Fig. 3406.

The high pressure cylinder has a piston valve and the intermediate and low pressure cylinders flat valves. Each cylinder has a link motion, and all three link motions are shifted from the same shaft, which is moved by a steam reversing gear. At a, [Fig. 3405], are the eccentrics for the link b, for the high pressure cylinder; b′, b′ are those for link b′, for the intermediate cylinder; and c′ c′ are those for the link c′, for the low pressure cylinder. From each link are rods e, [Fig. 3406], connected to arms on the shaft f f, to an arm on which is connected the rod g, from the worm wheel h, whose actuating worm i is on a crank shaft operated by the small steam cylinder j. The slide spindles d work in guides, and their cross heads c span the edges of the links, gibs being provided to take up the wear.

The gear for turning the engine when there is no steam in the main boilers is constructed as follows:

On the shaft of the wheel m, [Fig. 3405], is a worm n operating a worm wheel p, on whose shaft is a worm which operates the large worm wheel shown on the main crank shaft.

[Figs. 3407] and [3408] represent the compound engines of the steamship Poplar, concerning which The Engineer (from which the engravings are taken) says:

“Both the cylinders of these engines are fitted with piston valves, placed at the back of the cylinders and worked by the single eccentric valve gear, which has been so largely adopted and so successfully carried out by this firm in triple expansion as well as compound engines. It will be noticed that whilst this valve gear permits of the cylinders being close together, it allows of the crank shaft being made in two similar pieces, and affords exceptionally long main and crank pin bearings, of the former of which there are only three, instead of the usual four. In the case of the Poplar the cylinders are 29 in. and 55 in. in diameter and 33 in. stroke, and the crank pins are 11 in. long, whilst the centre main bearing, which does duty for both the engines, is 233⁄4 in. in length, each of the outer bearings being 18 in. in length, the diameter of the crank shaft being 91⁄2 in. Another very interesting feature about these compact little engines is the design of the front framework. Instead of the ordinary upright columns in front of each engine there is an arrangement which gives exceptional stiffness to the whole structure whilst affording the fullest possible accessibility to the main working parts, and which has the appearance of an arch, from the shoulders of which there are branches worked up to receive the feet of the cylinders, thus accommodating the close centres and providing for the support of the reversing wheel without in the least obstructing the gear below. The condenser is divided horizontally through the centre on a plan strongly advocated by the builders, the whole of the base of the engines being cast in one piece and made level on the under side, so as to enable it to receive support from, and be bolted to, the engine seating immediately beneath the crank shaft, as well as round the margin.”

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