The system of pestle and mortar was also in use, but as the process was necessarily very slow, this method was seldom resorted to.  An improvement on this system was invented by a Mr. Herbert, whose design it had been to construct a powerful and efficient machine which should combine cheapness and simplicity.  It consisted of three pieces of wood, viz., an upright piece fixed in the ground, from the lower and upper extremities of which there projected the two other pieces, the top one attached to the joint of a long horizontal lever, and the lower one to the joint of a vertical one.  The fixed upright post and the horizontal lever formed the press.  The bag of pulp being put between the upright one and the vertical, the pressure was obtained by suspending a negro or a weight from the lever.

In another press of the same or a similar kind, the bags were placed in a horizontal frame, and a loose beam of wood pressed down on it by a lever.

Another form of press had cambs and wedges; also a modification of it by Mr. Hall of Dartford, who applied the pressure by means of a steam-cylinder.  The cambs are arranged alternately, so that one is filled while the other is being pressed.  This brief notice will suffice to give an idea of such machines as are wrought by lever pressure.

We pass on, therefore, to later inventions and improvements.

First, The Dutch or stamper press, invented in Holland; second, the screw; and, third, the hydraulic:—­

(1.) The stamper press is something like a beetling-machine, in which wedges are driven in between the bags, containing, of course in a bruised condition, the seed to be pressed.

(2.) The screw press has an ordinary square-threaded screw, and it acts in the same way as press for making cider or cheese.

(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.