PRIMITIVE SLIDE-VALVE.
Fig. 6.
In its primitive form, the slide-valve was made merely long enough to cover the steam-ports when placed in the central position, as shown in [Fig. 6]. With a valve of this form, the slightest movement had the effect of opening one end so that steam would be admitted to the cylinder, while the other end opened the exhaust. By such an arrangement, steam was necessarily admitted to the cylinder during the whole length of the stroke; since closing at one end meant opening at the other. There were several serious objections to this system. It was very difficult to give the engine cushion enough to help the cranks over the centers without pounding, and a small degree of lost motion was sufficient to make the steam obstruct the piston during a portion of the stroke. But the most serious drawback to the short valve was, that it permitted no advantage to be taken of the expansive power of steam. For several years after the advent of the locomotive, the boiler pressure used seldom exceeded fifty pounds to the square inch. With this tension of steam, there was little work to be got from expansion with the conditions under which locomotives were worked; but, so soon as higher pressures began to be introduced, the loss of heat entailed by permitting the full-pressure steam to follow the piston to the end of the stroke became too great to continue without an attempted remedy. A very simple change served to remedy this defect, and to render the slide-valve worthy of a prominent place among mechanical appliances for saving power.