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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 sive action. Mr. Parkes reports a considerable economy in fuel.

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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.

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. "The 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

e

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

to aid him in the matter, advising them not to proceed with it. In fig. 136 a we give a lateral of the steam-carriage model as constructed by Symington; d the cylinder; e, boiler, supplied from the condenser; ff, direction pulleys; g, condenser; h, steam-pipe; i, water-tank; a, drum fixed on the hind axle; b, tooth and ratchet-wheels; c, rack-rods, one on each side of the drum, the alternate action of which upon the teeth and ratchet-wheels produces the rotatory motion.

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In a previous chapter, we described the high-pressure engine patented by Messrs. Trevethick and Vivian; in the patent they claimed its employment in "propelling wheel-carriages of every description." In 1804, Mr. Trevethick set an engine to work on a very indifferent railroad at Merthyr Tydvil Colliery, in Wales; this worked very satisfactorily, and up to 1830 or 1831, it was the form which, more or less modified, was working the railroads on which steam was employed. "The advantages," says Trevethick in his evidence before the Committee of the House of Commons on carriages for common roads, "gained by this improvement, were a detached engine, independent of all fixtures, working with five times the power of Bolton and Watt's engine, without condensing water, and the fire enclosed in the boiler surrounded with water, and a force-draught created by the steam for the purpose of working on the roads without a high chimney; and from this was copied all the boilers for navigation-engines, which without could not have been available, this being independent of brick-work, light, safe from fire, and occupying little room.' The following is a description of this engine: the boiler is cylindrical, as a a, fig. 137; b is the fire-place; e the fire-door; c the entrance of the flue, which is turned before entering the chimney. By this arrangement the economy of fuel effected was very considerable, the greater portion of the heat being given out by the furnace and flue to the water surrounding them. The lower part of the cylinder was placed within the boiler, as at d, and the upper portion was surrounded by a jacket, in the space between which and the cylinder the steam from the boiler was allowed to circulate freely; the loss of steam from condensation was by this means obviated. The steam was admitted above and below the piston by the four-way cock already described; and after working the piston, instead of passing it to the atmosphere, it was led by a pipe to the chimney. By this arrangement the draught of the furnace was greatly increased, and a convenient means established of getting rid of the waste steam. Had this plan been patented, the inventor would have probably reaped a golden harvest from this alone.

The piston-rod had a cross-head attached to it, as e, fig. 138, sliding in the parallel guides f, attached to the upper end of the cylinder, and steadied by a stay as at g; from both ends of this cross-head connectingrods d proceeded, and were connected to the crank e, fixed on the centre of the driving-wheel a; the axis of the driving-wheel passing with the carriage, and immediately beneath the cylinder. The principle of this arrangement is shown in fig. 151, p. 79, Mechanics and Mechanism. This engine, on its first trial, in which the propulsion was effected by the adhesion of the wheels on the rails, drew ten tons of bar-iron, besides the carriages, for nine miles, at the rate of five miles an hour, without stopping, and carrying its heavy load of fuel and water.

α

d

h

fig. 138.

a

g

We have now to notice the ingenious mechanism introduced to obviate an inconvenience in engine propulsion which only existed in imagination. With reference to this point, Dr. Lardner remarks: "It is a singular fact, that in the history of this invention, considerable time and great ingenuity were vainly expended in attempting to overcome a difficulty, which in the end turned out to be purely imaginary. To comprehend distinctly the manner in which a wheel-carriage is propelled by steam, suppose that a pin or handle is attached to the spoke of the wheel at some distance from its centre; and that a force is applied to this pin in such a manner, as to make the wheel revolve; if the face of the wheel and the surface of the road were absolutely smooth and free from friction, so that the face of the wheel would slide without resistance upon the road, then the effect of this force, thus applied, would be merely to cause the wheel to turn round; the carriage being stationary, the surface of the wheel would slide or slip upon the road as the wheel is made to revolve. But if, on the other hand, the pressure of the face of the wheel upon the road is such as to produce between them such a degree of adhesion as will render it impossible for the wheel to slide or slip upon the road by the force which is applied to it, the consequence will be, that the wheel will roll upon the road, and the carriage will be moved forward through a distance equal to the circumference of the wheel each time it performs a complete revolution. It is obvious that both of these effects may be partially produced; the adhesion of the wheel to the road may be insufficient to prevent stopping altogether, and yet it may be sufficient to prevent the wheel from slipping as fast as it revolves. Under such circumstances the carriage would advance, and the wheel would slip. The progressive motion of the carriage during one complete revolution of the wheel would be equal to the difference between the complete circumference of the wheel and the portion through which, in one revolution, it has slipped When the construction of travelling steam-engines first engaged the attention of engineers, and for a considerable period afterwards,

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