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thought to have sent you the description complete, but was "late last night before I finished so far, and to-day have a headache, therefore only send you a rough draft of part. "The drawing is made to -inch scale for 6-feet stroke, but "must be reduced to the 1-inch. This 5th method "makes an exceeding good motion, and may in many ways "be very useful, from its peculiar properties." And, two days later still," I wrote to you on Saturday, with drawings "of the 5th method of rotatives, and enclosed I send the complete specification of that method."

The drawings for this specification were made, in duplicate, by Mr. Watt's own hand; one of them "on stamped parch"ment for want of plain," and the other, " in an elegant "manner upon vellum, being the neatest drawing," he says, “I ever made: and [I] have improved the construction of "several of the machines, and represented their stands and "several other parts necessary. The double-toothed wheels," [the sun and planet motion], "admit of several different applications, one of which admits the rotative wheel to be "in the middle of an axis, and that was the original one." He says in another letter, before the specification was given in, "I have thought on some other methods by which rota"tive motions may be made, but they are inferior to those specified, and I feared the specification would have grown "four yards long."*

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While preparing the specification of the patent of 1781, for the five methods of producing a continuous rotative motion round an axis, from the vibrating or reciprocating motion of steam-engines, so as to give movement to mill-work, Mr. Watt was already arranging the contents of another patent of quite as great importance. The title of the new patent, which passed the Great Seal on the 12th of March, 1782, was a very general one; being "for certain new improvements "upon steam or fire-engines, for raising water, and other "mechanical purposes, and certain new pieces of mechanism

applicable to the same." But in the specification, which was enrolled on the 4th of July, 1782, are comprehended the following "new improvements :"

1. The use of steam on the expansive principle; together with various methods or contrivances, (six in number, some of them comprising various modifications), for equalising the expansive power.

2. The double-acting engine; in which steam is admitted to press the piston upwards as well as downwards; the piston being also aided in its ascent as well as in its descent by a vacuum produced by condensation on the other side.

3. The double-engine; consisting of two engines, primary and secondary, of which the steam-vessels and condensers communicate by pipes and valves, so that they can be worked either independently or in concert; and make their strokes either alternately or both together, as may be required. 4. The employment of a toothed rack and sector, instead of chains, for guiding the piston-rod.

5. A rotative engine, or steam-wheel.

1. It appears from one of Mr. Watt's letters, (to Mr. Boulton, 19th November, 1781), that he had first thought of the expansive engine in 1767; and had also explained it to Mr. Smeaton, at Soho, some years previous to 1781. Its principle is a curious one, and appears at first paradoxical ; for, in fact, by cutting off the supply of steam at a certain point before the steam-vessel or cylinder is full, the same effect is produced as if the steam-vessel had been entirely filled with steam: the expansion, or elastic force which the steam exerts, doing the same work that in the other case would have been done by a greater quantity of steam, and, therefore, a further expenditure of fuel. The proportion of steam specified by Mr. Watt as being most convenient for admission, in common use, was one-fourth of the contents of the steam-vessel; producing an effect equal to more than one-half the effect that would have been produced had steam been admitted to enter freely into the cylinder during the whole length of the stroke of the piston. But he adds that

any other proportion would produce similar [proportional] effects, and that in practice he did vary the proportions accordingly.

But the powers thus exerted by the steam being unequal at different periods of the stroke, while the resistance to be overcome, or work to be done, by the engine, was supposed to be equal throughout the whole length of each stroke, it was necessary to equalise the power of the engine; for doing which six different methods are specified. And, as of two of them, (viz. the first and third), there are two varieties, and of another, (viz. the fifth), there are four varieties, we have here no fewer than eleven varieties of such equalising machinery described; for particulars of all of which reference must be made to the specification, and relative plates.

To exhibit some of those "equalisers" in their embryo stage, we may give from Mr. Watt's correspondence the following extracts, written between the date of his making the affidavit to accompany the petition, and the enrolment of the specification. The extracts are all from letters written to Mr. Boulton, from Cosgarne, (in Cornwall), and are dated respectively the 9th, 11th, and 14th of February, 1782:—“ I "have nothing new to advise you of, except a new method of an equalising beam, by causing the gudgeon to change its "place, thus

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"The working-beam is made hollow on the under-side, and "rests upon a roller which has an axis through it, and this "axis has a wheel fixed upon each end of it, unconnected "with the roller, but connected together by means of the "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

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pump-end, and vice versa. 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 "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 soli

citing 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 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