the motion being regulated by a fly-wheel. This invention was employed by the patentee in several engines, and among others in one erected at Birmingham.

It will now be asked what Mr. Watt had been doing while these inventions were forcing themselves upon public attention. If we turn again to his single-action machine, two difficulties in the way of obtaining rotatory motion will be detected. The first and most evident is, that the motion of the engine was rectilinear. The second difficulty was, that the force of the engine was intermitting, and for a circular motion it was necessary that it should be constant and uniform. The work of his engine was confined to the descending motion of the piston-during the up stroke of the piston it was suspended. It was necessary, therefore, in order to make it applicable to a rotatory movement, either to change the principle of operation, or to devise some method of sustaining the motion of a revolving shaft while the power of the engine was suspended. Watt knew all this, and his mind had not been inactive. While others were apparently obtaining the advantage of patents, and superseding him, he possessed what they sought. The history he has given of the method by which he succeeded in obtaining a rotatory motion from the steam-engine, is so curious, and will be read with so much interest, that it must be quoted in full: "I had very early turned my mind to the producing continued motions round an axis, and it will be seen by reference to my first specification, in 1769,

that I there described a steam wheel, moved by the force of steam acting in a circular channel against a valve on one side, and against a column of mercury, or some other fluid metal, on the other side. This was executed upon a scale of about six feet diameter at Soho, and worked repeatedly, but was given up, as several practical objections were found to operate against it. Similar objections lay against other rotative engines, which had been contrived by myself and others, as well as to the engines producing rotatory motions by means of ratchet-wheels. Having made my reciprocating engines very regular in their movements, I considered how to produce rotative motions from them in the best manner; and amongst various schemes which were subjected to trial, or which passed through my mind, none appeared so likely to answer the purpose as the application of the crank in the manner of the common turning-lathe, (an invention of great merit, of which the humble inventor, and even his era, are unknown.) But as the rotative motion is produced in that machine only by the impulse given to the crank in the descent of the foot, and behooves to be continued in its ascent by the momentum of the wheel, which acts as a fly, and being unwilling to load my engine with a fly heavy enough to continue the motion during the ascent of the piston, I proposed to employ two engines, acting upon two cranks, fixed on the same axis, at an angle of 120° to one another, and a weight placed upon the circumference of the fly at the same angle to each of the cranks, by which means

the motion might be rendered nearly equal, and a very light fly would only be required.

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This had occurred to me very early; but my attention being fully employed in making and erecting engines for raising water, it remained in petto until about the year 1778 or 1779, when Mr. Wasbrough erected one of his ratchet-wheel engines at Birmingham, the frequent. breakages and irregularities of which recalled the subject to my mind, and I proceeded to make a model of my method, which answered my expectations; but having neglected to take out a patent, the invention was communicated, by a workman employed to make the model, to some of the people about Mr. Wasbrough's engine, and a patent was taken out by them for the application of the crank to steamengines. This fact the said workman confessed, and the engineer who directed the works acknowledged it; but said, nevertheless, that the same idea had occurred to him prior to his hearing of mine, and that he had even made a model of it before that time; which might be a fact, as the application to a single crank was sufficiently obvious.

"In these circumstances, I thought it better to endeavor to accomplish the same end by other means than to enter into litigation; and, if successful, by demolishing the patent, to lay the matter open to everybody. Accordingly, in 1781, I invented and took out a patent for several methods of producing rotative motions from reciprocating ones, among which was the sun and planet wheels. This contrivance was applied to

many engines, and possessed the great advantage of giving a double velocity to the fly-wheel; but is, perhaps, more subject to wear, and to be broken under great strains, than a simple crank, which is now more commonly used, although it requires a fly-wheel of four times the weight, if fixed upon the first axis my application of the double engine to these rotative machines rendered the counterweight unnecessary, and produced a more regular motion."

The idea of using two cylinders and pistons working alternately, which it will be recollected was the mode in which his first engine was made, did not long satisfy Mr. Watt's mind, and further thought convinced him that a single cylinder would accomplish all he desired, if a means could be devised of producing both the upward and downward motion by the expansion of the steam from the boiler. This he succeeded in doing in his double-acting engine, which may justly be considered as the perfection of his inventive power. The mode in which he accomplished his object was by bringing the upper and lower end of the cylinder alternately in connection with the boiler. During the descent of the piston, the upper part of the cylinder received steam, and by its expansion the downward motion was produced, instead of merely by the pressure of the atmosphere as had formerly been the case, the lower part being open to the condenser. For the up stroke, the arrangement was reversed. The lower part of the cylinder received steam, and the upper was exhausted by a union with the condenser.

In

both the upward and downward motion, therefore, the pressure of the steam was against a vacuum. For this contrivance Watt obtained a patent in March, 1782. Many changes in mechanical detail were required by this alteration of principle, and at no period was his ingenuity more severely taxed, but his genius triumphantly overcame every difficulty. Since his time, many improvements have been introduced, and the power of the engine has been greatly increased; yet in its general character, and in many of its details, the steam-engine is now essentially what Watt made it. Just as the animal body is constantly changed in its contour, while the anatomical structure retains in maturity the outline distinguishing it in youth, so the varieties in the engine are but different developments of form and condition upon the same elementary structure. But before we proceed to any more minute description of the double-action machine, another important invention must be explained. A valuable and economical change was made in the steam-engine by the adoption of the principle of expansion. As this invention has been sometimes claimed, on the point of priority, for Mr. Hornblower, it is necessary to give some evidence that Mr. Watt had previously used as well as thought of it. In a letter written from Glasgow to Dr. Small, of Birmingham, in May, 1769, he says: "I mentioned to you a method of doubling the effect of the steam, and that tolerably easy, by using the power of steam rushing into a vacuum, at present lost.

Open one of the steam valves, and admit