(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.

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.