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"axis. At beginning of the stroke the roller and beam

"stand thus

; but as the engine-end

"descends, the curve forces the roller to travel towards the "pump-end, and vice versâ. N.B. The wheels, and not the "roller, rest upon the fulcrum or support. I have also made "sketches of some equalising beams, which perform by means "of a roller acting upon a curve in the nature of the working "gear; the engine pulls by the arch C, and the pump is hung "to the arch P; the roller travels about the length of the stroke, and the curve permits a perfect equalisation ;”—“ "I "have filled one whole sheet, royal, with equalisers, and shall probably fill another before I am done;"-"I remark what you said in your last about equalisers, and had thought of

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"the same; below are two new ones. But the flyer is the "best of all, and will prove the true equaliser, and will have "much less friction than any other. It may, however, be "combined with some of the most simple ones; and the

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weight raised by the back-stroke may be placed so far "above the centre as in some measure to equalise itself.”

2. The double-acting engine, as appears from the letter to Mr. Boulton already referred to, where Mr. Watt calls it the "double cylinder," was also imagined by him about 1767. A large 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 double-acting 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 calum"niated 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.

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"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 derange"ment, wheels of cast-iron, with the teeth truly formed and finished, and properly proportioned to the work, were here

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"employed; and other machinery, which used to be made of "wood, was made of cast-iron, in improved forms; 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 con"struction 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."

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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. :—

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3. The double, or compound engine; the nature and advantages of which are thus shortly described by Mr. Watt:-" A 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 inven

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tion of the Parallel Motion, made at no distant date from that of this 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.

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5. Rotative engine, or steam-wheel. For the reason already mentioned when treating of the patent of 1781, viz. that “selfacting 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 reciprocating motions, and more difficult in "execution," it seems unnecessary to enlarge on this article, which is fully described in the specification.*

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STEAM TILT-HAMMER

CHAPTER XIX.

PATENT OF 1784 -PARALLEL MOTION MOTIVE STEAM-CARRIAGES- COUNTER -THROTTLE-VALVE

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LOCOGOVERNOR

STEAM BAROMETER OR FLOAT STEAM-GAUGE -INDICATOR MOST INVENTIVE PERIOD OF MR. WATT'S LIFE — DEATH OF HIS FATHERPATENT OF 1785-CONSUMPTION OF SMOKE.

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. Many of these battering rams will be "wanted if they answer." During his long 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, how"ever, 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 14 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,

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