357

Chap. 9.] Motive Pressure Engine.

engines a piston is alternately pushed up and down in its cylinder bysteam; and by means of the rod to which the piston is secured, motion iscommunicated to a crank and fly-wheel, and through these to the machi-nery to be propelled : it is the same with pressure engines when used tomove other machines, except that instead of the elastic vapor of water, acolumn of that liquid drives the pistons to and fro, as will be perceivedby an examination of the following figure.

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No. 162. Motive Pressure Engine.

E represents the lower part of the pipe which conveys the water downfrom its source into the air chamber C, from the lower part of which itpasses through a short tube and stop cock into the valve case, or “ sidepipe” D. This pipe is parallel with the working cylinder of the engine

A, and rather longer : it communicates with A through two passages forthe admission of the water to act upon both sides of the piston. The endsof D are closed by stuffing boxes, through which a rod in the direction ofits axis is made to slide, and upon this rod are secured two plugs, shownin the cut, that fill the interior of the pipe like pistons, and as the rod israised or lowered, alternately open and close the passages into the cylin-der. Suppose the position of the various parts of the engine as indicatedin the figure, and the stop-cock in the short tube that connects the “ sidepipe” to the air vessel be opened, the water would then rush into theupper part of the cylinder A, as shown by the arrows, and by its staticalpressure force down the piston; while any water previously below thepiston would escape through the lower passage into the side pipe (be-neath the plug) and run off to waste through the tube B, marked by dot-ted lines, and the circular orifices of which are also figured. When thepiston has reached the bottom of the cylinder, the rod to which the plugsare attached is drawn down, so as to close the upper passage and openthe lower one, upon which the water enters through the latter anddrives up the piston as before, the previous contents of the cylinder beingforced through the circular orifice in the upper part of the side pipe into

B. In this manner the Operation is continued and motion imparted to thebeam, crank and fly-wheel. The apparatus for moving the rod that opens