(24.) It is an established property of fluids, that they press equally in all directions; and air, like every other fluid, participates in this quality. Hence, it follows, that when the downward pressure or weight of the atmosphere is fifteen pounds on the square inch, the lateral, upward, and oblique pressures are of the same amount. But, independently of the general principle, it may be satisfactory to give experimental proof of this. [Illustration: _Fig._ 10.] Let four glass tubes, A, B, C, D (_fig._ 10.), be constructed of sufficient length, closed at one end, A, B, C, D, and open at the other. Let the open ends of three of them be bent, as represented in the tubes B, C, D. Being previously filled with mercury, let them all be gently inverted, so as to have their closed ends up, as here represented. It will be found that the mercury will be sustained in all, and that the difference of the levels in all will be the same.[7] Thus, the mercury is sustained in A by the upward pressure of the atmosphere; in B, by its horizontal or lateral pressure; in C, by its downward pressure; [Pg042] and in D, by its oblique pressure: and, as the difference of the levels is the same in all, these pressures are exactly equal. (25.) The same arrangement by which the pressure of the atmosphere is measured by a mercurial column of equivalent weight, also supplies the means of measuring the pressure or elasticity of atmospheric air, or any other gas or vapour, whether in a more or less compressed or rarefied state; and as instruments constructed on this principle are of considerable use in steam engines, we shall take this occasion to explain their principle and application. In the experiments described in (21), the space D B in the top of the barometer-tube, from which the mercury descended, is a vacuum. If, however, it were occupied by a quantity of air in a rarefied state, or any other gas or vapour, such gas or vapour would press on the surface of the mercury at D, with a force determined by its elasticity. In that case, the atmospheric pressure acting on the surface of the mercury C in the cistern, would be balanced by the combined forces of the weight of the mercurial column sustained in the tube, and the elasticity of the gas or vapour in the upper part of it. Now if we know the actual amount of the atmospheric pressure,—that is to say, the height of the column of mercury which it would be capable of sustaining,—we should then be able to determine the pressure of the rarefied air in the space C D.