It has been seen that water, like every other material substance,
resists the intrusion of other matter into the place which it occupies.
But many things, though they resist, can be easily squeezed or
=compressed= into a smaller volume. This, however, is not the case with
water, which like other liquids, is almost =incompressible=; that is to
say, an immense pressure is needful to cause its volume to diminish to
any appreciable extent. It may seem strange that anything so apparently
yielding as water should yet be almost as difficult to squeeze as so
much iron; but the apparent yieldingness of water is due to the ease
with which it changes its shape; and, if water is prevented from
changing its shape, it is very difficult to drive its parts closer
together. It has been ascertained that if water is confined in a closed
space, a pressure amounting to fifteen pounds on the square inch
diminishes its volume by only 1/20000th part. Take a common syringe, and
having seen that the plug or =piston= fits the =cylinder= of the syringe
well, put the nozzle into water and draw the piston up. Then turn the
nozzle upward and push upon the piston till a little of the water
squirts out, so as to make sure that the cylinder contains nothing but
water. Now put your finger on the opening of the nozzle firmly, so as to
stop any water from passing out, and then try to push the piston down.
You will find that you cannot make it stir without great force; and, if
the piston moves appreciably, it will be because some of the water has
escaped by the sides of the piston. In fact, if the piston presented a
square inch of surface, and fitted accurately, and the column of water
in the cylinder were one inch long, it must be pressed down by a weight
of 30,000 pounds (about thirteen tons) to make it move one-tenth of an
inch.