It should, however, be noticed, that in the hydrostatic press of Bramah, in common use, the same time is occupied in pumping against a small, as against a great resistance : in almost all cases the operation is commenced when the resistance is at a minimum : during the process the increase is gradual, and at the termination the resistance is at a maximum. As a remedy for this practical inconvenience, hydrostatic presses are generally made with two levers of different powers, with the view of changing the power at some time during the process. Notwithstanding this provision, however, the time and trouble attending the change, renders its assumed advantages a doubtful question of economy, and it is, consequently, rarely resorted to in practice.
To obviate these objections, Mr. James Murdoch has proposed a self-regulating hydrostatic press, in which the change of power proceeds in the same ratio as the resistance, without any care or interference on the part of the operator. The ingenious arrangement proposed will be comprehended by the annexed drawings, marked 142.1 and Fig. 2, together with the following description :— a represents the vacuum chamber, being similar to the exhausted receiver of an air-pump ; b a double-barrelled air-pump ; e a four-way cock, connecting a with d and e, the two force pumps, and its lower end communicating with the atmosphere; d is a section of one of the force pumps; f is the plunger, working through the stuffing-box m, and having a solid piston h keyed on to it, which works air-tight in the enlarged part of d; g is a valve opening upwards ; n the exit pipe, leading to the press, which is not here shown, it being of the usual construction ; o is a wheel, over which passes a chain connecting the two plungers ; it is fixed square on the axle t, as is the wheel p, which serves to turn the cock by means of a cord passing round it and r, which is a pulley playing loose on o, and having a projecting shoulder on its lower part ; s is a key fixed square on to, having shoulders at its lower end, and a weight at its upper end ; a is a rod attached to the plug of the cock c. The action is as
follows : the pistons in the situation shown in the enlarged chamber of the forcing pump d is now open to the vacuum chamber a, and the chamber of e is open to the atmosphere 11; the lower barrel of d is full of water. Upon rarefying the sir in the vacuum chamber a, by means of the air-pump b, the air in the chamber d likewise becomes rarefied, and the piston h will descend as soon as the pressure on it exceeds the pressure on the plunger f, and a portion of water is thus forced into the press by the pipe n. By the descent of the piston h, the wheel o revolves, and brings up the piston of the chamber e; the smaller wheel p is carried round at the same time, and turns the cock r, the shoulder on which, taking the shoulder on a, carries it round a little past the vertical line, when it (a) falls into the position of the dotted line, and opens e to the vacuum chamber, and d to the atmosphere. The air under the piston of e now becomes rarefied, and it descends in like manner as the other. The la will the vessel a is made in proportion to the chambers d and e, the better will the press accommodate itself to the changes of resistance.
Printing presses are described under the article PRINTING: see also OIL, COPYING-IIACRINE, &C.