therefore, that if water be supplied to the cylinder 1, it will descend through the tubes, and first filling the cylinder D and the tubes c, will gradually rise in the tubes B and E, will next fill the tubes K and the cylinder H. The grating of water-pipes c E K forms the furnace, the pipes c being the fire-bars, and the pipes E and K being the back and roof of the stove. The fire-door, for the supply of fuel, appears at M, fig. 116. The flue issuing between the tubes F is conducted over the tubes K, and the flame and hot air are carried off through a chimney. That portion of the heat of the burning fuel, which in other furnaces destroys the bars of the grate, is here expended in heating the water contained in the tubes c. The radiant heat of the fire acts upon the tubes K, forming the roof of the furnace, on the tube E at the back of it, and partially on the cylinders D and H, and the tubes B. The draft of hot air and flame passing into the flue at a acts upon the posterior surfaces of the tubes E, and the upper sides of the tubes K, and finally passes into the chimney.

As the water in the tubes CE K is heated, it becomes specifically lighter than water of a less temperature, and consequently acquires a tendency to ascend. It passes, therefore, rapidly into H. Meanwhile the colder portions descend, and the inclined positions of the tubes c and к give play to this tendency of the heated water, so that a prodigiously rapid circulation is produced, when the fire begins to act upon the tubes. When the water acquires such a temperature that steam is rapidly produced, steam-bubbles are constantly formed in the tubes surrounding the fire; and if these remained stationary in the tubes, the action of the fire would not only decompose the steam, but render the tubes red hot, the water not passing through them to carry off the heat. But the inclined position of the tubes, already noticed, effectually prevents this injurious consequence. A steam-bubble, which is formed either in the tubes c or K, having a tendency to ascend proportional to its lightness as compared with water, necessarily rushes upwards; if in c towards A, and if in K towards H. But this motion of the steam is also aided by the rapid circulation of the water which is continually maintained

in the tubes, otherwise it might be possible, notwithstanding the levity of steam compared with water, that a bubble might remain in a narrow tube without rising. To bring the matter to the test of experiment, I have connected two cylinders, such as D and н, by a system of glass tubes, such as represented at CEK. The rapid and constant circulation of the water was then made evident: bubbles of steam were formed in the tubes, it is true; but they passed with great rapidity into the upper cylinder, and rose to the surface, so that the glass tubes never acquired a higher temperature than that of the water which passed through them.

Every part of the boiler being cylindrical, it has the form which, mechanically considered, is most favourable to strength, and which, within given dimensions, contains the greatest quantity of water. It is also free from the defects arising from unequal expansion, which are found to be most injurious in tubular boilers. The tubes c and к can freely expand in the direction of their length, without being loosened at their joints, and without straining any part of the apparatus; the tubes E, being short, are subject to a very slight degree of expansion; and it is obvious that the long tubes, with which they are connected, will yield to this without suffering a strain, and without causing any part of the apparatus to be loosened.

When water is converted into steam, any foreign matter which may be combined with it is disengaged, and is deposited on the bottom of the vessel in which the water is evaporated. All boilers, therefore, require occasional cleansing, to prevent the crust thus formed from accumulating; and this operation, for obvious reasons, is attended with peculiar difficulty in tubular boilers. In the case before us, the crust of deposited matter would gather and thicken in the tubes c and K, and if not removed, would at length choke them. But besides this, it would be attended with a still worse effect; for, being a bad conductor, it would intercept the heat in its transit from the fire to the water, and would cause the metal of the tube to become unduly heated. Mr. Gurney of course foresaw this inconvenience, and contrived an inge

ing through the tubes such an acid as would combine with the deposit, and carry it away. This method was effectual; and although its practical application was found to be attended with difficulty in the hands of common workmen, Mr. Gurney was persuaded to adhere to it by the late Dr. Wollaston, until experience proved the impossibility of getting it effectually performed, under the circumstances in which boilers are commonly used. Mr. Gurney then adopted a method of removing the deposit by mechanical means. Opposite the mouths of the tubes, and on the other side of the cylinders D and H, are placed a number of holes, which, when the boiler is in use, are stopped by pieces of metal screwed into them. When the tubes require to be cleaned, these stoppers are removed, and an iron scraper is introduced through the holes into the tubes, which, being passed backwards and forwards, removes the deposit.

In these engines the draught through the furnace was produced by projecting the waste steam up the chimneys as is practised in railway engines; a method so perfectly effectual, that it is unlikely to be superseded by any other. The objection which has been urged against it in locomotive engines, working on turnpike-roads, is, that the noise which it produces has a tendency to frighten horses.

In the engines on the Liverpool road, the steam is allowed to pass directly from the eduction pipe of the cylinder to the chimney, and it there escapes in puffs corresponding with the alternate motion of the pistons, and produces a noise, which, although attended with no inconvenience on the railroad, would perhaps be objectionable on turnpike-roads. In the engine used in Mr. Gurney's steam-carriage, the steam which passes from the cylinders is conducted to a receptacle, which he calls a blowing box. This box serves the same purpose as the upper chamber of a smith's bellows. It receives the steam from the cylinders in alternate puffs, but lets it escape into the chimney in a continued stream by a number of small jets. Regular draught is by this means produced, and no noise is perceived. Another exit for the steam is also provided, by which the conductor is enabled to increase or diminish, or to suspend altogether, the draught

in the chimney, so as to adapt the intensity of the fire to the exigencies of the road. This is a great convenience in practice; because on some roads a draught is scarcely required, while on others a powerful blast is indispensable.

Connected with this blowing box is another apparatus of considerable practical importance. The pipe through which the feeding water is conducted from the tank is carried through this blowing box, within which it is coiled in a spiral form, so that an extensive thread of the water is exposed to the heat of the waste steam which has escaped from the cylinders, and which is enclosed in this blowing box. In passing through this pipe the feeding water is raised from the ordinary temperature of about 60° to the temperature of 212°. Fuel is thus economised and weight diminished; but there is another still greater advantage attending this process. The feeding water in the worm just mentioned, while it takes up the heat from the surrounding steam in the blowing box, condenses a part of the waste steam, which is thence conducted to the tank, from which the feeding water is pumped.

When steam is generated so rapidly as is necessarily the case in locomotive boilers, it rises with great violence in numerous bubbles from the bottom of the boiler to the surface of the water, and puts the liquid into a state of foaming turbulence not unlike the sea in a storm. As the steam rushes from the surface into the upper part of the boiler, under these circumstances, it carries with it a spray by which water is scattered in minute subdivision among the steam, and floats there like the spray which rises from the base of a cascade. If the steam be conducted immediately to the cylinder from the boiler in this state, it will carry with it the water which is thus suspended in it, which will pass through the cylinder, and finally be driven into the atmosphere upon the returning stroke of the piston. The hot water thus carried off possesses none of the mechanical properties of steam, and is wholly inefficient as a moving power, and is therefore an extensive source of the waste of heat. In every boiler, some means should be provided for the separation of the water thus suspended in the steam, before the steam is conducted to

remains above the surface of the water serves this purpose. The steam being there subject to no agitation or disturbance, the water mechanically suspended in it descends by its own gravity, and leaves pure steam in the upper part. In the small tubular boilers, this has been a matter, however, of greater difficulty. The contracted space in which the ebullition takes place causes the water to be mixed with the steam in a greater quantity than could happen in common boilers; and the want of the same steam-room renders the separation of the water from the steam a matter of some difficulty. These inconveniences have been attempted to be overcome by various contrivances. I have already described the rapid and regular circulation effected by the arrangement of the tubes. By this a regularity in the currents is established, which has a tendency to diminish the mixture of water with the steam. In addition to this, a method of separation is provided in the vessel 1, which is a strong iron cylinder of some magnitude, placed out of the immediate influence of the fire. A partial separation of the steam from the water takes place in the cylinder H; and the steam with the water mechanically suspended in it, technically called moist steam, rises into the separator 1. Here, being free from all agitation and currents, and being, in fact, quiescent, the particles of water fall to the bottom, while the pure steam remains at the top. This separator, therefore, serves all the purposes of the steam-room above the surface of the water in the large plate boilers. The dry steam is thus collected and ready for the supply of the engine through the tube N, while the water, which is disengaged from it, is collected at the bottom of the separator, and is conducted through the tube T to the lowest vessel D, to be again circulated through the boiler.

The pistons of the engine work on the axles of the hind wheels of the carriage which bears the engine, by cranks, as in the locomotives on the Manchester railway, so that the axle is kept in a constant state of rotation while the engine is at work. The wheels placed on this axle are not permanently fixed or keyed upon it, as in the Manchester locomotives; but they are capable of turning upon it in the same manner as ordinary carriage wheels. Immediately within