The example selected of a low-restore engine, is of the construction manu factured by Messrs. Bothwell, Hick, and Bothwell, of Bolton, in Lancashire. It is a portable engine of 12 horses' power ; the steam cylinder is 19i inches in diameter, having a four-foot stroke, and making 27 strokes per minute.
This engine consists, in the first place, of a largticast-iron plate, firmly bolted to stone or brick-work, on which the whole of the materials are fixed. The beam with all its appendages, is by this means supported, without being at all connected with the building, by a double diagonal frame, one-half surmounted by an entablature plate, to which the bearers or spring beams are attached, that receive the studs or centres of the radius_ rod of the parallel moticn ; the extreme ends of which are supported by a pillar resting on a bracketprojecting from the back of the cylinder. The pedestals in which the gudgeon of the beam works, rest on the entablature plate, and are firmly secured by bolts passing through the whole. The side walls on which the foundation plate acts, are so far asunder as to allow a sufficiently wide recess to receive the condensing cistern, with its air-pumps and condenser, hot and cold-water pumps, as well as to afford room for getting down to secure the ends of the bolts. Tbeor is sup ported by a standard placed directly over the crank shaft, and by a single pair of bevel wheels. The upper part of it is hollow, to receive • small rod, that is attached by a cross pin to a braes sliding socket, which is connected with the governor arms by two small links, and partakes of the motion commu nicated to them by the movement of the balls. The small rod has a communica tion with the throttle-valves, by means of the levers fixed to the ceiling of the engine-house.
The kind of boiler attached to this engine is of the waggon-shaped kind, a full description of which has already been given at page 198 of the first volume of this work.
The steam cylinder and its casing are cast together in one piece ; the space betwixt them is constantly filled with steam, which prevents any condensation taking place within the cylinder, and serves also as a conducting-pipe for the steam to the boxes E, containing the sliding-valves, (which are generally called D valves, from their resemblance in form to that letter,) through two separate opening!, for that purpose, in each of which is placed a throttle-valve, and on their spindles are levers, communicating by a rod with the governor, for regu lating the speed of the engine.
The sliding-valves are packed on their circular sides with a soft substance of hemp or flax, and in consequence of the steam being admitted to the under side of the top valve, and the upper side of the bottom valve, they of course require no more force to move them than what is necessary to overcome the friction of the packing, and the surface over which they slide. The weight of the valves and their rods are accurately counterbalanced by a movable weight or a lever under the cylinder, and are moved by an cc centric circle, or the fly-wheel shaft. By the arrangement of having two throttle valves, the least difference in weight between those parts of the en gine that are attached to the opposite ends of the working beam, can be regu lated, by allowing a little more steam to pees in the same time through either of the valves, as may be found necessary,—thereby equalising, as much as possible, the action of the engine. One pipe, 0, only is required in front of the cylinder, and that for theof conducting the steam from the upper side of the piston to the condenser. H, a vessel in which the condensation of the steam is effected after its escape from the cylinder, by admitting a quantity of cold water out of the condensing cistern I, through an injection cock, the opening of which is regulated by hand. The condensing cistern is supplied with water by the cold-water pump K. L is the hot water pump, used for raising water to supply the boner which water passes through a small valve, and down the same pipe that contains the damper-float. This valve is connected with a lever, having one of its ends connected by a rod passing through a pipe with a stone float, that rises and falls with the surface of the water in the hour, and thereby admitting a smaller or larger quantity of water, as may be requisite. This pipe, for the rod to pass through, has several advantages over the method of passing it through a stuffing-box on the boiler top; as, in case the hot-water pump by any accident should cease to act, and the water get low in the boiler, the steam would make its escape before any serious injury could —showing instantly that such was the fact, the moment it got below the end of the pipe. The friction be tween the rod and the water being so trifling, insures an almost uniform re gularity of action. N, a small cistern containing the blow-valve, for the par pose of allowing the air to escape from the cylinder, &c., previous to the engine being set to work.