drawing of it on parchment, now in our possession, made from a sketch by Mr. Watt, was laid before the Committee of the House of Commons when Mr. Watt was soliciting the Act of Parliament for the extension of the patent, in 1774-5. The reason of it not having been sooner secured by a patent, was the difficulty which its inventor "had encountered in teaching others the construction and use of the single engine, and in overcoming prejudices :"and the patent of 1782 was, even then, taken out only in consequence of Mr. Watt "finding himself beset with an host of plagiaries and pirates." In the same time, and with almost the same machinery, the engine on this new principle was enabled to do double the work of the single engine, independent of the additional saving resulting from the use of the expansive principles already explained, by which it could be used as a doubleacting expansive engine; in which case the fourth, fifth, and sixth of the contrivances for equalising the powers of the steam are specified as being peculiarly applicable.

One of the earliest double-acting engines completed for sale was one of those for the Albion Mills, erected in 1786, at the south-east corner of Blackfriars Bridge. "The mention of the Albion Mills," says Mr. Watt, "induces me to say a few words respecting an establishment so unjustly calumniated in its day, and the premature destruction of which, by fire, in 1791, was, not improbably, imputed to design. So far from being, as misrepresented, a monopoly injurious to the public, it was the means of considerably reducing the price of flour while it continued at work.

"It consisted of two engines, each of fifty horses' power, and twenty pairs of millstones, of which twelve or more pairs, with the requisite machinery for dressing the flour and for other purposes, were generally kept at work. In place of wooden wheels, always subject to frequent derangement, wheels of cast-iron, with the teeth truly formed and finished, and properly proportioned to the work, were here employed; and other machinery, which used to be made of wood, was made of cast-iron, in improved forms;

DOUBLE-ACTING AND COMPOUND ENGINES.

237

and I believe the work executed here may be said to [have] form[ed] the commencement of that system of mill-work which has proved so useful to this country. In the construction of that mill-work and machinery, Boulton and Watt derived most valuable assistance from that able mechanician and engineer, Mr. John Rennie, then just entering into business, who assisted in planning them, and under whose direction they were executed. The engines and mill-work were contained in a commodious and elegant building, designed and executed under the direction of the late Mr. Samuel Wyatt, architect."*

We may add, that Mr. Peter Ewart was then Rennie's apprentice, was sent for from Scotland expressly to assist in the erection of those mills, and was employed for four years upon them as a millwright; when his great mechanical talents and industry sufficiently recommended him to his future employers, patrons, and friends, Messrs. Boulton and Watt.†

We have here spoken of the "double-acting" engine, (a name sometimes applied to it on its first introduction,) to distinguish it from the next of the new improvements included in the specification of 1782, viz. :

" A

3. The double, or compound engine; the nature and advantages of which are thus shortly described by Mr. Watt: new compound engine, or method of connecting together the cylinders and condensers of two or more distinct engines, so as to make the steam which has been employed to press on the piston of the first, act expansively upon the piston of the second, &c., and thus derive an additional power to act either alternately or conjointly with that of the first cylinder."‡

4. Toothed rack and sector, instead of chains, for guiding the piston-rod. In consequence of Mr. Watt's beautiful invention of the Parallel Motion, made at no distant date from that of this

*Notes on Robison, p. 137.

+ See the Address of the President,' James Walker, Esq., ' of the Institution of Civil Engineers, to the Annual General Meeting, January 17, 1843.'

‡ Notes on Robison, p. 150.

i

specification, the rack and sector may be looked on as having
been only a temporary expedient to avoid the inconveniences
which had been found to result from the old system of chains
connecting the piston-rod with the beam.

5. Rotative engine, or steam-wheel. For the reason already
mentioned when treating of the patent of 1781, viz.: that "self-
acting rotative engines, not derived from the rectilinear motion
of a piston in a cylinder, instead of being more simple in their
construction, are more complex than those derived from recipro-
cating motions, and more difficult in execution," it seems
unnecessary to enlarge on this article, which is fully described in
the specification.*

* See the specification of this third steam-engine patent, printed in vol. iii. of the 'Mechanical Inventions of James Watt,' 1854, pp. 55 to 87; and the relative drawings, engraved in the same volume, Plates VIII., IX., X., XI., XII., XIII., XIV., XV., XVI., XVII., XVIII., and XIX.

CHAPTER XIX.

STEAM TILT-HAMMER-PATENT OF 1784-PARALLEL MOTION-LOCOMOTIVE STEAMCARRIAGES — COUNTER THROTTLE-VALVE-GOVERNOR-STEAM BAROMETER OR FLOAT-STEAM-GAUGE-INDICATOR-MOST INVENTIVE PERIOD OF MR. WATT'S LIFE-DEATH OF HIS FATHER-PATENT OF 1785—CONSUMPTION OF SMOKE.

Many of these During his long

A FAVOURITE employment of Mr. Watt in the workshops at Soho in the latter months of 1783 and earlier ones of 1784, was to teach his steam-engine, now become nearly as docile as it was powerful, to work a tilt-hammer for forging iron and making steel. So far back as 3 May, 1777, he had informed Mr. Boulton that "[John] Wilkinson is going to work in the forge way, and wants an engine to raise a stamp of 15 cwt. thirty or forty times in a minute. I have set Webb to work to try it with the little engine and a stamp-hammer of 60 lbs. weight. battering rams will be wanted if they answer." absence, and constant occupation in Cornwall, this labour seems to have been intermitted; and we do not find it resumed in earnest till November, 1782. Then "the rotative motion and mill part answered to every expectation, but the hammer-frame and anvil-block were not sufficiently secured, which, however, I have given orders for doing. And as the engine has a great overplus of power, I mean to increase the weight of the hammer to about 11⁄2 cwt., and to cause it to make 250 or 300 strokes per minute, by diminishing the height it rises to 9 or 10 inches.

The present facts are: cylinder, 15 inches diameter, and 4 feet stroke, 25 strokes per minute; hammer makes 6 blows per stroke of the engine; fly under 5 cwt., and 7 feet diameter; hammer 120 lbs., and 18 inches wide; it strikes a good blow, and forges iron very well. The camms were wood, and were cut all to pieces by the anvil-block sinking. I have ordered steel ones to be made, which I expect will stand it."* On the 30th of November he says, "I saw the tilt go admirably from 16 to 25 strokes per minute, and it could have gone much faster, but our men could not work the iron under it. Joseph said that yesterday they made it go 28 strokes per minute, which is much more than the engine should do by my calculations; but in the midst of our glory, the hammer helve broke: it appears to have been rotten. The steel camms answer very well, and the whole will answer better when made to have a less lift and more strokes, as it will then answer for a common tilt for steel; at present the blow is so strong, that we dare not attempt to hack a piece of iron under 11⁄2 inch square, otherwise it knocks it to pieces. By the help of some more weight on the outer end of the beam, it goes so regular that you cannot tell when the engine is going out or when coming in." On the 12th of December,-"I went out to Soho yesterday forenoon, hoping the engine would be ready for trial, but it was not. In the evening they wrought it 2 hours, 240 blows per minute, rise of hammer 8 inches." On the 13th,"We have tried our little tilting-forge hammer at Soho, with success. The following are some of the particulars:-cylinder 15 inches diameter, 4 feet stroke, strokes per minute 20. The hammer-head, 120 lbs. weight, rises 8 inches, strikes 240 blows per minute. The machine goes quite regular, and can be managed as easily as a water-mill. It requires a very small quantity of steam, not above half the contents of the cylinder per stroke. The power employed is not more than of what would be required to raise the quantity of water which would enable a

* To Mr. Boulton, 28 November, 1782.