WEAK POINTS OF THE LINK-MOTION.
This attribute which the link-motion possesses, of accelerating the release and compression along with the cut-off, is very detrimental to the economical operating of locomotives that run slow. High-speed engines need the pre-release to give time for the escape of the steam before the beginning of the return stroke; and the compression is economically utilized in receiving the heavy blow from the fast-moving, reciprocating parts, whose direction of motion has to be suddenly changed at the end of each stroke, and in helping to raise the pressure promptly in the cylinder at the beginning of the stroke. A locomotive, on the other hand, that does most of its work with a low-piston speed, would not suffer from back pressure if the steam were permitted to follow the piston close to the end of the stroke; and a very short period of compression would suffice. If the engine, whose motion we have been considering, instead of releasing at sixteen inches, could allow the steam to follow the piston to twenty-two inches of the stroke, after cutting off at six inches, a very substantial gain of power would ensue. And this would be well supplemented by avoiding loss of power, did compression not begin till within two inches of the return stroke.