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He now applied his boiler to the working model above mentioned; when it appeared, that the quantity of steam expended at every stroke; exceeded many times what was sufficient to fill the cylinder, and deducing from thence the quantity of water required to form as much steam as would supply each stroke of the engine, he proceeded to examine how much cold water was used for injection, and what heat it gained; which, to his very great surprise, he found to be many times the number of degrees which could have been communicated to it by a quantity of boiling water equal to that of which the steanı was composed. Suspecting, however, that there might be some fallacy in these deductions, he made a direct experiment to ascertain the degree of heat communicated by steam to water ; when it clearly appeared, that one part of water, in the form of steam, at 212°, had communicated about 140 degrees of heat to six parts of water. The fact, thus confirmed, was so contrary to all his previous conceptions, that he at first saw no means of explaining it. Dr Black indeed had, some time before, made his discovery of latent heat; but Mr Watt's mind being otherwise engaged, he had not attended sufficiently to it, to make himself much acquainted with the doctrine : but upon communicating his observations to the Doctor, he received from him a full explanation of his theory; and this induced him to make further experiments, by which he ascertained the latent heat of steam to be above 900 degrees.

The causes of the defects of Newcomen's engines were now evident. It appeared that the steam could not be condensed so as to form an approximation to a vacuum, unless the cylinder, and the water it contained, were cooled down to less than 100°; and that, at greater degrees of heat, the water in the cylinder muit produce steam, which would in part relift the pressure of the atmosphere. On the other hand, when greater degrees of exhaustion were attempted, the quantities of injection water required to be increased in a very great ratio ; and this was followed by a proportionate destruction of steam on refilling the cylinder.

Mr Watt now perceived, that to make an engine in which the destruction of steam should be the least poflible, and the vacuum the most perfect, it was necessary that the cylinder ihould condense no fteam on filling it, and that, when condensed, the water, forming the steam, fhould be cooled down to 100 degrees, or lower. , In reflecting on this defideratum, he was not long in finding that the cylinder must be preserved always as hot as the steam that enters iv; and that, by opening a communication between this hot cylinder when filled with steam, and another vefsel exhausted of air, the steam, being an elastic fluid, would rush into it, until an equilibrium was establidhed between the two

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vessels; fiće, no part is condensed until the whole effect has been obtained in the cylinder ; and when it has acted there, it is so condensed in the separate vefsel that no resistance remains : Accordingly, the barometer proves a vacuum, nearly as perfect as by the exhaustion of the air-pump. The whole of the steam and heat is usefully employed; and the contrivance appears scarcely to admit of improvement.

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Such is the history of this valuable invention, which we have extracted from Dr Black and Professor Robison's testimonials, who were privy to Mr Watt's discovery; as well as from some early letters of his own to confidential friends, to, which we have had access.

We have entered thus minutely into the subject, from a desire to do that justice which is due to Mr Watt, by showing that this great improvement was not the effect of accident, or of casual observation, but the result of deep reflection, of great ingenuity, and much philosophical investigation.

It did not, at the early period we have been speaking of, escape him, that great benefit might be derived from the direct application of the power of steam to driving mills, instead of using it to raise water to act on a wheel, as had heretofore been done ; and with this view, he invented and executed the model of a steam wheel, for giving a circular motion to an axis.

His occupations in the business of a civil engineer which he had now taken up, perhaps also the indifferent state of his health, his want of funds, and his apprehension of the prejudices and opposition he might have to encounter, preveated his applying for a patent for the invention we have described, until the year 1769. He had, we believe, previous to, or about that time, erected an engine for his friend Dr Roebuck of Kinneil, near Borrowstounness, which, upon a large scale confirmed his expectations ; the proportionate saving of fuel being from two thirds to three fourths of that of engines on Newcomen's construction). Dr Roebuck, whose spirit for enterprise and improvement in the arts is well known, foresaw all the advantages likely to result from this invention, and became associated in the prospects which it opened. But some of his own projects having failed, he soon after disposed of his interest to Mr Boulton the celebrated founder of Soho manufactory, with whose aid Mr Watt, in 1774, solicited and obtained an act of Parliament for the extension of the term of his patent for twenty-five years; and the business of making steam-engines was soon after commenced by the firm of Boulton and Watt.

In executing his invention on a large scale, Mr Watt felt the necessity of improving the construction of several of the parts of Wewcomen's engine. With this view, he induced Mr Wilkinson

to

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toothed sector on the end of the working beam, working into a trundle, which, by means of two pinions with ratchet wheels, produced a rotative motion, in the same direction, by both the ascending and descending stroke of the arch ;, and, by shifting the ratchets, the motion could be reversed at pleasure. This engine had no fly-wheel, and went sluggishly and irregularly. Who the inventor was, we do not know.

A patent was taken out in 1769, by a gentleman of the name of Stewart, for an engine which produced a rotative motion, by a chain going round a pully, and also round two barrels furnished with ratchet wheels, with a weight suspended to the free end of the chain, which served to continue the motion during the return of the engine. In 1778, Mr Matthew Washbrough also obtained a patent for communicating a rotative motion from the steam engine, by a method which was virtually the same as that at Hartley ; only, he had added a fly-wheel, which we believe was then for the first time employed in the steam engine, though it is evident, from the letter we have quoted from Mr Watt to Dr Small, that the former had conceived the idea long previous to this period. Two or three of these engines were erected; but, owing to the defective mode of communicating the motion, were subject to such irregularities and accidents, as rendered them of little use.

The idea of communicating motion from the beam of the steam engine to a crank, in the same manner as is done in the common foot-lathe, had, as we are informed, early occurred to Mr Watt; but we believe he did not seriously set about reducing hiş ideas to practice until the year 1778 or 1779. In the first model he then made, in order to equalize the power, two cylinders, acting upon two cranks were fixed upon the same axis, at an angle of 120° from each other; and a weight was placed upon the circumference of the fly-wheel, at an angle of 120 from each of the cranks; which weight was to be so adjusted, as to act when neither of the cranks could do so, and consequently to render the power nearly equal. This model performed to satisfaction; but Mr Watt having neglected to take out a patent immediately, the essential part of the contrivance was communicated, as we understand, by a workman employed to make the inodel, to the persons engaged about one of Washbrough's engines ; and a patent was taken out for the application of the crank by the engineer there employed. This did not dishearten Mr Watt; and, without troubling himself with setting aside a patent which, so ong as it continued attached to the common atinospheric engine, uld do him little harm, he set about other modes of effecting same thing; and, in 1781, took out a patent for several new

methods

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