Page:Motors and motor-driving (1902).djvu/299

rod is placed in the middle of the link. As the engine revolves the eccentrics simply push each end of the link up and down alternately, giving practically no motion to the slide valve, but by moving the reversing handle so that the curved link takes position somewhere between those shown on fig. 25 and fig. 23, the travel or distance which the slide valve moves up and down is reduced, so that the steam is cut off from the piston before the end of its stroke. This process is known as 'notching up,' and it simply means that the steam, instead of being admitted to the piston almost to the end of each stroke, is cut off before the stroke is completed, the expansion of the steam being sufficient to finish the working stroke. This results in a distinct economy in steam, and is one of the first things which a driver of a steam car should learn to do, for it is often unnecessary when running fast on the level or down slight slopes to drive with the steam admitted right to the end of each stroke. This results in economising the steam, which in its turn means that less fuel is used. The backward and forward eccentrics are so set with relation to the crank that the steam is admitted and released at the proper time. When running forward, the forward eccentric does all the work of moving the slide valve up and down, and when running backward the backward eccentric performs it. In other words, the eccentric rod nearest the slide valve rod is the one which works the eccentric, and when we get into the position of 'mid gear' (fig. 23) no steam is admitted at all, as both eccentrics are working the link up and down, but not moving the valve, the block b occupying the same relation to the curved link that the boy does to a see-saw when he stands in the middle of it. The box on the side of the cylinders, in which the slide valve works, is known as the 'steam chest.' The arrows show the passage of the steam from the boiler into the cylinder and out of it, e, the exhaust outlet, being marked for clearness in fig. 23. Although we have spoken throughout as if the eccentrics were so set in relation to the crank, and the slide valve so proportioned that the steam was all pushed out