112

MODERN STEAM PRACTICE.

worm-wheel and pinion. This gear is very generally adopted, asthe link is locked at any position without any other mechanism forholding it up. Some arrangements for direct action have the eccen-tric rods too short; where it is not convenient to get in a length ofrod that is the radius of the link, at least six times the throw ofthe eccentric, indirect means are preferable for working the slide-valve; and certainly the return eccentric rod System, as before de-scribed, seems as good an arrangement for indirect motion as can bedevised. It is essential that the suspending rod for the link should bemade as long as convenient: this is a very necessary point to attendto when arranging link motions, as when the rod is too short theversed sine of the chord of the arc that it describes becomes verygreat, causing the link to have an up and down motion, which sen-sibly affects the working of the valves, more especially when theeccentric rods are short. Indeed, when such faults are both com-bined in one arrangement there is no truthful action of the valvewhatever, and in all cases this can be avoided with proper attention.It is quite unnecessary to describe such malformations.

The modes adopted for guiding the lifting rod in a vertical mannermust next be considered, and this apart from the starting-wheel,as its position varies very much, and is simply arranged in the bestlocality for handling the engines, which undoubtedly is on the samelevel as the stoke-hole, although some engineers place the startinggear on the top of the condenser, or about the same height as it,thus getting a good view of the machinery in motion. With bevel-wheels and cross shafts it is a very simple matter to place the start-ing gear in the most convenient Situation; but all means should beas direct as circumstances will admit of. In guiding the lifting rodfor the link in a vertical manner a simple kind of parallel motion issometimes used. The lifting rod is suspended downwards, the bot-tom pin is fixed to the middle of a short link, the ends of the linktaking an arm placed above and another below the lifting centre;one of these arms merely vibrates on a fixed pin, while the otheror bottom one is keyed on the reversing shaft that passes across theengine. This shaft is likewise fitted with an arm for taking thereversing rod, passing along to some convenient part on the con-denser. In this example the reversing rod is attached to a cross-head moving in suitable guides, the starting-wheel actuating a screwcut in its shaft, the cross-head having a female screw to correspond.It is quite essential that all link motions should be balanced with a