communicates with the steam-boiler. So that if we suppose the piston to be in an ascending direction, and the steam of course entering the cylinder beneath, a communication will at the same time be formed between the upper side of the piston and the atmosphere, while the steam that had previously been employed to depress the piston is now allowed to escape. When the piston has reached the top of the cylinder, the cock is turned, and its action reversed, the steam now entering above the piston, while a communication is formed for its escape beneath.

The remaining parts of the high-pressure engine, as constructed by Messrs. Trevithick, may very easily be understood. The boiler consists of a large cylinder of cast iron, made very strong, and placed with its axis horizontally upon short feet or pillars of cast iron; the boiler has a flanch at one of its ends to screw on the end or cover, which has the requisite openings for the fire-door, the man-hole, the exit for the smoke, and the gauge-cocks. The fire is contained within the boiler in a cylindrical tube of wrought iron, which is surrounded with water on all sides; one end of this tube is attached to the end or cover of the boiler, and is divided into two parts by having the fire-grate extended across it; the fire-door closes the opening in the upper half, which is the fire-place, the lower half forming the ash-pit; the tube extends nearly to the end of the boiler, where

it is reduced in size, then doubles, and returns back in a direction parallel to the first tube or fire-place, to form the flue, till it arrives at the end of the boiler, through which it passes at the side of the fire-door, and it is then conducted from it into the chimney, thus carrying off the smoke.

At the part where the flue enters the chimney, is a small door to remove any soot that may have accumulated. On the top of the boiler is a safetyvalve, kept down by a lever and weight, to allow the steam to escape in case it becomes so strong as to endanger the bursting of the boiler. The cylinder of the engine stands in a perpendicular direction, and is enclosed within the boiler, except a few inches of its upper end, at which the fourpassaged cock is situate, and the flanch which screws on the lid, with the stuffing-box for the piston-rod to pass through. The boiler has a projecting neck, into which the cylinder is received, and it is fastened in its place by a flanch round the upper end of the neck of the boiler, which is united by screws to a similar flanch projecting from the cylinder. The upper end of the piston-rod is fastened to the middle of a crossbar, which is placed in a direction at right angles to the length of the boiler, and guided in its ascending and descending motion, by sliding between two perpendicular iron rods, fixed to the boiler, parallel to each other, being connected together at top, and firmly supported there

by two diagonal stays extending from the other end of the boiler, and secured to the flanch, which screws on the end of the boiler. At the ends of the cross bar of the piston-rod, the two connectingrods are jointed, and the lower ends of them are connected with two cranks, fixed upon an axis, extending across beneath the boiler, and under the centre of the cylinder; the axis is supported in bearings made in the legs which support the boiler, and the fly-wheel is fixed in it. One of the cranks is formed by a pin, which is fixed into the arm of the fly-wheel at the same radius as the opposite crank. The fly-wheel is situate close to the side of the boiler, and the pin for the other crank is fixed into the arm of a large cogwheel, fixed on the axis of the fly-wheel at the opposite side of the boiler. This cog-wheel communicates the power of the engine to other cogwheels. As the piston is alternately forced up and down by the pressure of the steam, it carries the cross-bar with it, and by the connecting-rod turns the two cranks, together with the fly-wheel, and other connecting machinery.

The Double-cylinder Expansion Engine, constructed by Mr. Woolf, possesses considerable advantages over that invented by Mr. Hornblower, with whom, it appears, the idea of constructing an engine with two cylinders originated: the only novelty, however, in Mr. Woolf's engine, consists in the application of high-pressure steam, and in

the proportioning of the cylinders to the elasticity of the vapour employed. Having already explained the principle upon which Mr. Woolf's engine is constructed, it may be enough for our present purpose to describe its action, with reference to the annexed diagram.

A and B represent the two cylinders, in the larger of which the steam is allowed to expand itself, after passing from B. The steam, which in the first instance is of considerable elasticity, is admitted to the cylinder B, by the tube and valve 6, and entering the cylinder above the piston 3, impels it to the bottom. When this is effected, a communication is opened between the upper

part of the cylinder B, and the under side of the piston 4, in the larger cylinder. The communication between the cylinder B and the steampipe 6, is now reversed, and the steam is made to press on the under side of the piston 3; a communication being at the same time formed between the upper part of the cylinder A, and the pipe leading to the condenser 5. So that if we suppose the pistons 3 and 4 connected by means of their rods with one end of an ordinary working beam, the upward and downward strokes of each will be performed at the same time.

We have hitherto considered the steam as passing direct from the boiler to the cylinder B; this, however, is in reality effected by a more circuitous route, as it is in the first instance admitted to the steam-case of the larger cylinder, by the pipe 1, and passing round a similar case, encircling the cylinder B, it is then made to enter the cylinder by the tube 6. The pipe at Fig. 2, is merely intended to form a communication for carrying back to the boiler any water that may be produced by condensation in the steam-case, before the engine arrives at a proper temperature for working.

Having described the nature of Mr. Woolf's engine, it may now be advisable to examine the boiler, by which he proposed to generate steam of sufficient elasticity for the use of the small cylinder, which requires vapour of great expansive force.