until advantage should be experienced; and they even erected some Engines on several mines at their own expense, to be paid provided they answered the expectations, which they, as manufacturers, held out were obtained by their adoption. And it was subsequently stated that a sum not less than forty-seven thousand pounds had been expended in the speculation, by Bolton and Watt, before they began to receive any remuneration.

The mine of Chacewater was among the first at which they were introduced in Cornwall. Playfair has stated, that here three of the largest-sized engines were erected; and it gives a high idea of the enormous expenses of the common engines, when we are told that for each of those machines the proprietors engaged to pay 800 pounds annually, as a compromise for the patentee's "third part of the saving" made in coals by using Mr. Watt's Engine instead of Newcomen's.

The expansive power of steam suggested by Mr. Watt in 1769, and subsequently partially adopted to equalize the motion of the piston, was introduced as a means of saving steam in an engine at Soho manufactory in 1776, and in 1778 at Shadwell water-works, and afterwards particularly described in his specification of a patent in 1782. The parts of the Expansive Engine are in every respect the same with those we have already described; but the cylinders require to be made of greater dimensions.

If we now suppose the piston b, in the Twentyfifth Figure, to be in the situation there represented, and the vacuum made under this piston by the usual means of opening a communication with the cylinder and the condenser; let the cock s be opened,

and allow the steam from the boiler to enter and press down the piston until it reaches the middle of the cylinder, to about the position of a; and let the cock 1 be now turned, and allow no more steam to enter; the quantity which already has been admitted will press the piston to the bottom of the cylinder. It is also remarkable that this quantity of steam will do more work than if it had been allowed to flow into the piston to the limit of its stroke. Suppose, for instance, the boiler contains 100 cubic feet of steam, and that this quantity is to be expended in raising a certain quantity of water-say 100 cubic feet, when the communication is allowed to be open between the cylinder and boiler during its entire stroke; if this communication be shut off when the cylinder is only half filled with steam, the same quantity of 100 cubic feet of steam will raise 170 cubic feet of water. If the cylinder be only one third filled with steam, its effect will be greater than if it were filled one half; for the same quantity would raise 210 cubic feet of water; or, what is the same thing.

When the cylinder is quite full, its performance

pands by variations of pressure, in the same ratios that air would do. In practice it has been found, that under the usual pressure the encrease is not great after the steam is rarefied four times. It will be very obvious that this principle can be applied to both double and single engines; and that the common engines can be made to act expansively, or not, by the mere alteration of the tappets in the plugframe.

In both these forms of the apparatus the steam acts not only to form the vacuum, but to depress the piston. But still, during the operation of the counterpoise it produces no effect; and where it was required to move machinery, this suspension of impulse was a great drawback on its utility. This, however, was not objected to its general merit, when used as a mover of pumps; and the more so, as it was common to the Atmospheric Engine: but before it could be considered as a general first mover, this interyal required to be much shortened, if it could not be altogether filled up.

We have seen that a contrivance was used at the Hartley colliery for the production of a continuous motion from a reciprocating one; but this did not obviate the inconvenience of the returning stroke; and it was left for Mr. Watt to make this other step towards the perfection of the machine, and he accomplished it by a very slight extension of his first idea. He had introduced steam acting against a piston to press it downwards; he now formed a communication between both sides of the piston and the boiler, and also with the condenser, and made the

steam act to press the piston upwards as well as downwards.

The mechanism was now, as far as the principle went, perfect; and it was freed, for the first time, from the enormous dead weight of counterpoises, which had hung on it from the first attempts of Newcomen; and the equally enormous load which was used in the construction of the various parts, for the purpose of equalizing the motion.

The cylinder a, in the Twenty-seventh Figure, is enclosed in a jacket or casing like the single engine, having a similar interval, which may be filled with steam or air. The piston b is attached to the leverbeam by the rod x. 1, 2, 3, 4, are the valves which admit steam to the cylinder, or open a communication between the upper and under sides of the piston, and the condenser. g is the pipe leading from the valves to the condenser. mm, the levers or spanners, which are elevated or depressed by the tappets or pins n, z, in the plug-frame, and open or shut the valves to which they may be connected. h is the condenser; L, a pipe connecting it with air-pump i, and a second air-pump E. e, the piston-rod of this second pump, attached like the other, l, to the lever-beam. F, a pipe from the cold water-pump q, to supply the reservoir in which the condenser and its pumps are placed. k, a trough or reservoir into which the water heated by the condensation of steam in the condenser, which is raised by the air-pump, is pumped back by M, into the boiler. G, a pulley; and H, an endless chain moving over it, also going round a pulley fixed on the upright axis of the conical pendulum or governor z. The other pul