The atmosphere that surrounds this earth is like a shell that is two hundred to three hundred or more miles in thickness. We live at the bottom of an immense ocean of gaseous matter, which envelops everything, and presses upon everything with a force which appears, at first, perfectly incredible, but whose actual amount admits of easy proof. Gravity being, so far as is known, common to all matter, it is natural to expect that gases, being material substances, should be acted upon by the earth's attraction, as well as solids and liquids. This is really the case, and the result is the weight or pressure of the atmosphere, which is nothing more than the effect of the attraction of the earth on the particles of air. The amount of pressure exerted upon every square inch of the surface of the earth, and the objects thereon, is from fourteen to fifteen pounds. This enormous force is borne without inconvenience by the animal frames, by reason of its perfect uniformity in every direction; and it may be doubled, or even tripled, without inconvenience. An important law which connects the volume occupied by a gas with the pressure made upon it, is expressed by Mariotte in the following manner. This law is usually called Mariotte's law: "The volume of gas is inversely as the pressure; the density and elastic force are directly as the pressure, and indirectly as the volume." This law has been found to be true no matter how rarefied the air. The atmosphere, like everything else on earth or connected therewith, and like all other planets known, and the earth itself, is composed of elements, as we shall see presently. The atmospheric air is composed of gases, elementary substances, known by the names of Nitrogen and Oxygen, with variable proportions of carbonic acid and watery vapors, and usually a trace of ammonia. Besides these, there may occasionally be other substances present, depending upon local causes, as the odoriferous principles of plants and the miasmata of marshes, etc., etc. Nearly three-fourths of the atmosphere is composed of nitrogen, while about one-fourth or less is oxygen. The following is the relative proportion: By weight. By measure. Nitrogen 76.9 79.3 Oxygen 23.1 20.7 ----- ----- 100 100 Its specific gravity is unity (1), being the standard with which the density of all gaseous substances is compared. It is 814 times lighter than water, and nearly 11,065 times lighter than mercury; 100 cubic inches weigh 31 grains.