application to this purpose. He proposed to employ an engine on Savery's plan, and added machinery to open and shut the cocks. Two or three large engines have been constructed in this country, which have since been employed in Holland with the most beneficial effects; and there is no doubt but that their value, when duly appreciated, will be sufficiently obvious. This is more particularly the case in those tracts of low and swampy ground, whose outfall lies at a considerable distance, and which has previously to pass through ground of a higher level. In some instances it has been found necessary to cut drains or rather trenches of from ten to twenty feet in depth, and this too for several miles in length.

The late Mr. Savory, of Downham, who gave considerable attention to this branch of civil engineering, states the cost of an engine of twenty-horse power fitted up for this purpose at fifteen hundred pounds, and that this will do as much work as a mill with a forty-feet sail, when in full velocity. The advantages that may be derived from the use of steam in the fens or marsh country, appears, from the same authority, to be of the first importance. In case of intense frost, the uniform velocity, with the opportunities of communicating heat, would prevent the engine from freezing, to which, from the uncertainty of winds, the other engines are very much subject. The consequence is, that a great fall of snow coming at the same time that the mills have not been in a state to prepare the ditches to receive the overplus water which it occasions, an inundation generally takes place in the fens; and, as the waters rise very rapidly under these circumstances after a thaw, it frequently occurs, that when the mills are set at liberty from the effects of ice, they are for some days incapable of successfully opposing the accumulation of water. On the other hand, by adopting the means of steam, the engines

would be working in full effect during the continuance of a frost, and the ditches being kept proportionably low, would at all times be capable of discharging the water, and thus prevent inundation.

IMPROVEMENTS

EFFECTED BY

MR. WATT, AND OTHERS,

DOWN TO THE PRESENT TIME.

CHAP. II.

Boulton and Watt-Cartwright-Smeaton-Horn blower -High-pressure Engine-Woolf's Improvements—Rotatory Engines-Kempel-Sadler-Cooke-Bell-crank Engine-Employment of the Steam Engine in North America, and the Colonies-Locomotive Engines.

IN the engine usually ascribed to Newcomen, the steam was not employed as an impelling power, but was used for producing a vacuum beneath the piston, which was afterwards forced down by the pressure of the atmosphere; and it was left to the masterly and towering genius of an otherwise obscure mechanic, to quadruple the force of this stupendous machine, and thus by one step, perfect the labours of the preceding century.

Mr. Watt's attention was first drawn to this subject, by the examination of a small model of an atmospheric engine, belonging to the University of Glasgow, which he had undertaken to repair. In the course of his experiments with

it, he found the quantity of fuel and injection water it required, much greater in proportion than in the larger engines; and it occurred to him, that this must be owing to the cylinder of this small model exposing a greater surface in proportion to its contents than was effected by larger cylinders. This he endeavoured to remedy, by employing non-conducting substances for those parts of the engine which came in immediate contact with the steam. After a variety of experiments, the results of which we shall presently describe, he succeeded in constructing a working model, capable of producing a force. equal to fourteen pounds on every inch of the piston, and which did not require more than one third of the steam used in the common atmospheric engine to produce the same effect.

It will be evident that this was as near an approximation towards perfection as could possibly have been expected; and indeed much more than was likely to be effected in a large engine, as the vapour left beneath the piston possessed only one-fifteenth part of the elastic force of the steam employed to form the vacuum.

Having discovered that the great waste of caloric in the old engine, arose from the alternate heating and cooling of the cylinder, by the admission and subsequent condensation of the heated steam, Mr. Watt perceived that to make an engine in which the destruction of steam should be the least possible, and the vacuum the most perfect, it was necessary that the cylinder should remain uniformly at the boiling point; while the water forming the steam was cooled down to the temperature of the atmosphere. To effect this, he employed a separate condensing vessel, between which, and the hot cylinder, a communication was formed by means of a pipe and stop-cock.

Mr. Watt's first great improvement in the engine of Newcomen may be best understood by reference to the

annexed diagram, in which a represents the cylinder, and b its plug or piston made to fit air-tight. The pipe d is

furnished with a stop-cock, by means of which the elastic vapour is occasionally admitted; a similar pipe, furnished with a stop-cock, at f, passes from the other side of the cylinder, and enters the vessel g; e is a reservoir to contain water. If we now suppose the piston at the bottom of the cylinder, and steam admitted by the pipe d, its expansive force will elevate the piston, and the whole internal cavity of the tube will be filled with condensable vapour. On closing the steam-cock, and opening that connected with the vessel g, a portion of the vapour will immediately expand itself, and coming in contact with the cold sides of the vessel, a portion of its heat must be absorbed by the water at e. A new supply of steam then descends, and is also condensed; and, indeed, the same