end are turned conical seatings, in which are fitted rings of metal k k, cut open on one side, leaving a lap-joint to prevent escape of steam. These rings are capable of being adjusted by bolts passing through the side plates 11, and are thus easily adjusted. The cranks gg are keyed on the shaft at right angles to each other, equidistant from a line drawn through centre of shaft and centre of piston; these cranks convey the power to the lower cranks, ii, by rods or drag-links h h. The vibrating rods ff are carried on the pedestal jj. The ends of the cylinder a do not require to be bored, as the whole wear takes place on the plates d and e. The plate e is dovetailed in and fitted fast; d, being loose in its parallel recess, which allows it to follow up the piston as it wears; the plate d is kept up to the face of the piston by springs behind it, or by admitting steam into the recess at the back of it. This plate serves another useful purpose; this is the prevention of priming in the cylinder: as the water increases in the cylinder, it forces back the plate, and rushes from one side of the piston until it escapes. Steam is admitted to act on the piston by means of a valve n through the steam-ports mm, open to the top and bottom of the piston alternately; the valve is worked by an eccentric o keyed on the crank-shaft p. This valve is on the equilibrum principle, and exhausts through the back, and works between two parallel planed surfaces; the wear that takes place being accommodated by a ring of metal o, similar to that employed for packing the piston. This form of engine is being employed in numerous instances, and with marked economical effect.

As the concluding portion of the present division of our treatise, we propose briefly describing the principles of action of two varieties of engine which have been successfully introduced into practice; these are the "Cambrian" and the "Disc Engine."

In the Cambrian engine the piston has a semi-rotatory motion given to it by the following arrangement: Let a a, fig. 133, be the external casing or cylinder; cb the arms of a piston vibrating on the axle d; the steam space is divided into compartments by the triangular abutments ef; the pipe g admits steam to the compartment m, and h into s; the steam is exhausted through o. By passages cut in the pistonshaft, diagonally, as in x; steam is admitted from the space s into t, and from the space m into n. The steam, on being admitted to one of the spaces, as m, passes into the opposite space n, and thus presses on both ends of the piston b and c, but on opposite sides; the strain on the working parts is by this arrangement much reduced. By this pressure on the alternate sides of the piston a reciprocating motion is produced in the piston-shaft d, which is communicated to the crank-shaft in the usual manner. A large number of engines on this principle have been successfully introduced.

fig. 133.

The movement of the "disc engine" is very peculiar; the most lucid exposition of its principle we have met with is that given by a "practical engineer," himself a well-known and able inventor, in the pages of the Expositor. We here append it: "The vessel in which the piston moves, the fixed recipient for the action of the steam, is the section of a hollow

sphere, such as would remain after two equal opposite segments were cut off. In this is fitted the piston, called from its form and peculiar movement a disc. The centre of the disc coincides with that of the sphere; and as its diameter is equal to that of the inside of the sphere, it can have no direct movement like the common engine piston; but it may perform an oscillatory motion, such as a top or a teetotum describes when their spinning force is nearly

a

α.

fig. 134.

f

fig. 135.

exhausted; that is to say, each point in the periphery successively dips; and the lowest point seems to proceed round the periphery, though there need not necessarily be (nor is there in this engine) any absolute rotation. Like a wave each point in the disc in its turn rises and falls; and like the wave also, there is no onward motion. To understand the action more perfectly, we refer to the following diagram (fig. 134): a a is the spherical case we have described, b is the disc, and c a ball concentric with the axis d; ef are two conical covers, g is a crank, into which the end of the axis d is inserted. If the crank be now turned round, it will be seen that every part of the disc b will successively be brought into contact with the cones at two opposite radial lines; but the rotation of the axis of the crank need not necessarily cause the disc to perform any other than the oscillatory one we have described, and, as we have said, it cannot do so. There is a slot in the disc thus (fig. 135), and there is a partition in the engine extending from the outside to the ball and fitting the two cones. When we turn the two cranks, therefore, the oscillatory motion will be performed by the disc and axis, the side of the slot rubbing up and down on the surface of the partition." It is difficult to describe the way in which the disc receives the effort of the steam; but it may be sufficient to state, that the struggle or force of the steam to enter and escape, passing through an entrance made in one of the conical covers on one side of the partition to the exit-pipe placed on the other side of the partition, forces the disc partially round, and acting on the ball c, makes the lever d rise and fall in the direction of the arrows, and thus communicates motion to the crank g.

The first patent for the disc-engine was taken out by a Mr. Dakeyne in 1830. His engine was not, however, put in practice. Henry Davies was the next inventor who turned his attention to this engine. He took out three patents, each combining successive improvements in its action, his last patent being taken out in 1844, in which his improvements had reference to working the engine expansively. He introduced a variety

of improvements in the details of this engine; indeed he may with all truth be termed its inventor. In this form of engine, in order to insure the utmost efficiency of working, it is necessary that the contact between the surfaces of the conical ends and the sides of the disc should be as perfect as possible, to prevent the passage of the steam between the surfaces of the plate or disc and the cones. To make this more perfect, Davies formed a series of ribs or cogs on each side of the disc, radiating from the central ball to the outside of disc, and a similar series of cogs in the interior surfaces of the conical ends; these cogs on the disc and cones being so arranged that they work into one another like the teeth of pinions, the cogs being ground so as to insure as perfect contact as possible. He fitted up the sides of the slot in the disc with metallic packing, making them rub on the sides of the partition; by this arrangement he was enabled to work the engine expansively. The disc engine, as thus improved by Davies, was carried into practice pretty extensively, and a company was formed at Birmingham for introducing it on a large scale. From some cause this company ceased soon to exist, and the disc engine fell into comparative obscurity, until Mr. Bishopp, in 1844, introduced a variety of improvements, and, aided by the admirable mechanical resources of eminent engineering firms, he has succeeded in placing it in a comparatively high position among economical and compactly working engines. Mr. Bishopp has dispensed with the cogs or ribs on the disc and cones, and substituted a series of strips of metallic packing, forced outwards in contact with the face of the disc, which is quite plain in its surface, by a series of springs. To insure the perfect action of the sides of the slot in the disc against the partition, Mr. Bishopp adopts a semicircular bow which extends over the engine, its two extremities being attached to the opposite ends of the axis. A pin is carried by the bow, a rectangular truss being attached to this pin, and moving from side to side in a groove made in the outside of the engine, and which groove is concordant with the plane of the partition. By this arrangement, the centre line of the slot in the disc always moves in the same plane; the packing presses equally on the face of the partition, and any degree of expansion used as may be required. "Thus improved, these engines," remarks an authority, "are now no longer experimental. They have been adopted (1851) in about fifty cases, and are found to be both economical and durable." We have before us both the reports of Messrs. Terrey and Parkes, both of whom pronounce in their favour. Mr. Terrey, alluding to some comparative experiments made at Lewisham with a disc engine, and one erected by Messrs. Penn and Son, states that he is of opinion, from what he has seen of the improved disc engines, that their performance is equal to that of the best engines of the construction in common use, in the like conditions of pressure of steam and extent of expansive action. Mr. Parkes reports a considerable economy in fuel.

CHAPTER V.

RAILWAY LOCOMOTION AND LOCOMOTIVE ENGINES.

PREVIOUS to describing the modern mechanism of the locomotive engine, so called par excellence, in contradistinction to the "steam-carriage for common roads," which properly is also entitled to the distinctive appellation of locomotive, we propose giving a rapid sketch of the history of its introduction, and a notice here and there of the most striking of the machines from time to time introduced, ending in the comparatively perfect machine now in daily use on our railways. We must premise, however, that the nature of our treatise does not admit of our going into the history of the introduction of railways, or an explanation of their construction; it is with the engine, its history and construction, that we have alone to deal. The subject of railways belongs more exclusively to the treatise on “civil and mechanical engineering,” and which may hereafter be added to the series of works of which the present forms a part.

"The

It is difficult to decide to whom the honour is due of having suggested the use of the steam-engine for the purpose of propelling carriages. Savery hints at its use in this way, at least he considered that it was possible to apply it. Dr. Robison, the gentleman who was the means of directing the attention of Watt to the steam-engine, "threw out the idea of applying the power of the steam-engine to the moving of wheel-carriages;" but other occupations withdrew his attention from the subject, and nothing further was effected. In the patent taken out by Watt in 1784, he described the application of the steam-engine to the propulsion of carriages. boiler of this apparatus he proposed should be made of wooden staves joined together, and fastened with iron hoops like a cask. The furnace to be of iron, and placed in the midst of the boiler, so as to be surrounded on every side with water. The boiler was to be placed on a carriage, the wheels of which were to receive their motion from a piston working in a cylinder ; the reciprocating motion being converted into a rotatory one by toothed wheels revolving with a sun and planet motion, and producing the required velocity by a common series of wheels and pinions. By means of two systems of wheel-work differing in their proportion, he proposed to adapt the power of the machine to the varied resistance it might have to overcome from the state of the road. A carriage for two persons might, he thought, be moved with a cylinder of seven inches in diameter, when the piston had a stroke of one foot, and made sixty strokes per minute. Watt, however, never built a steam-carriage." Such is the account given by one authority. Another, however, affirms that Watt did at least construct a model, of which we give a diagram in fig. 136, illustrative of its construction; and further states, that Messrs. Bolton and Watt constructed a steam-carriage, which was made to run on the roads of Cornwall in the years 1785-1786. We are, however, inclined to think that the model of the locomotive carriage, as here attributed to Watt, and which the writer states was made by Mr. Murdoch, Watt's assistant, was not only made by him, but owed its creation to the inventive genius of Murdoch himself. In a life or biographical sketch of Murdoch, read some two years ago at the Institution

of Mechanical Engineers, it is there stated that Murdoch constructed the model of a steam-carriage while residing at Redruth in Cornwall, and the details and general arrangement of which resembles those in the diagram

fig. 136.

now given very closely. Leaving this matter to be decided by more competent authorities, we hasten to the other points of the present division.

Another claimant for the honour of having introduced the first steamcarriage is the celebrated William Symington, the engineer now acknowledged to be the first introducer of a practically-working steamboat. As early as 1784, it occurred to him that steam might be applied to the propulsion of

f

fig. 136a.

carriages. He commenced experiments, with a view of perfecting the idea; and in 1786, submitted to the inspection of the professors, and other scientific gentlemen of Edinburgh, a working model of a steam-carriage. This gave such proofs of practicability, that he was urged to carry the machine into practice. Such, however, were the difficulties to be overcome in this, that he conscientiously stated his scruples to those anxious