(3.) _The hydraulic press_. Here the pressure is produced by means of a
piston driven up by the force of water, the immense power of which is,
in great part, due to its almost total incompressibility. This is by far
the most perfect form of press. Its power must be familiar to all who
remember the lifting of the tubes of the Britannia Bridge, and the
_launching of the Great Eastern_.

An oil-mill is in form something like a flour-mill. The operation
begins at the top, where the seed is passed through a flat screw or
shaker and then through a pair of rollers, which crush it. These rollers
are of unequal diameter, the one being 4 feet, and the other 1 foot; but
they are both of the same length, 1 foot 4 inches, and make fifty-six
revolutions in a minute. By this arrangement it is found the seed is
both better bruised and faster than when, as was formerly the case, the
rollers were of the same diameter. A pair of rollers will crush 4-1/2
tons of seed in eleven hours, a quantity enough to keep two sets of
hydraulic presses going.

After the seed is crushed in this way, it is passed under a pair of edge
stones. These stones weigh about seven tons, are 7 feet 6 inches in
diameter and 17 inches broad, and make seventeen revolutions a minute.
If of good quality, they will not require to be faced more than once in
three years, and they will last from fifteen to twenty. They are fitted
with two scrapers, one for raking the seed between the stones, the other
for raking it off at the proper period. One pair of stones will grind
seed sufficient for two double hydraulic presses, and the operation
occupies about twenty-five minutes. The seed is now crushed and ground,
but before it is passed on to the press it is transferred to the
heating-kettle.