of an iron girder, marked A, and massive timber principals, BB; also to place, what may not be inaptly termed, the bressumer, C. The principals have a purchase on the girder, shown by dotted lines, and instead of being placed at intervals, as in roofs, they are all fixed and bolted close together from one end of the roadway to the other, forming a complete unbroken mass of timber. On the bressumer, at intervals, pieces of timber are morticed in, forming joists or purloins, DDD, on which are fixed substantial planking, and the road paved as before. These are further strengthened by counter principals; the struts, marked N°. 1, are also fixed close together; those marked N°. 2 have an interval of one width between them. It will be seen that the greatest precaution was necessary in the construction, owing to the bulky and weighty materials continually passing over the bridge from the hop market, tanners and merchants' wharfs, which are situated contiguous to this building. Another difficulty laboured under was to keep the truss sufficiently high, to allow of vessels to pass under; in doing this the span became exceedingly flat, and added to the difficulty of forming the centering. On the whole this is reckoned to be a good specimen of mechanical and professional skill; and the truss to be the best calculated to answer the desired purpose; which is evident by the one previously executed on the opposite end of the same bridge. CHRISTOPHER DAVY, Teacher of Architectural Drawing, &c. Lond. Mech. Inst. 11, Furnival's Inn. History of the Steam Engine, Chap. VI. A short description of Trevithick's Loco-motive Engine was given at page 110. It appears that the more general adoption of this machine was prevented by a fear that the wheels would not adhere sufficiently to the surface over which it passed, but that they would slip round without producing loco-motion when any considerable load was attached to the machine. To obviate this imagined difficulty Mr. Blenkinsop, of Middleton Colliery, near Leeds, obtained a patent in 1811, for the application of a rack or toothed rail laid down on one side of the railway from end to end. Into this rack a toothed wheel is worked by the steam engine: the revolution of which wheel produces the necessary motion without any of the slipping alluded to. The accompanying figure will convey to our readers an idea of Mr. Blenkinsop's plan. The boiler is placed on a wooden or castiron frame y. Through its interior passes a wrought-iron tube of sufficient diameter to hold the fire and grate; this tube is carried out at the further end of the boiler, when it is bent upwards and continued sufficiently high to form the chimney %. a a are two working cylinders fixed in the boiler, and which work in the usual way; the piston rods are connected by cross heads to the connecting rods bb. These connecting rods are brought down on each side of the boiler and there joined to the cranks cc, (there being corresponding cranks on the other side of the machine) which are placed at right angles to each other, consequently the two cranks on the first shaft are horizontal and at their greatest power, at the time the other two are passing the centre. Upon these shafts are fixed (under the boiler) two small toothed wheels, which give motion to a larger toothed wheel e, fixed upon a third axle. A toothed wheel, f, is firmly keyed to the end of the central axle, and revolves with the wheel e. The teeth of ƒ correspond with, and work into a rack, RR, stretched along one side of the railway. Motion, therefore, is given by the pistons to the wheels dd, which they communicate to the wheel ƒ by e: a progressive movement is given to the carriage by the teeth of f taking hold of the rack. By this means the load can be drawn up a greater acclivity than by the machine of Messrs. Trevithick and Vivian, the only objection being that the power is applied on one side only, which must have a tendency to force the flanges or projecting rims of the supporting wheels, against the edges of the rails, by which an extra friction would be produced. This, however, is a trifling inconvenience, and is not found in practice to deduct much from the effect of the engines, several of which have, since the date of the patent, been in constant use in drawing coal waggons between Middleton Colliery and Leeds. In the year 1813 Mr. William Brunton, of Butterly Iron Works, also obtained a patent for a mode of giving motion to carriages by a Aver novel contrivance. The present engraving represents this Loco-motive Engine, which he terms his "mechanical traveller." "The boiler was nearly similar to that of Mr. Blenkinsop's semi-circular (circular); there was a tube passing through it to contain the fuel." The cylinder A was placed on one side of the boiler; the piston-rod is projected out behind horizontally, and is attached to the leg ab, at a, and to the reciprocating lever a c, which is fixed at c; at the lower extremity of the leg a b, feet are attached by a joint at 6; these feet lay a firmer hold upon the ground, being furnished with short prongs, which prevent them from slipping, and are sufficiently broad to prevent their injuring the road. On inspecting the drawing it will be seen that when the piston rod is projected out from the cylinder, it will tend to push the end of the lever or leg a from it, in a direction parallel to the line of the cylinder; but as the leg a b is prevented from moving backwards, by the end b being firmly fixed upon the ground, the re-action is thrown upon the carriage, and a progressive motion given to it, and this will be continued to the end of the stroke. Upon the reciprocating line a c is fixed at 1, a rod, 1, 2, 3, sliding horizontally backwards and forwards upon the top of the boiler; from 2 to 3 it is furnished with teeth, which work into a cog wheel, lying horizontally; on the opposite side of this cog-wheel a sliding rack is fixed, similar to 1, 2, 3, which, as the cog wheel is turned round by the sliding rack, 2, 3 is also moved backwards and forwards. The end of this sliding rod is fixed upon the reciprocating lever dc, of the leg de, at 4. When, therefore, the sliding rack is moved forwards in the direction 3, 2, 1, by the progressive motion of the engine, the opposite rod 4, is moved in the contrary direction, and the leg de is thereby drawn towards the engine; and, when the piston rod is at the farthest extremity of the stroke, the leg de will be brought close to the engine; the piston is then made to return in the opposite direction, moving with it the leg a b, and also the sliding rack 1,2,3; the sliding rack acting on the toothed wheel, causes the other sliding rod to move in the contrary direction, and with it the leg de. Whenever, therefore, the piston is at the extremity of the stroke, and one of the legs is no longer of use to propel the engine forward, the other, immediately on the motion of the piston being changed, is ready in its turn, to act as a fulcrum or abutment for the action of the moving power, to secure the continual progressive motion of the engine. The feet are raised from the ground during the return of the legs towards the engine, by straps of leather or rope fastened to the legs at ff, passing over friction sheeves, moveable in one direction only, by a ratchet and catch worked by the motion of the engine. The feet are described of various forms in the specification, the great objeet being to prevent them from injuring the load, and to obtain a firm footing, that no jerks should take place at the return of the stroke, when the action of the engine came upon them; for this purpose they were made broad, with short spikes to lay hold of the ground.* The next attempt we find to produce a loco-motive steam engine is in the patent of Messrs. Dodd and Stephenson, of Newcastle upon Tyne, a description of which we extract from Mr. Wood's work on rail roads. The patent was dated February 28, 1815, and consisted of the application of a pin upon one of the spokes of the wheels that supported the engine, by which it travelled upon the rail road, the lower end of the connecting rod being attached to it by what is termed a ball and socket joint; the other end of the connecting rod being attached to the cross-beam, worked up and down by the piston. ab represents the connecting rod, the end a attached to the cross beam, and the end 6 to one of the spokes of the wheel; in like manner the end d of the connecting rod c d, is attached to the beam of the other piston, and h and c to a pin fixed in the spokes of the wheel B, By these means, the reciprocating motion of the piston and connecting rod is converted by the pin upon the spokes acting as a crank into a rotatory motion, and the continuation of this motion secured by the one pin or crank being kept at right angles to the other, as shewn in the drawing. To effect this, the patentees had two methods; to crank the axle on which each of the wheels were fixed, with a connecting rod between, to keep them always at the angle, with respect to each other; or to use a peculiar sort of endless chain, passing over a toothed wheel on each axle. This endless chain which is now solely used upon these kind of engines, consisted at first of one broad and two narrow links, alternately fastened together at the ends with bolts; the two narrow links were always on the outside of the broad link; consequently, the distance they were separated laterally would be equal to the breadth of the broad link, which was generally about two inches, and their length three inches. The periphery of the wheels fixed upon the axles of the engine, were furnished with cogs, projecting from the rim of the wheels, (otherwise perfectly circular Wood on Rail Roads. |