(21.)

The amount of the pressure of the atmosphere on each square inch of horizontal surface on which it rests, is obviously the weight of the column of air extending from that square inch of surface upwards to the top of the atmosphere. This force is measured by the following means:—

Figs. 8., 9.

Take a glass tube, A B ( fig. 8.), above 32 inches long, open at one end A, and closed at the other end B, and let it [Pg040] be filled with mercury (quicksilver). Let a glass vessel or cistern C, containing a quantity of mercury, be also provided. Applying the finger at A, so as to prevent the mercury in the tube from falling out, let the tube be inverted, and the end, stopped by the finger, plunged into the mercury in C. When the end of the tube is below the surface of the mercury in C ( fig. 9.), let the finger be removed. It will be found that the mercury in the tube will not, as might be expected, fall to the level of the mercury in the cistern C, which it would do were the end B open, so as to admit the air into the upper part of the tube. On the other hand, the level D of the mercury in the tube will be nearly 30 inches above the level C of the mercury in the cistern.

The cause of this effect is, that the weight of the atmosphere rests on the surface C of the mercury in the cistern, and tends thereby to press it up, or rather to resist its fall in the tube; and as the fall is not assisted by the weight of the atmosphere on the surface D (since B is closed), it follows, that as much mercury remains suspended in the tube above the level C, as the weight of the atmosphere is able to support.

If the section of the tube were equal to the magnitude of a square inch, the weight of the column of mercury in the tube above the level C would be exactly equal to the weight of the atmosphere on each square inch of the surface C.