THE GROWTH OF THE STEAM-ENGINE. 143 



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 pro- 

 portions 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 

 i?the pump-rod. The condenser is seen at G, and the air-pump at II. 

 The steam-cylinder is " steam-jacketed," and is surrounded by a cas- 

 ing, 0, 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 admis- 

 sion 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, w r here 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 w r hich 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 dauger 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 educ- 

 tion 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 

 mos't 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 effec- 

 tive 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, (/, and is carried on 



