Such were my motives for making an engine, and the work was accomplished in the year 1793." The priLcipal parts of this engine are represented by Fig. 1 a plan, and Fig. 2 an elevation, in Plate CCLXXIX. It is mounted upon a strong frame of wood, the upper part serving as a box to preserve it, and which at certain places opens for use. This stand does not, like those of engines hitherto made, form a part of the machine ; it only serves to support it at a convenient height, and is not, excepting the platform EE, at all represented in the Plate. The lowest part of the engine is a heavy tripod of cast brass, nearly in the same state in which it came from the mould. Two of its branches are denoted at A, A, in the plan ; the third is similar, but mostly covered by the work above. In Fig. 2. the tripod is also represented by AA, below which three finger-screws that support it upon the plat form are seen. These screws, marked B in the elevation, serve the purpose of levelling the engine. They work in the tripod with their heads downwards, and are planted in the broad part where the two branches meet. At about two inches from the centre of the tripod, and at equal dis tances from each other, are fixed three conical tubes, ex tending downwards nearly three feet. Two of them ap pear at C, C, the other is hidden behind the axis of the engine plate. They are connected at the lower ends by a strong piece of brass F, forming together with the centre of the tripod a frame wherein the axis revolves.
The engine plate itself, represented by Gs in Figs. 1, 2, and 5, was cast in one piece of brass, all except the cir cular limb. The form of the twelve radii respecting depth is seen in the elevation, and a central part of four inches diameter, equal in thickness to their greatest depth, con nects them ; but the broad circular centre-piece seen in the plan, as well as the circular ring shewn in the middle, are no thicker than the limb. The limb, three inches broad and half an inch thick, is formed without soldering of one piece of fine plate brass. It is rabbetted upon the extre mities of the radii, so as to bring its upper surface into the same plane with them, and there by rivets made permanent ly fast.
The axis of the plate is a strong conical tube D, four inches in diameter at the upper end, and half as much be low. Its length is determined by the three cones of the tripod. At the upper end it is immoveably fixed to the centre of the plate ; the lower end terminates in an obtuse point of steel. There is fastened in the upper end of the axis an arbor of hardened and tempered steel, which, having passed through the plate from below, stands full two inches above its surface, and ends in an obtuse point similar to that below. Upon the two points, when revolv ing in the lathe, the surface of the plate was generated, and its outer edge made perfectly circular.
A screw for making the teeth in the limb had previous ly been made. It had 20 threads in an inch ; and as it
was intended that by one of its revolutions it should car ry the plate through 1.,” angle of 10', it followed, that the circumference of the plate should be 108 inches. From the measure of the screw, therefore, the dimension of the exterior border of the plate was derived, first by computation nearly, and afterwards by trial with the screw itsel A strong collar of bell-metal had been soldered upon the axis, and, when the limb was turned, made concentric with it. The position of this collar is, respecting height, coincident with the body of tlic tripod. The centre of the tripod is hollow, in order that the collar should pass through, hut does not form a socket for it to work in. Instead of that, two narrow pieces of steel are fixed ver tically at an angle of 120° with each other. Against these, the collar is pressed by a steel spring planted at 120° dis tance from them. In this triangular bearing the axis is supported at top, while the piece F receives tie point at the lower end, and supports the whole weight.
To that branch of the tripod which is in front, a strong plate of brass is screwed fast, as represented in the plan. It extends inwards half way to the centre, and outwards somewhat beyond the border of the wheel. Its breadth is rather more than the length of the screw arbor. I mme diately above this, and in contact with it, is an exactly si milar plate. To the under side of the latter are screwed 4luee oblong pieces, the thickness of which is equal to that of the lower plate. These, one of them at the middle of the inner end of the plate, and the other two at the outer end near the edges, are received by slits cut in the lower plate, which are about one-eighth of an inch longer than the pieces, and allow a motion of the upper plate, in the direction of the radius equal to that quantity, but afford it no lateral play.
The dividing screw is fastened to. the upper plate, and partakes of its motion, the use of which is occasionally to disengage the screw from the teeth of the limb. Two pieces, which connect the screw with the upper plate, are seen, one or both of them, in all the figures, and marked with II ; they extend towards the centre, as far as the plate, and form edge-bars to sti engthen it. The shape of these pieces is best shewn in Fig. 4. especially as to the manner how they are brought from below, fin placing the screw even with the edge of the wheel, and how the screw arbor is centered in them. The arbor of the screw is cylindri cal ; and a portion of each end forms a cylinder of smaller diameter. The shoulders, which near each end of the ar bor, limit those parts, prevent lateral play in the pieces last described ; for the smaller parts work freely in the holes of those pieces, the shoulders being in contact with their inner edges.