Expel the air out of a pair of bellows, then close the nozzle and
valve-hole beneath, and considerable force will be requisite to
separate the boards from each other. This is caused by the pressure
or weight of the atmosphere, which, acting equally upon the upper and
lower boards externally, without any air inside, operates like a dead
weight in keeping the boards together. In like manner, if you stop
the end of a syringe, after its piston-rod has been pressed down to
the bottom, and then attempt to draw it up again, considerable force
will be requisite to raise it, depending upon the size of the syringe,
being about fourteen or fifteen pounds to every square inch of the
piston-rod. When the rod is drawn up, unless it be held, it will fall
to the bottom, from the weight of the air pressing it in.
Or, fill a glass tumbler to the brim with water, cover it with a
piece of thin wet leather, invert it on a table, and try to pull it
straight up, when it will be found to require considerable force. In
this manner do snails, periwinkles, limpets, and other shells adhere
to rocks, &c. Flies are enabled to walk on the ceiling of a room, up a
looking-glass, or window-pane, by the air pressing on the outside of
their peculiarly-constructed feet, and thus supporting them.
To the same cause must be attributed the firmness with which the oyster
closes itself; for, if you grind off a part of the shell, so as to make
a hole in it, though without at all injuring the fish, it may be opened
with great ease.