The Life of James Watt

acting upon two cranks fixed upon the same axis at an angle of 120 from each other, and to place a weight on 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 in itself nearly equable. Or to place "the two cranks at right angles to each other, and to load "the outer end of the working-beams of the two cylinders "with weights equal to half the power of the respective cylinders, which might act during the time of the ascents of the respective pistons.

"Of this I caused a model to be made, which performed "to satisfaction. But, being then very much engaged with "other business, I neglected to take a patent immediately, ❝and, having employed a blackguard of the name of Cart

wright, (who was afterwards hanged), about this model, he, "when in company with some of the same sort who worked "at Wasborough's mill, and were complaining of its irregu"larities and frequent disasters, told them he could put them "in a way to make a rotative motion which would not go "out of order nor stun them with its noise, and accordingly explained to them what he had seen me do. Soon after "which, John Steed, who was engineer at Wasborough's mill, "took a patent for a rotative motion with a crank, and applied it to their engine. Suspicions arising of Cartwright's treachery, he was strictly questioned, and confessed "his part in the transaction when too late to be of service to "us. I had afterwards a conversation with Steed upon the subject, who said that he had made the invention before he "was informed of Cartwright's communication, and had taken "his ideas from the common foot-lathe; and that Cartwright's "information had no other effect than to accelerate his taking "out the patent. How this was, I know not; it is certainly possible that Steed might have invented it; but what I have "related is the fact as far as concerns my invention.

"Finding the door thus closed upon us, and circumstances "making us unwilling to go to law with the patentees, which "could only have had the effect of throwing the invention.

open to the public if we had succeeded, we judged it better "to let it remain with them,-who did not seem capable of doing us much harm,-than to let it get into the hands of men more ingenious, being sensible at the same time that "their rotative motion could not do much good, without it "were attached to our engines. We therefore thought it "better to take a patent for several contrivances on the same principle, which the former patentees never thought proper "to call in question. The revolving wheels was the one which we principally adopted; but the advantages of the crank in "point of simplicity have made it now to be generally used, "and the other to be laid aside, perhaps for ever."

With regard to the fifth method specified, that, namely, of the "sun and planet wheels," which perhaps was the most ingenious and elegant, as well as the most practically useful, of all the five, it is to be observed that, although its invention dated from an earlier period in Mr. Watt's life, it seems to have been revived in the interval between taking out the patent and giving in the specification. Living at that time in Cornwall, Mr. Watt went to Penryn on the 25th of July, 1781, and made the affidavit to accompany his petition for the patent; and on the 3rd of January, 1782, he writes to Mr. Boulton:-"I wrote to you on the 31st, since which I "have tried a model of one of my old plans of rotative engines, revived and executed by Mr. M[urdock], and which "merits being included in the specification as a fifth method; "for which purpose I shall send a drawing and description "next post. It has the singular property of going twice "round for each stroke of the engine, and may be made to

go oftener round, if required, without additional machinery. "The wheel A is fixed at the end of an axis which carries a

fly; the wheel B is fixed fast to the con

necting-rod from the working-beam, and "cannot turn on its axis, and is confined by "some means, so as always to keep in con"tact with the wheel A; consequently by "the action of the engine it goes round it and

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"causes it to revolve on its axis; and if the wheels are

"equal in the number of their teeth, A will make two "revolutions while B goes once round it." Two days

later,* "I send you the drawings of the 5th method, and 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 4-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."†

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,

*To Mr. Boulton, 5 January, 1782. See the specification printed in vol. iii. of the Mechanical Inven'tions of James Watt,' 1854, pp. 36 to

54; and the relative drawings engraved on Plates III. IV. V. VI. and VII. of the same volume.

which passed the Great Seal on the 12th of March, 1782, was quite a 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.

Its

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. 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

"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

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The Life of James Watt: With Selections from His Correspondence