Cornwall. It is upon Mr. Watt's double-action principle, and is loaded to exert its utmost force. The steam-cylinder is sixty-three inches in diameter, and raises water equally in the ascent or descent of the piston. The weight of water in the pumps is 82,000 pounds; and with this load it makes 6 double strokes per minute, of 7 each, or it gives to the load 100 feet motion per minute. To effect this, which is equivalent to raising 8,261,500 pounds, lifted one foot high, or the power of 250 horses, it is necessary to consume about thirty-one pounds of coal per minute.

In 1797, the ingenious Mr. Cartwright, well known for the value and variety of his scientific avocations, invented a mode of applying the vapour of alcohol, or other ardent spirit, as a substitute for common steam. In addition to the saving to be effected by this plan, Mr. Cartwright intended to employ his engine as a still, by which the whole cost of fuel would be saved.* Such a method, however,

* It is curious, that a distinguished chemist of the present day should have suggested the construction of an engine on the following plan, which, it will be seen, is nearly similar to that described in Mr. Cartwright's specification.

"Since the vapour of alcohol, having the same elastic force as the atmosphere, contains 44 of the latent heat of ordinary steam, and since its elastic force is doubled at the 206th degree (six below the boiling heat of water) with perhaps one-third of additional caloric, might we not, in particular circumstances, employ this vapour for impelling the piston of a steam engine? The condensing apparatus, could be, I imagine, so constructed, as to prevent any material loss of the liquid, while more than a quadruple power would be obtained from the same size of cylinder at 212o, with an expenditure of fuel not amounting to one half of what aqueous vapour consumes; or the power and fuel would be as three to one, calling their relation in ordinary steam one to one. A considerable engine could thus also be brought within a very moderate compass. Possibly, after a few operations of the air-pump, the incondensible gas may be so effectually withdrawn, that we might be permitted to detach this mechanism, which, though essential to common engines, takes away onefourth of their power. In a distillery in this country, or on a sugar-estate

supposes a capability of blending the business of a distiller with a variety of trades, to which it is totally inapplicable. A scheme somewhat similar to this, and to which we shall afterwards more fully revert, has lately been attempted by Colonel Congreve, in which he proposes to burn a large portion of chalk mixed with the coal, and thus convert the furnace into an efficient lime-kiln. From this view of the subject, we think it will be seen, that however plausible or ingenious this invention may appear in theory, there are insuperable objections to its general employment. We are still, however, greatly indebted to Mr. Cartwright for the mechanical arrangement of the engine described in his patent, as it furnishes the first hint of an elastic metal pis− ton, which has since been found of the greatest use in high-pressure engines.

The first Portable Steam Engine appears to have been constructed by Mr. Smeaton, who employed it for draining foundations and other temporary works. It had a pulley, or wheel, to receive the chain which communicated motion from the piston to the pump-rod, instead of a beam. The pivots of the wheel were supported by two inclined beams connected at top, whilst the cylinder and pump were bolted down to the groundsills. Thus, the whole machine being supported by one frame of wood, it could without trouble be set to work in the open air. The boiler, which required no setting in brick work, was in the shape of a tea-kettle, and the fire-place being in the centre, was

in the colonies, a trial of this plan might perhaps be made with advantage. While exercising its mechanical functions of grinding, mashing, or squeezing the canes, it would be converting ordinary into strong spirit for rectification, or for the convenience of carriage. Might not such an engine be erected on a small scale, for many purposes of domestic drudgery? It would unquestionably furnish a beautiful illustration in philosophy, to make one small portion of liquid, by the agency of fire, imitate the ceaseless circulation and restless activity of life."-Phil. Tran. vol. cviii. p. 393.

surrounded on all sides by water, thus presenting the greatest possible surface to the action of the flame. Portable steam engines are now employed not only in the erection of bridges, and in underground excavations, but are also usefully applied to the purpose of propelling vessels and carriages: the latter application is of a very recent date. Steam navigation, however, from its great national importance, will deservedly find a place in a separate division of this work.

The principle of the high-pressure steam engine depends on the power of steam to expand itself very considerably beyond its original bulk, by the addition of a given portion of caloric, thus acquiring a considerable elastic force, which, in this case, is employed to give motion to a piston. One of the greatest advantages attendant on employing the repellent force of steam, as in this form of the engine, consists in an evident saving of the water usually employed in condensation; and this, in locomotive engines, for propelling carriages, is an object of considerable importance.

Leupold has furnished a description of a high-pressure engine, in a very valuable work on machines, published in 1724. He ascribes the invention to Papin, and his apparatus consists of two single cylinders placed at some distance from each other, each of which is provided with a piston made to fit air-tight, and connected with a forcing pump.

When high-pressure steam is admitted at the bottom of the first cylinder, it is forced upwards, carrying with it the lever of the pump; at the same time that the steam or air is expelled from the other. On this operation being repeated, or rather reversed, the steam is allowed to enter the second cylinder, which is also connected with the boiler, while the steam in the first cylinder is allowed to escape into the air. From this it will be evident that the process

D

of condensation forms no part of the principle of the highpressure engine; and that the expansion of gunpowder might be made to produce a precisely similar effect.

The amazing force to be produced by the expansion of highly elastic vapour, did not escape the penetrating notice of that towering genius, which was now directing all its energies towards its improvement. Accordingly, we find in Mr. Watt's first patent, the following clause, which expressly describes this engine: "I intend, in many cases, to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner as the pressure of the atmosphere is now employed in common fire-engines. In cases where cold water cannot be had in plenty, the engines may be wrought by the force of steam only, by discharging the steam into the open air after it has done its office."

Messrs. Trevithick and Vivian were the first to employ the high-pressure engine to advantage, as they found it admirably adapted for the purpose of propelling carriages. In this case the steam, after having performed its office, was thrown off into the air; and the condenser, together with the necessary supply of cold water which must have accompanied it, was by this means dispensed with. For the purpose of motion, the high-pressure engine certainly possesses considerable advantages, not the least of which are its cheapness and portability; the danger, however, attendant on the use of steam, acting with a force equal to from forty to eighty pounds on the square inch, must inevitably form an insuperable bar to its general introduction to our manufactures.

Mr. Woolf's improvements, which are of considerable importance, are founded on the same principle as those of Mr. Watt, namely the power of steam to expand itself, or increase its volume in a very considerable degree, after its

passage from the boiler. From a variety of experiments made on this subject, he ascertained that a quantity of steam having the force of five, six, seven, or more pounds on every square inch of the boiler, may be allowed to expand itself to an equal number of times of its own volume, when it would still be equal to the weight of the atmosphere, provided that the cylinder in which the expansion takes place, have the same temperature as the steam possessed before it began to increase.

The most economical mode of employing this principle, consists in the application of two steam-cylinders and pistons of unequal size to a high-pressure boiler; the smaller of which should have a communication both at its top and bottom, with the steam vessel. A communication being also formed between the top of the smaller cylinder and the bottom of the larger cylinder, and vice versa. When the engine is set to work, steam of a high temperature is admitted from the boiler to act by its elastic force on one side of the smaller piston, while the steam which had last moved it, has a communication with the larger or condensing cylinder. If both pistons be placed at the top of their respective cylinders, and steam of a pressure equal to forty pounds the square inch, be admitted, the smaller piston will be pressed down, while the steam below it, instead of being allowed to escape into the atmosphere, or pass into the condensing vessel, as in the common engine, is made to enter the larger cylinder above its piston, which will make its downward stroke at the same time as that in the smaller cylinder; and, during this process, the steam which last filled the larger cylinder, will be passing into the condenser to form a vacuum during the downward stroke.

To perform the upward stroke, it is merely necessary to reverse the action of the respective cylinders; and it will