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 is 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

* To understand the power of a high-pressure engine, and the amount of caloric absorbed to produce a given force of steam, it is necessary to compute this force at the various thermometine degrees of heat, commencing at the boiling point of water. This has been most accurately calculated by Dr. Ure, who, as well as Mr. Dalton, made a variety of experiments to illustrate this highly interesting subject, the result of which will be found in Appendix (B).

scribed by Mr. J. Cooke in the Transactions of the Irish Academy. It consisted of a wheel, with moveable valves or flaps on its circumference, turning freely on joints, which admitted their openings only to a line passing through the centre of the wheel. These, when closed, lay exactly on a level with its outer extremity, but when open fell down perpendicular to it. The wheel, thus formed, was enclosed in a case, which fitted it exactly, while the upper valves were close and the lower extended. The steam in its passage from the boiler to the condenser, pushed the extended valves, and thus gave motion to the wheel. A working model of this engine, without the condenser, was, we believe, exhibited before several members of the academy. In this instance, however, it must have acted as a high-pressure engine, discharging the steam at each division of the wheel; and we very much doubt whether it would be possible to pack the hanging valves sufficiently tight to admit of their readily falling to the position described by Mr. Cooke.

Several other attempts have also been made to produce a continuous rotatory motion, the most important of which will be found in the analysis of patents prefixed to this work; but the only one which appears at all likely to succeed in practice has lately been invented by Mr. Masterman, and to this we would more particularly call the

* Vol. iii. p. 113. A. D. 1789.

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 pîston, 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 be effected by the pressure of the steam in the top of the small cylinder, acting beneath the piston in the great cylinder; thus alternately admitting the steam to the dif ferent sides of the smaller piston, while the steam last admitted into the smaller cylinder, passes regularly to the different sides of the larger piston,

tion at the right angle, and the axis of it is supported by bearings screwed to the cistern; and at the opposite end to that upon which the cylinder is placed, the horizontal arm of the triangle forms the working arm of the beam, to the extremity of which the power of the cylinder is applied. The connecting rod is jointed at the upper end of the perpendicular arm, and extends to the crank, which is supported in bearings screwed to the cistern at the same end at which the cylinder is placed, the centre of motion being at the same level with the top of the cistern; and beneath the cylinder, the hypothenuse of the triangle of the beam forms a brace to strengthen it. Two of these beams are used, and are applied on opposite sides of the cistern, upon the same axis of motion, and are united together by cross rods, thus forming two connecting rods and cranks upon one axis of motion, the fly-wheel being placed at one extremity of the axis. To connect the piston-rod with the ends of the arms of the beam, or the base of the triangle, a rod is fixed across the top of the piston rod; and to the two ends of this two other rods are linked, which descend to the beam, and are pointed to it at the ends. By this means, the ascent and descent of the piston-rod produce a corresponding action of the beam upon its centre of motion; the upper end of the perpendicular arm moving backwards and forwards, thus by means of the connecting rods turning the cranks. The rods which descend from the bar, which is

To produce a continuous rotatory motion, is, however, little more than a return to the simple principles on which the earliest engines were constructed. We have already stated, that the Italian philosopher Brancas, directed steam of considerable repellent force against the vanes of a wheel, which was employed to give motion to a stamping press, though the force obtained must have been very inconsiderable. The principle of this invention was also applied to a very ingenious, though we fear useless rotatory engine, constructed by Kempel.

It consisted of a hollow cylinder, furnished with two arms, at the end of which were transverse apertures; and this was connected with a large æolipile or boiler, by means of a moveable socket. The steam employed to drive the arms was of considerable elasticity, and rushing out at the apertures with considerable violence, produced a rotatory motion.

About the same period, Mr. Sadler, of Oxford, took out a patent for a similar machine, though we are not aware that it was ever usefully employed, the rarity of steam being such, that even if none be condensed by the cold of the atmosphere, the impulse must necessarily be exceedingly feeble, and the expense of steam, to produce any serviceable effect on the machinery, exceedingly great.

A very ingenious, and not less simple mode of producing a continuous rotatory motion, is de