The accompanying engraving is a transverse section of this engine through the centre of the cylinder. At a is the foundation plate, to which all the parts of the engine are directly or indirectly attached; b is an external cylinder fixed to the foundation plate; c is a small cylinder, revolving within the external one, on a shaft d, whose centre is placed-so far above that of the external cylinder, that their circumferences may touch one another at the upper point h1; and the space between them thus gradually increases from A1 to the lower point h2. The shaft d passes through steam-tight stuffing-boxes in the cylinder ends, and revolves in bearings in the frames, which ar firmly bolted to the foundation plate, and stayed to the cylinder; e e are two sliding pistons, consisting each of two arms, connected together by four rods passing over the shaft. Their breadth is equal to that of the outer cylinder, and their joint length over their extremities is necessarily somewhat less than its diameter, owing to the eccentricity of the revolving cylinder.
These pistons slide freely at right angles to one another, through passages made in the circumference of the revolving cylinder, their sliding motion being caused by the pressure of one of their extremities on the ascending side of the outer cylinder (whichever side that may be); and the eccentricity of the revolving cylinder, through which they slide. As their length is always slightly varying during the course of a revolution, the difference is made up by metallic packing placed between the two thicknesses of plates, of which the arms of the pistons are composed. This Backing is pressed by springs towards the sides and circumference of the outer cylinder, as will be readily understood by reference to the figure. In the passages in the inner cylinder, through which the piston slides, there are also metallic packings, which are pressed on the flat surfaces of the pistons by springs, and prevent the steam passing to the interior. There are, besides, two steel rollers at the inside of the packings, which are pressed up to the flat sides of the pistons by screws, for the purpose of diminishing the friction of their sliding motion; but these rollers are not necessary, except in large engines.
The rim of the inner cylinder is made to project into metallic packing-boxes in the cylinder ends, whereby the steam is entirely prevented from passing into the interior of the inner cylinder. A packing-box is also placed at the point of contact to prevent the steam passing to either side. From what has been stated, it will be perfectly understood, that the steam only acts on the projecting part of the sliding pistons between the inner and outer cylinders. The steam, in coming from the boiler through the steam pipef, has first to pass the slideg, which is worked by the handle h. After passing that slide, it enters the steam-tight jacketj, the bottom of which is the slide face, having the four cylinder ports klm and n, and the eduction port g on it. A slide o, worked by a handle p, passes over these ports for the purpose of reversing the motion of the engine; on this slide there are two ports, o1 and o2. In the position in which the slide is shown in the engraving, the port o2 is open to the steam port l, the port n is closed, and the two ports m and k are open to the eduction portg; so that when the slide is in this position, the engine will necessarily move in the direction indicated by the arrows.
Now, by moving the slide along until the port o1 is above thesteam port k, then the port m will be closed, and m and / open to the eduction, so that the steam will act at the opposite side of the cylinder, and consequently the motion be reversed. It will here be observed that the lower cylinder ports m and n are never used for admitting steam, but only for leading off the used steam. The object in placing them so low in the cylinder is to allow the vacuum to act upon the pistons sooner. It must be kept in mind, therefore, that in whatever direction the shaft revolves, the steam is always admitted at one of the upper ports k or l, and the used steam let off at its opposite lower ports. All the ports where they lead into the cylinder are divided by bridges placed diagonally across them, so that the pistons may pass freely over them. From the relative position of the two cylinders, and the distance between their circumferences gradually increasing from contact at the upper point h1 to the greatest distance at the lower point h2 (which in this case is 1/6 of the diameter of the external cylinder, but may be varied according to circumstances), it follows that in whatever direction the engine revolves, the area of that part of the pistons which is acted on by steam and vacuum gradually increases, so that the principle of expansion is carried out to its full extent without the aid of expansion valves and gear.
The steam passing through the eduction passage g is conducted by the eduction pipe r to the condenser s; tis the injection slide placed at the lower end of the eduction pipe, and conducting the water up the pipe, so as to act fully on the steam in passing downwards; it is worked by a lever and rod, connected to the handle u, which is placed in proximity with the other starting handles h and p. The air-pump is a double-acting one. It has a metallic packed piston, which is worked from the main shaft by a crank and connecting rod; and the piston-rod is kept parallel by two slide guides bolted on the air-pump cover. The pumps are worked from the main shaft by an eccentric, connected by a rod and lever to a rocking shaft d, on which are keyed two levers, which are connected by rods to the bilge and feed-pumps. The latter has its valve chestj bolted on the hot well