If the driving wheels be five feet in diameter their circumference will be fifteen feet seven inches. To drive a train with a velocity of thirty miles an hour, it will be necessary that the engine should be propelled through a space of forty-five feet per second. To accomplish this with five-feet wheels they must be therefore made to revolve at the rate of very nearly three revolutions per second; and as each revolution requires two motions of the piston in the cylinder, it follows that each piston must move three times forwards and three times backwards in the cylinder in a second; that steam must be admitted six times per second from the steam-chest to each cylinder, and discharged six times per second from each cylinder into the blast-pipe. The motion, therefore, of each piston, supposing it to be uniform, must divide a second into six equal parts, and the puffs of the blast-pipe in the chimney must divide a second into twelve equal parts. The motion of the slides and other reciprocating parts of the machinery must consequently correspond.

This motion of the reciprocating parts of the machinery being found to be injurious to it, and to produce very rapid wear, attempts have been made to remedy the defect, and to obtain greater speed with an equal or diminished rate of motion of the piston, by the adoption of driving wheels of greater diameter, and on several of the great lines of railway the magnitude of the wheels for the passenger-engines have been increased to five feet and a half and six feet diameter; but such engines have not been sufficiently long in use to afford grounds for forming a practical estimate of their effects. Experiments of a much bolder description have, however, been tried on one of the great lines of railway by the adoption of driving wheels of much greater diameter. In some cases their magnitude has been increased even to ten feet; but from various experiments to which these engines have been submitted by myself and others, as well as from the experience which appears to be obtained from the results of their ordinary work, it does not appear that any advantages have attended them, and they have been accordingly for the most part abandoned.

The pressure of steam in the boiler is limited by two safety

valves, represented in fig. 97. at N and o. The valve at N is under the control of the engineer, but the valve at o is inaccessible to him. The structure of the safety-vale repre

The valve A, which is made of brass, is mitred round the edge at an angle of 45°, and has a spindle, or stalk B, cast upon it, projecting downwards from the middle of it. The valve-seat c is also made of brass, and cast with a flange at the bottom to attach it to the boiler. The mitred surface of the valve is ground into the valve-seat, so as to rest in steam-tight contact with it. Across the valve-seat, which is two and a half inches in diameter, is cast a thin piece D, seen in plan in fig. 113. and in section in fig. 112. which extends from the top to the bottom, and has a longitudinal hole through it, in which the spindle B of the valve works: by this hole it is guided when it rises from its seat. A projection E is cast upon the seat of the valve, in which a standard F is inserted. This standard is forked at the top, and receives the end of a lever G, which turns in it upon a centre. A rod H is jointed to this lever by another pin at three inches from the former, and the lower end of this rod, ground to a point, presses upon the centre of the valve A. At the other end of the lever, which is broken off in fig. 112., at a distance of three feet from the centre pin, inserted in the fork of the pillar F, the rod of a common spring-balance w, fig.101., is attached by a finger-nut n. The bottom of this springbalance is secured on to the fire-box. This balance is screwed up by the finger-nut on the valve-lever until the required pressure on the lever is produced through the medium of the rod н, this pressure being generally fifty pounds per square inch above the atmosphere. When the pressure of the steam in the boiler exceeds this, the valve A is raised from its seat, and the steam escapes.

It is evident that the sliding weight by which the pressure

of the safety-valve is sometimes regulated in stationary engines would not be admissible in a locomotive engine, since the motion of the engine would constantly jolt it up and down, and cause the steam to escape. One of the disadvantages attending the use of the spring-valve is that it cannot be opened to let the steam escape without increasing its force, so that the steam, when escaping, must really have a greater pressure than that to which the valve has been previously adjusted. The longer the lever is, the greater will be this difference of pressure, inasmuch as a given elevation of the pin governing the rod H would cause a proportionally greater motion in that end of the lever attached to the spring.

The second safety-valve o is enclosed in a case, so that it is inaccessible, and its purpose is to limit the power of the engineer to increase the pressure of steam in the boiler. This valve is similar in construction to the former, but instead of being held down by a lever, is pressed upon by several small elliptical springs placed one above another over the valve, and held down by a screw which turns in a frame y, fixed into the valve-seat. By this screw the pressure on the valve can be adjusted to any required degree; and if the open safety-valve be screwed down to a greater pressure, the steam will begin to escape from this second valve.

Also in the case where the boiler produces surplus steam faster than its escape can be effected at the valve N, the pressure will sometimes be increased until the valve o is opened, and its escape will take place from both valves.

The whole weight of the engine bears upon those parts of the six axles R', fig. 99., which project beyond the wheels. Boxes are formed in which these parts of the axles turn, and through the medium of which the weight of the engine rests upon them. Over these boxes are constructed oil or grease cups, by means of which the axles are constantly lubricated. It is usual to lubricate the axles of the engine itself with oil: the axles of the tender, and other coaches and waggons, are lubricated with a mixture of oil and tallow. In the middle of the box in which the axle turns, and between the two oilcups, is cast a socket, in which the end of the spindle on

which the spring presses rests. The springs are composed of a number of steel-plates, laid, in the usual manner, one above the other, increasing in length upwards. In the engine here described, the plates forming the springs of the driving wheels are thirteen in number, each of which is four inches in width, andths of an inch in thickness. The springs upon the other wheels are three inches in width. The springs of the driving wheels are below the axle, while those of the smaller wheels are above it.

Buffers D' are placed behind the tender, which act upon a spring c (fig. 100.), to break the collision, when the waggons or carriages strike upon the tender, and similar buffers are attached to all passenger-coaches. Some of these buffers are constructed with a system of springs similar to c, but more elastic, and combined in greater number under the framing of the carriage, so that a considerable play is allowed to them. In some cases the rods of the buffers are made to act upon strong spiral springs inserted in the sides of the framing of the carriage. This arrangement gives greater play to the buffers; and as every coach in a train has several buffers, the combined effect of these is such, that a considerable shock given to either end of the train may be rendered harmless by being spent upon the elasticity of these several systems of springs.

In order to give notice of the approach of a train, a steamwhistle z', fig.97. 101., is placed immediately above the fire-box at the back of the engine. This is an apparatus composed of two small hemispheres of brass, separated one from the other by a small space. Steam is made to pass through a hollow space constructed in the lower hemisphere, and escapes from a very narrow circular opening round the edge of that hemisphere, rushing up with a force proportionate to its pressure, The edge of the upper hemisphere presented downwards encoun ters this steam, and an effect is produced similar to the action of air in organ pipes. A shrill whistle is produced, which can be heard at a very considerable distance, and, differing from all ordinary sounds, it never fails to give timely notice of the approach of a train.

The water tank r", fig. 98. 100., which is constructed on the tender, is formed of wrought-iron plates of an inch thick,

riveted at the corners by angle iron already described. This tank is 9 feet long, 63 feet wide, and 24 feet deep. The top is covered with a board к", and a raised platform N" is constructed behind, divided into three parts, covered with leads, which open on hinges. The middle lid covers an opening to the tank by which water is let in: the lids at either side cover boxes in which are contained the tools necessary to be carried with the engine. The curved pipe p", fig. 98., leading from the bottom of the tank to the pipe o", is of copper. The pipe o", connecting the latter with the feed-pipe K', fig. 99., is sometimes formed of leather or India-rubber cloth, having a spiral spring on the inside to prevent it from collapsing. It is necessary that this pipe " should have a power of yielding to a sufficient degree to accommodate itself to the inequalities of motion between the engine and tender. A metal pipe is sometimes used, supplied with a double ball and socket, and a telescopic joint, having sufficient play to allow for the lateral and longitudinal inequalities of motion of the engine and tender. The weight of an engine, such as that here described, supplied with its proper quantity of water and fuel, is about 12 tons: the tender, when empty, weighs about 3 tons; and when filled with water and fuel its weight is 7 tons. The tank contains 700 gallons of water, and the tender is capable of carrying about 800 weight of coke. This supply is sufficient for a trip of from thirty to forty miles with an ordinary load.

(198.) It is not usual to express the power of locomotive engines in the same manner as that of other engines by the term horse-power. Indeed, until the actual amount of resistance opposed to these machines, under the various circumstances in which they are worked, shall be ascertained with some degree of precision, it is impossible that their power or efficiency can be estimated with any tolerable degree of approximation. The quantity of water evaporated, and passed in steam through the cylinders, supplies a major limit to the power exerted; but even this necessary element for the calculation of the efficacy of these machines has not been ascertained by a sufficiently extensive course of observation and experiment. Mr. Stephenson states, that the engine which