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 stopcock. To understand the action of this engine, we may employ a common syringe, connected with a boiler, as in the atmospheric engine, and furnished with a pipe passing into an air-tight vessel, immersed in water for the purpose of condensation. If the piston be then raised, and the communication with the condenser cut off, the steam will speedily expel the air; when this is effected, the further admission of steam must be prevented, and the communication with the condenser opened. The steam will now expand itself, passing down the pipe and entering the condenser; the moment, however, that it comes in contact with the sides of the cold vessel, it will be condensed and a vacuum formed; and this process will continue to proceed, so long as any steam remains beneath the piston. The only objection that offered itself to this admirable mode of condensation, arose from the 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 onefifteenth 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 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 stopcock. To understand the action of this engine, we may employ a common syringe, connected with a boiler, as in the atmospheric engine, and furnished with a pipe passing into an air-tight vessel, immersed in water for the purpose of condensation. If the piston be then raised, and the communication with the condenser cut off, the steam will speedily expel the air; when this is effected, the further admission of steam must be prevented, and the communication with the condenser opened. The steam will now expand itself, passing down the pipe and entering the condenser; the moment, however, that it comes in contact with the sides of the cold vessel, it will be condensed and a vacuum formed; and this process will continue to proceed, so long as any steam remains beneath the piston. The only objection that offered itself to this admirable mode of condensation, arose from the Soon after the completion of his first model, Mr. Watt erected an engine for his friend Dr. Roebuck of Kinneil, near Borrowstownness, with whom he was afterwards associated in the manufacture of his improved engine: the latter gentleman, however, in 1774, disposed of his share of the business to Mr. Boulton, of Soho*. * From this nursery of ingenuity has originated some of the noblest and most striking chefs d'œuvre of mechanical art yet witnessed. The following account of this celebrated manufactory is from the pen of Dr. Darwin. It was written in 1768, and when the manufactory, although "big with promise," was little more than a type of its present magnitude. "Soho is the name of a hill in the county of Stafford, about two miles from Birmingham, which a very few years ago was a barren heath, on the bleak summit of which stood a naked hut, the habitation of a warrener. "The transformation of this place is a recent monument of the effects of trade on population. A beautiful garden, with wood, lawn, and water, now covers one side of this hill; five spacious squares of building, erected on the other side, supply workshops or houses for about six hundred people. The extensive pool at the approach to this building, is conveyed to a large water-wheel in one of the courts, and communicates motion to a prodigious number of different tools. The mechanical inventions for this purpose are superior in multitude, variety, and simplicity, to those of any manufactory in the known world. "Toys, and utensils of various kinds, in gold, silver, steel, copper, tortoise-shell, enamels, and many vitreous and metallic compositions, with gilded, plated, and inlaid works, are wrought up to the highest elegance of taste, and perfection of execution in this place. "Mr. Boulton, who has established this great work, has joined taste and philosophy with manufacture and commerce; and from the various branches of chemistry, and the numerous mechanic arts he employs, and his extensive correspondence to every corner of the world, is furnished with the highest entertainment, as well as the most lucrative employment." We have already stated, that Dr. Falck described an engine with two cylinders as early as 1779, which he called a double acting engine; but similar advantages were obtained by Mr. Watt in an engine with a single cylinder. To effect this, he applied the power of the steam to press the piston upwards as well as downwards, by forming the vacuum alternately above and below the cylinder. When it became necessary to connect the piston and beam for an upward, as well as for a downward stroke, a double chain acting on an arch head was substituted for the single one previously employed; and this was speedily superseded by a rod connected with the working beam, by means of a parallel motion. The Expansion Engine was also invented by Mr. Watt, and though not generally employed until 1778, appears from a letter written by him to a gentleman of Birmingham, to have suggested itself as early as 1769. The principle of this invention, consists in shutting off the farther entrance of steam from the boiler when the piston has been pressed down in the cylinder, for a certain proportion of its total descent, leaving the remainder to be accomplished by the expansive force of that steam already introduced. To regulate the time of closing this valve, and as such the precise amount of steam admitted, Mr. Watt employs a plug-frame with moveable pins, which may be placed in such a manner, that the steam-valve will shut when the piston has de |