crank fitted on the end of axis a. Packing is introduced into cavities at the end of each projection, as at b, to keep them steam-tight during the revolutions. There is much leakage in an engine of this kind, and the friction is great.

From the great variety of rotatory engines of this class which have been introduced from the time of Watt till now, it is quite an impossibility for us to notice even a small proportion of their number; we must refer

the reader to larger treatises, where several of the most ingenious are illustrated. We propose only giving illustrations of one or two of the most recently introduced, and which, from their admirable arrangement of

fig. 123.

parts, and their general efficiency, are likely to be introduced on a comparatively extensive scale.

A form of rotatory engine, which has been spoken highly of by competent authorities, is that invented by Mr. Isaiah Davies of Birmingham. Through the courtesy of William Johnson, Esq., editor of the Practical Mechanic's Journal, we are enabled to present our readers with a description and illustration of this (as well as of the one following this) ingenious rotatory steam-engine. In fig. 122 we give a transverse section of the engine, which is of the duplex construction. Fig. 123 is an external endelevation corresponding, showing the arrangement of the cam motion for working the valves. Fig. 124 is a plan view, showing an engine or cylinder in elevation; the other in section. a is the main shaft of the engine, which

both directly receives and transmits the power. Two pistons b, are carried loose on this shaft on three feathers; by this arrangement the pistons are carried round by the revolution of the shaft, but are allowed a certain amount of play laterally; that is, they can move backward and forward a slight extent on the shafts. It is in this point that the main objections to rotatory engines have been overcome; this side to side movement of the pistons preventing all wear and injurious binding of the piston: and by means of the end set-up plates the surest adjustment of parts can be obtained, thus obviating the excessive wear found in other engines of this class on the surface of the piston and the ends of the cylinder. The pistons, b, are cylindrical for the greater portion of their circumference, but have each projections cast as in fig. 123, these receiving the actuating pressure of the steam. The axes of the pistons coincide with the centres of the cylinders; and the diameter of the former being less than that of the latter, an annular space is left all round as at c. The pistons are placed on the shaft in such a manner that the projections are exactly opposite each other. The cylinders, cc, are bolted down on the same axial line upon a cast-iron

foundation-plate, carried by a light stone foundation. Each cylinder is flanged at both ends, and the two are bolted together by bolts passing through a central partition plate placed between them to divide the spaces of each, the shaft being passed through a central hole in the plate. The transverse section in fig. 122 explains the valvular arrangement employed for the induction and eduction of the steam. The steam-chamber, d, is cast in one piece with the cylinder, and is provided with a movable cover and adjusting-screws, by which to adjust a metal plate placed at the back of the steam-valve to take off the pressure of the steam. The steam-valve is worked by a spindle, e, standing out in front of the engine, and which is actuated by a cam motion of a peculiar nature, presently described. The flat steam-stop, f, which acts as an abutment or movable partition, against which the steam re-acts in urging round the piston, is in one piece with the flat steam-valve. The supply steam is brought by the pipe c, which is bolted by flanges to the lower side of a receiving-chest containing an inverted d or single cup-valve, employed for stopping or reversing the motion of the engine. The valve is worked by a small hand-wheel, 2, on a screwed spindle, working into a socket on the spindle of the valve. By this means the valve may be adjusted to any desired position with the greatest nicety. As arranged in the diagram, the steam passes as shown by the arrows from the lower valve-chest up through the uncovered port k; and thence by the port at the front of the working valve-chest to the space above the top plate, which bears upon the upper face of the steam-valve. From this point it passes by the narrow gridiron slots in the plate to a similar series of slots in the upper side of the steam-valve; these slots extending across one half of the valve surface, and communicating with a narrow channel cored out of the centre of the valve's thickness; and finally terminating in a larger port, which in fig. 122 is represented as conducting the inductionsteam into the cylinder above the end of the valve forming the stop-piece. The exhaust steam is meanwhile returning from that portion of the cylinder which is continued between the back of the projection and the lower side of the stop-piece f, by an exactly similar set of ports and slots, occupying the remaining half of the valve, as seen more clearly in fig. 124. The main exhaust channel, in the thickness of the valve, communicates, by means of three narrow slots in the lower surface of the valve, with a corresponding series in the bottom-supporting surface of the valve-chest, and the steam exhausts thence into a passage leading into a hollow of the reversing valve, and finally escapes by the exhaust port l into a waste pipe bolted to the side of the stop valve-chest. It is easy to understand how the passage of the steam may be entirely reversed, by setting the reversing valve to cover the front port and open the back one, when the steam will be immediately admitted, by what were before the exhaust valves, to that portion of the cylinder below the steam stop-piece f; the exhaust likewise taking the place of the former steam-ports on the upper side of the valve. The steam may also be entirely shut off from the cylinder, by placing the reversingvalve in the centre of its stroke, so as to cover both ports on its face. action of this valve is essentially different from that of ordinary steam slide-valves, for it has to fulfil conditions of a very different character. It has to remain in one position for a long period during each revolution of the engine, to admit steam to urge round the piston; it must afterwards be quickly withdrawn, merely to allow of the passage of the projecting part of

K

The

P

the piston; after which it is again passed inward. To produce these peculiar movements, Mr. Davies has applied an ingenious arrangement of cams, which work inside the frame n (see the elevation in fig. 123), in one piece, with an actuating rod partially supported by a link o, jointed to a stud on the foundation plate; the extremity of the actuating rod being jointed to the end of a short lever p keyed on the valve rocking-shaft. The rocking-shaft works in pedestals carried by the foundation-plate, and has keyed on it a lever with a forked upper end, connected by short links to a cross-head on the valve-spindle e. In an improved form of this engine, the inventor has substituted double-acting pistons for the single-acting ones, as in fig. 122, having two projections instead of one. The two projections are set opposite each other on the pistons, the latter being so placed on the shafts that the projections of each stand at right angles to one another; thus balancing both their own weight and the actuating pressure of the steam. Steam-stops, with the requisite valves, are provided for each projection on opposite sides of the engine; the two stops for one piston being simultaneously worked by one cam motion. In place, therefore, of describing the single motion of the engine in fig. 122, we shall explain that adapted for the engine with duplex projections. Fig. 125 exhibits a side elevation of this arrangement, in which the two dotted lines a, b, c, d are supposed to stand in planes coincident with the centre lines of the piston projections. The throw of the cams having been settled, the first thing to be done in setting out their curves is, to form the dotted square e, o, p, h, with diagonals drawn through it. To find the h diameter of the anti-friction rollers k l, against which the cams work, the throw of the cams is added fig. 125. to the thickness of the roller studs, allowing about one-eighth of an inch clearance between the extremity of the cams and the roller studs. The true curve of the actuating cam surface is a matter of great nicety, as upon its exactitude depends the correct working of the valve-stops against the piston projections. The breadth of the end m, n, of the smaller cams, is determined by the piston projections themselves, as the two must correspond. On moving the rollers in the direction of the arrows, it is evident that no rectilineal motion will take place until the point n is gained; and the nature of the movement subsequent to this is thus determined. On the horizontal line a b, a semicircle with a radius equal to half the throw is delineated at a, and this is divided into any given number of equal parts; and from these points ordinates are drawn to the diametrical line. Thus, from the centre of the shaft the arc ad is described, being bisected by one of the diagonals before mentioned.

That portion of the arc contained between the point o and the diagonal line is divided into the same number of parts as these micircle at a, drawing radial lines to each, and describing concentric arcs, commencing at the ordinates of the semicircle a. A line then traced through the points of intersection of these arcs with the radii will indicate the centre of motion of the traverse of the roller d, the periphery of which gives the required shape of the cam. The large cam p is of course formed so as to act in concert with the small one, which we have just described. In fig. 122, it will be seen that the actuating projections of the pistons which work up

against the circumference of the cylinders are fitted with a packing-piece of metal m, placed obliquely across the face of the piston, so as to facilitate its passage across the slot in the cylinder. This stop-piece is kept up to its working face by the pressure of the steam behind it, a slight bladespring being provided as an additional safeguard. The action of the cam motion is such, that at each revolution, when the projection m approaches the steam slide the latter is drawn back at such a rate as to keep its innerfaced end just clear of the approaching curved surface; and just at the instant of the passage of the face of the projection, the slide for an instant stops, and is then similarly pushed inwards, to fill up the space gradually left by the receding of the projection. The working-face end of the steam slide is fitted with a tongue-piece of brass, with a spring behind it, to work upon the cylindrical portion of the piston. The action of this tongue is clearly shown in the details of valve in fig. 126. To support the slide, fig. 122, against the pressure of the steam when stretched across the annular space of the cylinder, it is let into a groove in the centre partition and end set-up plates, thus providing it with a solid foundation. The adoption of the system of divided steam-ways in the slide removes all objectionable frictional wear at that part, as it is only for a quarter of an inch movement in each stroke that the steam-pressure in the valve is unbalanced. The instant the communication is opened on both sides of the stop, the pressure is equipoised, and the remainder of the valve's stroke is performed under a pressure not greater than that resulting from the mere weight of metal. At each revolution, a portion of the steam contained between the after curved side of the projection and the cylinder, which steam has before done its duty in carrying round the piston, is shut in by the steam-stop, to be again made use of in the succeeding stroke or revolution. Fig. 126 is an enlarged view of the steam-valve and partition-slide. In this view it is working in the same conditions as in fig. 122, the steam entering above the partition and escaping from below it, as before explained. Again referring to fig. 124, which, as a combined view of the whole engine, gives the clearest explanation of the arrangement, we shall now show how the difficulties attending the end wear of the piston have been effectually got rid of. A favourite argument of many writers, holding views adverse to rotatory engines, was that of supposing two plane circular discs of metal to be working together, revolving upon coincident centres. Experience had gone to show that the plates in these conditions would inevitably wear untrue, by reason of the much greater space passed through by the circumference of the discs, as compared with the space near the centre of motion. After working some time, the surfaces were no longer planes, but by the law of relative velocities became cones, the centres of which remained in contact, whilst the circumferential portions parted. Applying this result to the end wear of rotatory engines, it was held to be an invincible argument against their success; for the evil only increased by continued working. We shall now explain how this evil does not obtain in the engine under consideration. The end covers are double, the outer

fig. 126.

a