Presses are contrivances for compressing or squeezing together
substances that may require to be so treated, as in the case of
extracting the oil from seeds, &c. The earliest presses were simply
heavy stones or pieces of metal, put on one after the other; but
the great inconvenience and loss of time incurred in putting on and
taking off these, soon led to the screw and lever, which form the
usual screw press. The screw is fixed at one end in a socket and
is turned round by a long bar of iron or wood, and as the “worm”
works in a corresponding hollow screw which is fixed, it ascends or
descends slowly but with great power. But by far the most powerful
contrivance of this kind is the “hydraulic” press; this machine is
not only used as a press, but also to raise great weights, and for
many other purposes. The hydraulic press consists of a strong iron
cylinder having a solid piston exactly fitting to it, this piston is
raised by forcing water under it by means of a pump; the principle
depends upon the peculiar property which water and every other fluid
has, of exerting, when confined in a given space, an equal pressure
upon every part of that space; thus if one pound pressure be made
upon one square inch, the water will press with one pound power
upon every square inch of surface that it comes into contact with;
for example, suppose a cylinder, the piston of which is one foot
measurement on the face--this foot contains 144 square inches--and
from the bottom of the cylinder a tube should be made to rise a few
feet above the piston, and that this tube should have an area of one
inch; then one pound weight of water poured in at the top of this
tube would raise 144 pounds weight placed on the piston, for these
144 pounds would press but one pound on each inch, and the pound of
water would have the whole of its weight on the one inch of the tube,
they would therefore balance each other. But instead of pouring in
the water, let a piston be fitted to the tube; a man with his hand
can easily exert 100 pounds pressure on this, and the result would
be that he would raise 14,400 pounds or nearly six-and-a-half tons,
and if to this small piston a handle and valves be fixed so as to
make a pump of it he can easily pump in water at a pressure of two
or three hundred pounds to the square inch; and if instead of the
large piston containing one foot area it has three or four feet, then
the weight raised would be very great; indeed there is no limit to
the power of this instrument but the strength of the material used.
It must however be observed that when the piston descends, say six
inches, it does not raise the six-and-a-half tons six inches, but
only a hundred-and-forty-fourth part of that distance, so that the
piston would have to be raised and depressed six inches 144 times
in order to raise the six-and-a-half tons six inches. But this is
such a saving and concentration of labour that the application of
the hydraulic press is becoming more in demand every day.