It is single-acting, and has a steam-jacket and a plug-rod valve-gear, J K. The improvements are principally in the form and proportions of its parts, and in its adaptation to high steam and "short 'cut-off.' "
A is the steam-cylinder, B C the piston and rod, D the beam, and E the pump-rod. The condenser is seen at G, and the air-pump at H. The steam-cylinder is "steam-jacketed," and is surrounded by a casing, O, composed of brickwork or other non-conducting material. Steam is first admitted above the piston, driving it rapidly downward and raising. the pump-rod. At an early point in the stroke the admission of steam is checked by the sudden closing of the induction-valve, and the stroke is completed under the action of expanding steam assisted by the inertia of the heavy parts already in motion. The necessary weight and inertia are afforded in many cases, where the engine is applied to the pumping of deep mines, by the immensely long and heavy pump-rods. Where this weight is too great, it is counterbalanced; and where, as when used for the water-supply of cities, too small, weights are added. When the stroke is completed, the "equilibrium-valve" is opened, and the steam passes from above to the space below the piston, and, an equilibrium of pressure being thus produced, the pump-rods descend, forcing the water from the pumps and raising the steam-piston. The absence of the crank or other device which might determine absolutely the length of stroke compels a very careful adjustment of steam admission to the amount of load. Should the stroke be allowed to exceed the proper length, and should danger thus arise of the piston striking the cylinder-heads, the movement is checked by buffer-beams. The regulation is effected by a "cataract," a kind of hydraulic governor, consisting of a plunger-pump with a reservoir attached. The plunger is raised by the engine, and then automatically detached. It falls with greater or less rapidity, its velocity being determined by the size of the eduction orifice, which is adjustable by hand. When the plunger reaches the bottom of the pump-barrel, it disengages a catch, a weight is allowed to act upon the steam-valve, opening it, and the engine is caused to make a stroke. When the outlet of the cataract is nearly closed, the engine stands still a considerable time while the plunger is descending, and the strokes succeed each other at long intervals. When the opening is greater, the cataract acts more rapidly, and the engine works faster. This has been regarded until recently as the most economical of pumping-engines, and it is still generally used in Europe in freeing mines of water.
43. Fig. 18 represents a lighter, cheaper, and almost equally effective machine, known as the Bull Cornish or Direct-Acting Cornish engine. It was first designed by the competitor of Watt, by whose name it is known. As is seen by reference to the engraving, its cylinder a is directly above the pump-rods c, d, g, and is carried on
