In relation to the effects of vapor on the atmosphere, it is stated that, on the supposition that the vapor is uniformly dis tributed through the atmosphere, which will not be far from the, truth if considered with refer ence to the principal zones of the earth, we can calculate the whole weight of water contained. Thus we find, if the water were at the boiling point, its elastic tension or pressure would be equal to the, pressure of the atmosphere, and in this case it would support thirty inches of mer cury, or its equivalent, 407.4 inches of water; and since transparent vapor observes the same law of expansion and contraction by variations of pressure and temperature that dry air does, it is clear that we shall have the following relation for any other temperature, namely, as thirty, inches is to the quantity of mercury expressing the elasticity of the air at any temperature, so is 407.4 inches of water to the whole weight of the aqueous vapor, provided the weight of vapor is the same as that of the air. It has, however, been
proved that vapor is only five-eighths ' of the density of air, and 'therefore, the quantity found by the foregoing relation must be reduced in this ratio. If we assume that the dew-point is on an average of 6° below the temperature of the air, and allowing the temperature of the tropical regions to be 82°, we shall have the following proportion: 30 : 0.897 : : 407.4 : 12.81. This last number must, however, be multiplied by I, and this will give us 7.61 inches. From this it will appear that if the atmospheric columns at the equator were to discharge their whole watery store, the moisture precipitated would cover the the earth to the small depth of 7.61 inches; and from a similar calculation we find that, if the columns of air resting upon the city of Washing ton were to precipitate at once all their moisture, the quantity of water would be indicated by about three inches of the gauge. To supply, therefore, thirty or forty inches of rain in the course of a year it is necessary that the vapor contained in the atmosphere should be very fre quently renewed, and that, consequently, locali ties which can not be reached by moist winds must be abnormally dry.