such a reciprocating engine. It is stated that he intended to obtain this end by the use of the crank, and was preparing to patent its application, but that, while the matter was under consideration, one Pickard, a workman in Watt's employ, revealed the secret to a man of the name of Wasbrough of Bristol, who was endeavouring to obtain rotary motion by various complex contrivances, which he made the subject of a patent of 1779 (No. 1213); that these being unsuccessful he joined himself to Pickard, who in 1780 took a patent (No. 1263) for the use of the crank in the steam engine. Watt was seriously inconsistent in his observations on this crank question, and his biographers—or some of them—have allowed themselves to follow him in his inconsistency; for while on the one hand he put himself forward as a meritorious inventor, and the intending patentee of the use of the crank, and complained bitterly of his invention having been stolen, on the other hand he writes in respect of Pickard's patent that ‘the true inventor of the crank rotative motion was the man who first contrived the common foot-lathe. Applying it to the engine was like taking a knife to cut cheese which had been made to cut bread.’ Thus Watt, while intending to patent the use of the crank, must in his own mind have known that such use was a mere ‘obvious application,’ and was therefore not capable of being made the subject of a valid patent. On finding that he was shut out by Pickard's patent from the use of the crank, Watt devoted himself to devising other means for converting a reciprocating into a rotary motion. He devised five different modes, the subject of his patent of 1781 (No. 1306), none of which, in his opinion, were amenable to the charge of involving the use of cranks; but there is no doubt that two of them were absolutely cranks. There does not appear to be any record of four of these devices having been used; but the fifth device, the ‘Sun-and-Planet’ wheel, was largely employed by Watt for converting the reciprocating motion into rotary motion.
Watt's engines, as actually made (the writer of this article remembers one of them perfectly), had the sun and the planet wheels of equal size, the planet being confined to its orbit by a link loose upon the sun-wheel shaft—the natural and proper means of doing it. But whether Watt feared that such a construction might be held to amount to a crank, or what other cause may have influenced him, cannot now be determined; but the fact is that in his specification he made a most extraordinary provision for confining the planet wheel to its orbit, by inserting a pin in continuation of the axis of the planet wheel, into a circular groove. The sun and planet wheels of the proportions used by Watt—that of equality of diameter—had a certain value besides that of steering clear of Pickard's patent, in that they gave two revolutions of the sun shaft, which was also the fly-wheel shaft, for each double reciprocation of the engine, so that the speed of a slow-going engine was at once augmented in the very engine itself, and, moreover, the fly-wheel had its value quadrupled. Some attempt was made to agree with Pickard for the use of the crank; but Watt's pride revolted from buying back that which he said was his own invention, and he explains that he had no wish to destroy Pickard's patent, thus throwing the use of the crank open to the public, and depreciating therefore the value of Watt's own substitute, the sun and planet.
Up to the present time it will have been noticed that, in all cases of Watt's engines, there was only one working stroke made during the passage to and fro of the piston in the cylinder, the return stroke being due to the action of a counter-weight. But, having now in these engines a close-topped cylinder with a piston-rod working through a stuffing-box, and having valves by which connection was made alternately between the under side of the piston and the steam boiler, and between the underside of the piston and condenser, it followed almost as a consequence that by additions to these valves the functions of the steam and vacuum might be repeated on the upper side of the piston, and that thus the engine would have a working stroke in both directions, rendering it independent of counterweights, and eminently adapting it for operation upon a crank, or upon its equivalent, to produce rotary motion. This was one of the subjects of Watt's patent of 1782 (No. 1321), and not only was this construction of great utility for giving comparative uniformity of rotary motion, but also it was one which obviously doubled the work that could be obtained out of a given dimension of cylinder. This patent also embraced another most important principle in the use of steam, one upon which practically the whole improvement, made since Watt's days to the present, in the economy of fuel depends—namely, the employment of steam expansively.
A few words of explanation to the non-technical reader may perhaps be necessary. Assume a cylinder of such a diameter as to have 1 square foot = 144 square inches of area, and assume the stroke of the piston in
