THE STEAM-ENGINE. 



255 ^ 



What are the 

 advantages and 

 disadvantages 

 of high-press- 

 ure engines f 



High-pressure engines are generally used in all situations 

 where simplicity and lightness are required, as in the case of 

 the locomotive ; also in situations where a free supply of 

 water for condensation can not be readily obtained. As they 

 use steam at a much higher pressure than the condensing en- 

 gines, they are more liable to accidents arising from explosions. High-press- 

 ure engines are less expensive than low-pressure, since all the apparatus for 

 condensing the steam is dispensed with, the only parts necessary being the 

 boiler, cylinder, piston, and valves. 



_,. . . 579. It is not necessary in the steam-enrino that the steam 



when IS steam •' ° 



said to be used should flow continuously from the boiler into the cylinder 

 expansively? during the whole movement of the piston, but it may be cut 

 off before it has fully completed its ascent or descent in the cylinder. The 

 eteam already in the cylinder immediately expands, and completes the move- 

 ment already begun, thus saving a considerable quantity of steam at each 

 movement. Steam employed in this way is said to be used expansively. 



To carry out this plan to the best advantage, the 

 expansive force of the steam must be greatly in- 

 creased by working it under a high pressure. 



580. In many engines the supply 

 of steam to the cylinder is regu- 

 lated by an apparatus called the 

 Governor. This consists, as is rep- 

 214, of two heavy balls, C and C, 

 connected by jointed rods, D D', with a revolving 

 axis. A. When the axis is made to revolve rap- 

 idly, the centrifugal force tends to make the balls 

 diverge, or separate from one another in the same 

 manner as the two legs of a tongs will fly apart 

 when whirled round by the top. This divergence 

 draws down the jointed rods, but a slower motion of the axis causes the 

 balls, on the contrary, to approach each other, and thus push them up. 

 These movements of the jointed rods in turn raise or lower the end of a bar, 

 E, which acts as a lever, and moves a valve which increases or diminishes the 

 quantity of steam admitted from the boilers into the cyhnder — thus preserv- 

 ing the motion of the engine uniform. 



In stationary engines, also, a large and heavy fly-wheel is often used, which 

 by its momentum causes the machinery to move uninterruptedly, even if tha 

 pressure of steam be less at one point than at another * 



• Fig. 215 illnstrates the principal parts of a condensing steam-engine and its mode of 

 action. 



Upon the left of the fijjure is the cylinder, which receives the steam from the boiler. 

 A part of the side of the cylinder is cut away in order to show the piston, which moves 

 alternately up and down accordinij as the steam is admitted above or below it. By the 

 rod A the piston triin-mits its alternating movements to the walking-beam, L, which is an 

 enormous lever accurately balanced on its center, and supported by four columns. The 

 walking-beam, L, communicates its motion by means of a connecting-rod, I, to the crank. 



How isthe mo- 

 tion of steam- 

 engines regu- 

 lated ? 



resented in Fie 



