WHICH IS THE MOST APPROVED MACHINE FOR TRYING THE HYDRO-MECHANICAL PRESS. In order effectually to prove the strength of beams, as calculated by the principles laid down in the foregoing part of this work, it is necessary to have at hand some very powerful and commodious instrument, one that is easy of management and delicate in its indications, so that the straining force can be removed, and the beams suffered to restore themselves, when they have attained the degree of flexure intended to be produced, and which theory assigns to them. Of all the engines now in use, the HydroMechanical Press is the most convenient for this purpose, not only on account of its immense power, but because of the accuracy with which it indicates the instant when the desired effect has been produced. This very ingenious and powerful machine is founded on the following hydrostatical principle: viz. When a mass of fluid, in a state of equilibrium, is subjected to the action of any forces, every particle of the fluid is pressed equally in every direction. Therefore, any number of pistons of different sizes are ap if plied to apertures in the sides of a vessel full of water, the forces with which the pistons are pressed, will be in equilibrio if they are proportional to the areas of the pistons on which they act. By attentively considering the above principle, the late Joseph Bramah, Esq. of Pimlico, succeeded in rendering a law of nature subservient to the most important purposes; such as working cranes, pulling up the roots of trees, trying the strength of materials, packing goods and the like. This is done by forcing an incompressible fluid through a small tube, into a cylinder of considerable strength, furnished with a solid moveable water-tight piston, which we shall designate the ram, merely to distinguish it from the piston of the forcing pump. Into the bottom of this cylinder the tube is inserted, and communicates with a forcing pump placed in a cistern which contains the water; the power is applied to a lever attached to the pump, and the piston, pressing on the surface of the water, communicates its force (through the intervention of the fluid) to the ram, or piston of the cylinder, to the top of which the work to be performed is applied. It is the proportion between the diameter of the forcing pump and the diameter of the cylinder that constitutes the principal feature of the machine, and on which its excellence chiefly depends; for since the one can be increased and the other decreased at pleasure, it is evident there can be no limit to its power. If the diameter of the pump and that of the cylinder be equal, a force of one pound on the piston will transmit a pressure of one pound only to the ram; but if the diameter of the pump be half that of the cylinder, a force of one pound on the piston will transmit a pressure of four pounds to the ram, and ten pounds will transmit forty; that is, the force and the effect, are to each other directly as the square of the diameter of the pump, to the square of the diameter of the cylinder. This is the principle, and the theory is developed in what follows. Notation. Put d the diameter of the forcing pump, f=the force with which the piston descends; D= the diameter of the cylinder, P the pressure on the ram; the diameter of the safety valve, and p the pressure thereon. Then, the several particulars of the Hydro-Mechanical Press are exhibited in the following analogies: |