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cally engaged on the theme by which his great fame was obtained. There was in the college an old model of a steamengine by Newcomen, which was constantly used to illustrate the lectures of the professors. It had got out of order—indeed it had never acted properly, and Watt, whose ingenuity appears at that time to have been highly appreciated, was employed to put it into working condition. He did so, most satisfactorily; but the business did not rest there. His sagacious mind soon perceived that the ill-working of Newcomen's machine was owing to its demanding two almost irreconcilable conditions-the requirement of water at a high temperature; and a perfect vacuum, which could only be obtained by an injection of cold water, which had the effect of lowering the temperature of the steam on its coming in contact with the sides of the cylinder and piston, after these had been cooled by the introduction of the cold water. This discovery laid the foundation of his plan for a separate condenser, which he afterwards carried out so successfully. By the middle of the year 1765, his invention was completed; and the effect of it was that a great saving was effected in the cost of fuel, as it did away with a large amount of wasteful expenditure of steam power. During the progress of this invention, however, one great anomaly struck him for he found that steam of only 47 or 48 degrees of heat was sufficient to make water rise to the boiling heat of 212 of Farenheit's thermometer. On mentioning this strange circumstance to Dr. Black, that scientific person immediately showed him the cause of it, and then developed the qualities of latent heat, which he had lately discovered. Another great improvement introduced by Watt was the employment of steam instead of atmospheric air, to drive down the piston to the bottom of the cylinder. This was effected by letting the steam from the boiler enter above and below the piston alternately-the vacuum below the piston being also produced by the property of steam. Three years

elapsed from the time when these great discoveries were made by Watt before an opportunity was allowed him of carrying them into effect, on a sufficiently large scale to prove their working capabilities. This was at length accomplished by his introduction to Dr. Roebuck, when, after some negociations, a patent was taken out, and a partnership formed, Watt agreeing to cede to Roebuck two-thirds of all advantages to be derived from the invention. An experimental engine on a large scale. was next constructed, the success of which, with the exception of a few practical difficulties that presented themselves, was most complete.

A few years afterwards Dr. Roebuck became embarrassed in his circumstances, and in 1773 a partnership was entered into by Watt with Mr. Matthew Boulton of Soho, near Birmingham, in

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whose extensive establishment he found that assistance from able artizans and extensive capital which his most ardent wishes could desire. In the following year an application was made to Parliament for an extension of the patent, and in

1775 an Act was passed extending the term, which, according to the original patent, would have expired in 1783, for a period of seventeen years longer. Watt now applied himself vigorously to the perfection of his invention in all its practical details, and the result was the construction, on a large scale, of what is now known as his single acting steam-engine.

During the progress of those numerous minor improvements in the steam-engine which were continually being effected by Watt, one of the most interesting of which was that ingenious mechanical combination known as the parallel motion, he attempted to remedy the irregularity of action caused by the suspension of the power of the engine during the ascent of the piston rod; but while occupied in making various experiments, a workman in Watt's employ commu nicated the nature of the means by which Watt sought to effect the object in view to a person of the name of Wasborough, who at once adopted it, and took out a patent for the application of the crank to steam-engines.

To avoid litigation, Watt abandoned his idea of using a crank, and substituted for it a contrivance known as the sun and planet wheel. Neither of these inventions, however, served to maintain a regular rotatory motion such as was desired; but this was afterwards effected by means of a fly-wheel.

To remedy the irregularity of motion, produced by the unequal supply of steam from the boiler, Watt invented the throttle-valve, which, being placed in the pipe through which the steam is conveyed from the boiler to the cylinder, the opening and partial closing of it, by means of a lever, increased or reduced the supply of steam, according as it was required.

It being necessary that this lever should be moved by the man in charge of the engine, Watt foresaw that any want of proper vigilance and skill on his part would render the throttle-valve ineffectual for the uses to which it was designed.

He therefore connected the lever, by means of which its motions were regulated, with an apparatus, founded on the principle of the regulator employed in windmills, to which he gave the name of the governor. This had the effect of enabling the engineer at all times to regulate the monster of motion under his care to such a nicety, that although there is the power of some millions of horses now continually engaged, an accident is an infrequent occurrence. Such a degree of security, indeed, has been obtained, that it is said to be by no means uncommon, on the presence of visitors, for the director of an engine at the Cornish mines, where the most powerful machines in the world are employed, to step on the valve of his vast machine and stop it, until, perhaps, if checked one moment longer, it would blow him and all around into ten thousand atoms—a foolish feat which no one with a proper regard to the lives of others would venture to put in practice.

Before proceeding further with our history, we think it desirable to bring before the reader at one view the high state of perfection to which the steam-engine had been advanced by the superior intelligence and energy of Watt, that the reader may see how the many beautiful contrivances, emanating from the master mind of this greatest among mechanicians, when brought together, form one complete and harmonious whole. This object will, we think, be effected by the study of the diagram and description of the double-acting steam-engine here given.

The steam from the boiler is conveyed to the cylinder a through the steam-pipe в, the supply being regulated by the throttle-valve c, which valve is under the direct influence of the governor D. On one side of the cylinder, at the upper and lower ends, are attached two square hollow boxes, marked E, which communicate with the cylinder by means of a passage in the middle of each. These boxes have each two valves, by means of which they are divided into three compartments.

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The top compartment in both boxes communicates with the steam-pipe, and the lower one with the eduction-pipe leading to the condenser. These valves move in pairs—that is, the upper induction-valve F and the lower exhaustion-valve ƒ move together, and the same with the upper exhaustion-valve G and the lower induction-valve g. The piston R, being accurately

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