ists in his employing the steam both to elevate and depress the piston. In the engines of Newcomen and Beighton, the steam was not the impelling power, it was used merely for producing a vaccum below the piston, which was forced down by the pressure of the atmosphere, and elevated by the counterweight at the farther extremity of the great beam. The cylinder, therefore, was exposed to the external air at every descent of the piston, and a considerable portion of its heat being thus abstracted, a corresponding quantity of steam was of consequence destroyed. In Mr. Watt's engines, however, the external air is excluded by a metal plate at the top of the cylinder, which has a hole in it for admitting the piston rod; and the piston itself is raised and depressed merely by the force of steam.

When these improvements are adopted, and the engine constructed in the most perfect manner, there is not above part of the steam consumed in beating the apparatus; and, therefore, it is impossible that the engine can be rendered more powerful than it is at present. It would be very desirable, however, that the force of the piston could be properly communicated to the machinery without the intervention of the great beam. This, indeed, has been attempted by Mr. Watt, who has employed the pistou-rod itself to drive the machinery; and Mr. Cartwright has, in his engine, converted the perpendicular motion of the piston into a rotatory motion, by means of two cranks fixed to the axis of two equal wheels which work in each other. Notwithstanding the simplicity of these methods, none of them have come into general use, and Mr. Watt still prefers the intervention of the great beam, which is geberally made of hard oak, with its heart taken out, in order to prevent it from warping. A considerable quantity of power, however, is wasted by dragging, at every stroke of the piston, such a mass of matter from a state of rest to a state of motion, and then from a state of motion to a state of rest. To prevent this loss of power, a light frame of carpentry has been employed by several engineers instead of the solid beam. Cast iron beams have been adopted with great success. We have already glanced at Mr. Cartwright's engine; but think it right to develop a little farther its construction, as it evinces much ingenuity. See Pl. 165, fig. 2, where a is the cylinder, which is supplied with steam from the boiler through the pipe b; c is the piston in the act of going up; d is the pipe that conducts the steam into the condenser e, which consists of two cylinders, one within the other, leaving a small space between them, into which the steam is admitted; while the inner cylinder is filled with cold water, and also the external cylinder surrounded by the same; so that, by this means, a very large surface of steam is exposed, though no water is suffered to come into actual contact with it.

To the bottom of the piston, c, is attached a rod, with another piston, e, working in the pipe d. When the piston e arrives at the bottom of the eylinder, a valve which is in the piston is opened by its pressing against the bottom, and opens a communication with the condenser, whilst the spring k, fixed to the rod of the piston, shuts the valve which admits the steam from the boiler. The steam, therefore, being thus condensed, runs into the lower pipe f. The piston e, arriving at the bottom of the pipe in which it works at the same time with c, presses upon the condensed water, shuts the valve f, and forces the water up VOL. XI.

the pipe g, into the box h. The air which is disa engaged from the water rises to the top of the box, and, by its elasticity, forces the water through the pipe i, which carries it back again into the boiler. When the air accumulates in the box to such a degree as to depress the water, the ball cock falls with it, and opeus a valve in the top of the box, which suffers some of the air to escape.

When all the steam is condensed, the motion of the fly attached to the machine brings the piston up again, its valve now remaining shut by its weight. On arriving at the top, it presses up the steam-valve, which admits the steam from the boiler to force it down as before.

I and m are two cranks, upon whose axis are two equal wheels working in each other, for the purpose of converting the perpendicular motion of the piston-rod into a rotatory motion, for working the machinery attached to it.

But the most valuable part of this engine is in the construction of the piston, which Mr. Cartwright made wholly of metal, and so as, by means of springs, to fit the cylinder very exactly. This not only saves the expence and trouble of packing, which they are obliged frequently to renew in all other engines, but also saves a great deal of steam, on account of the more accurate fitting of the piston.

As it is evident, from its construction, that the whole of the steam is brought back again into the boiler, it affords the means of employing ardent spirit instead of water, and thus saving a great deal of fuel.

This machine seems to be peculiarly applicable to purposes requiring only a small power, as it is not expensive, and occupies little room.

Besides these there are several other truly ingenious constructions of the steam engine, by the Hornblowers, by Trevethick, Woolf, Murray, Bramah, Maudsley, &c. for descriptions of some of which the reader may consult the 2d vol. of Gregory's Mechanics, and for many others, the successive volumes of the Repertory of Arts and Manufactures. We cannot here find room to describe more than one other variety, which is the

High Pressure Steam Engine.-In this construc tion the operation is solely by the expansive force of the steam, which is not condensed in the manner of the atmospheric or Boulton's engines. Steam being raised in a boiler, and heated far beyond the boiling point, acquires a great expansive force, and exerts an immense pressure to escape from any vessel it is confined in this steam being admitted into one end of a cylinder, while a communication is opened with the other end to the open air, will exert a pressure upon the piston of the cylinder, and move it from one end to the other. Such is the principle of this engine. Its construction is detailed in plates 163 and 164; the former containing a perspective view of the whole engine, and the latter a section and elevation of it; the same letters of reference being used throughout. AA is the boiler, being a cylinder of cast iron with a flaunch at one end to screw on the end a. The fire is contained in a cylinder BB, fig. 2, of wrought iron, included within the former and surrounded with the water, the fire-grate CC extending across it, and the space beneath forming the ash-pit. At the farther end, the fire-place is connected with an iron tube, (shewn by the dark circle in fig. 2), which returns through the lid a of the boiler, and turns up at DD, forming the chimney which conveys away

H

the smoke. In the pipe D is a small door b to remove the soot which may accumulate there. E is a hole covered by a lid to clean the boiler, and F another of the same kind, to let off the water; ef are the gauge cocks for ascertaining the height of the water in the boiler; one is placed a small quantity above the intended water line, and the other as much below it: if, therefore, the cocks, when opened, both emit water, it shews the boiler has too much water, or if on the other hand both give steam, the water is too low; as was the case with respect to Watt's engine. On the top of the boiler at G a valve is fixed, and kept down by a kind of steelyard, loaded with a weight: if the pressure of the steam exceed 60 lbs. per square inch, so as to endanger the bursting of the boiler, this valve is lifted up by the pressure, and allows the excess of steam to escape. His the steam cylinder fitted into the boiler; it is provided with a piston g, and a lid with a stuffing box, through which the piston rod h passes, and is at its upper end fastened to a cross bar I, guided in a vertical motion by sliding upon two parallel iron rods KK, firmly fixed to the cylinder, and supported by stays LL extending to the other end of the boiler. At the ends of the cross rod I two connecting rods MM are jointed, and these at their lower ends are connected with two cranks, upon an axis N, extending across beneath the cylinder, and supported in bearings made in the legs which support the boiler; oue of these cranks is shown at i fig. 3, the other is only a pin fixed into one of the arms of a fly wheel 00. Now as the piston is alternately forced up and down by the pressure of the steam, it moves the rod I, and by the connecting rods M turns the cranks, the fly wheel by its momentum regulating the motion: the machinery the engine is adapted to is turned by a cog wheel fixed on the end of axis N, beyond the crank i. It now remains to shew the means by which the steam is brought to act alternately on different sides of the piston: for this, see figs. 4 and 5, where H represents the cylinder, and k the flaunch by which it is fixed into the boiler. On one side of the cylinder above this flaunch a protuberance is cast to contain the four following passages: one at opening beneath the flaunch, and therefore bringing steam from the boiler to the cock (represented by a small circle) which admits it either into the upper or lower ends of the cylinder; the former through the passage m, and the latter by a pipe n extending down close by the side of the cylinder to the bottom. The last passage o is to allow the escape of the steam when it has passed through the engine. These four passages meet each other at right angles round the cock, which has two curved passages in it to connect the passages at proper times. When in the position fig. 4, the steam entering at 7 passes through n down to the bottom of the cylinder, and tends to force the piston upwards at the same time the passage m from the top of the cylinder is open to o, allowing the steam contained in the upper part of the cylinder to escape to the open air as the piston rises by turning the cock a quarter round, this action is reversed; steam from I being admitted through m, into the top of the cylinder, and the steam from the bottom escaping through n at o, the motion of the cock is produced by a short handle p, figs. I and 3, fixed on it, and this is moved by pins q, projecting from a small rod r fixed to the rod I, and consequently having the same motion, it moves the cock every time the piston is at the top or

bottom of its motion, and by that means reverses the movement.

The steam escaping at the orifice o passes down through a pipe P, which is conducted into the open air. This pipe is inclosed within another R, leaving a small space all round between them from this pipe is a small passage at the upper end leading into the end of the boiler; into this external pipe R at the lower end a pipe is joined, this brings cold water from a small pump worked by the engine, this water passes out into the boiler at the upper end, to supply the loss of water in boiling; but in its passage, this water is heated nearly to boiling by the waste steam in the pipe P passing through it, and cominunicating a heat which would otherwise be lost; to regulate the velocity of the engine, a small cock s, figs. 4 and 5, is placed in the passage 1, by which the quantity of steam admitted to the cylinder can be regulated, or suddenly cut off if the engine is wanted to be stopped. In engines which have a regular and constant load or resistance of work opposed to them, this cock is moved by its handle, till the engine has acquired its intended velocity; but in engines where the work performed is irregular, an apparatus shewn in fig. 1 is introduced; it is called the governor or regulator, and is equally applicable to any other kind of steam engine, or to a water wheel. It consists of a vertical axis SS, to which two pendulums tt, with iron balls at their ends, are suspended by a joint at v: above this the pendulums have short arms w w, like a pair of shears, and these arms are connected by iron rods with a ring z, having liberty to slide up and down upon the axis. This ring is encompassed by the end of a lever y, which is connected by a similar lever z, which by a wire moves the handle of the regulating cock before mentioned. The governors are turned by a pair of bevelled wheels at T receiving motion, by an endless strap V from a pulley W fixed on the axis of the fly wheel 00. The operation of the machine is as follows: by the revolution of the axis S, the centrifugal force of the balls it causes them to recede from the axis, and this by the joints ww draws down the end of the levers y and z, opening the regulating cock to admit just such a quantity of steam as will work the engine at the proper velocity: now if by part of the resistance the engine has to overcome being removed (or by the steam being hotter and consequently more powerful) its velocity is increased, the balls being turned quicker fly out farther from the centre, and close the cock till it comes to the right velocity again; if from opposite causes the engine moves too slow, the balls approach each other and open the cock to give the engine a greater supply of steam; so that this governor always regulates the engine to work with the same velocity under all circumstances as indeed was observed in the description of Watt's machine, and is merely repeated here, to show the application to the present engine.

On the Power of Steam Engines, and the Method of

computing it.

From the account which has been given of the steam engine, and the mode of its operation, if must be evident that its power depends upon the breadth and height of the cylinder, or, in other words, on the area of the piston and the length of its stroke. If we suppose that no force is lost in overcoming the inertia of the great beam, and that the lever by which the power acts is equal to