Watt was faced with a dilemma, and he overcame it by a series of studies in the properties of steam which constitute, perhaps, the highest achievement of this workman-philosopher.

Out of all his experiments two conclusions were drawn by him; first, that the lower the temperature of condensation of steam the more perfect the vacuum thereby formed; second, that the temperature of the cylinder should be as nearly as possible equal to that of the steam admitted to it. In Newcomen’s engine these two conditions were obviously incompatible, and the problem was,—how could they be reconciled? Early in 1765, while walking one Sunday afternoon in Glasgow Green the idea flashed upon him of condensing the steam in a separate vessel. The steam was generated in a separate vessel, why not produce the vacuum separately? With a view to trying this effect he placed a hollow air-tight chest beneath the steam cylinder, connected with it by a pipe having a stop-cock in it. This new or lower vessel was immersed in a cistern of cold water. Upon trial being made, it was found that by this simple contrivance as perfect a vacuum as desired was produced; the speed of the engine was greatly increased, the expenditure of fuel radically reduced, the walls of the steam cylinder were maintained at a high and constant temperature, and the whole arrangement promised great success. The new vessel Watt called a Condenser.

Fresh difficulties now arose. As the engine worked, the condenser gradually filled with the condensed steam and had to be emptied periodically. The water in which it was immersed became so hot, by absorbing the heat of the steam, that it frequently required changing. Watt promptly called in aid two new auxiliaries, two organs whose motion was derived from the main beam of the engine: the Air Pump and the Circulating Pump. By these expedients the action of the condenser was rendered satisfactory, and an engine resulted which had a fuel-consumption less than half that of Newcomen’s engine.

Much, he saw, yet remained to be done to obtain economical expenditure of steam. In particular the open-topped cylinder, whose walls were chilled at every descent of the piston by contact with atmospheric air, was an obvious source of inefficiency. He therefore determined not to expose the walls to the atmosphere at all, but to enclose all the space above the piston; and, thinking thus, he conceived the idea of replacing the air above the piston by steam, an equally powerful agent. The cylinder he proposed to maintain at a constant high temperature by means of a layer of hot steam with which he encased it, which he called a steam jacket. And so the atmospheric engine as left by Newcomen evolved into the single-acting steam engine of Watt;—an engine in which steam was still used below the piston, only to displace air and provide a vacuized space for the downward motion of the piston; but in which steam now acted positively above the piston, in lieu of atmospheric air, to drive it down. It was still a sufficiently primitive form of prime mover. The piston was still lifted by the counterweight at the other end of the timber cross-beam; the engine had not yet developed the organs necessary for producing a satisfactory rotary motion. This step was shortly to follow.

In 1769 Watt obtained his patent for the “double impulse,” as it was called; and by this step, by the transition from a single- to a double-acting engine, the possibilities of such machines for every variety of application first came into general view. This stage of the development showed to the full the ingenuity of Watt’s mechanical mind. By the invention of the slide-valve he distributed steam to the top and to the bottom of the cylinder, and in appropriate phase with these actions opened the two ends to the condenser; so that the piston was actuated positively and by an equal force on both up and down strokes. The chain by which the piston had been suspended was no longer adequate; it was replaced by a rod. A straight-line motion was required for the top end of the rod; so he formed a rack, to gear with the circular end or horse-head of the beam. But this noisy mechanism was soon superseded by another contrivance, the beautifully simple “parallel motion,” in which two circular motions are combined to produce one which is rectilinear. This was patented in ’84.

Four years before this, that ancient mechanism the crank and connecting rod had been applied, together with a flywheel, to transform the reciprocating motion of a steam engine into a rotary motion; and the non-possession of this invention of James Pickard’s proved for a time a stumbling-block to Watt in his further development of his engine. Watt would have nothing to do with it. By now he had joined his fortunes with those of Mr. Boulton, of Soho, Birmingham, a man of great business ability, in conjunction with whom he was engaged in constructing engines in large numbers to suit the varying conditions of the mines in Cornwall and the North. Considerable ingenuity was expended by him in trying to circumvent the troublesome crank of Pickard, and many devices were produced, the most noteworthy being the “sun-and-planet wheels,” which enabled him with some sacrifice of simplicity to obtain the rotary motion desired.

Watt seemed to be borne along by the momentum of his own discoveries; every inquiry yielded him valuable reward. For some time he had studied the possibility of reducing the violence with which the piston, now positively steam-driven on both sides, came to the end of its stroke. This problem led him to the discovery of the advantage of using steam expansively: of cutting off the inflow of steam before the piston had travelled more than a fraction of its stroke, and letting its inherent elastic force impel it through the remainder of its journey, the steam meanwhile expanding and thus exerting a continuously decreasing force. Later came the throttle valve, and the centrifugal governor for controlling the speed of rotating engines; there was no end to his ingenuity. And so complete was his inquiry into the possible sources of improvement of the steam engine, that he even considered means of regulating the force which the piston exerted on the crank throughout its working stroke, a force which was compounded of the steam pressure itself and of the mass-acceleration of the piston and other moving parts.

Another cardinal invention followed: the Indicator. The principle of the indicator is now applied to every form and kind of piston engine. It is a reproduction on a small scale of the essential part of the engine itself; a small piston, held by a spring and moving in a cylinder connected by a pipe with the cylinder of the engine itself, shows by the degree of compression imparted to the spring the gaseous pressure actually present at any moment in the engine cylinder. By recording the position of the indicator piston on a paper wrapped round a rotating drum whose motion represents the motion of the engine’s piston, a diagram is obtained which by its area measures the work done by the steam during the stroke of the engine.

This instrument was designed by Watt to give his firm some standard of work which would serve as a basis for the power of each engine, on which to charge their customers; their engines being sold by the horse-power. But its usefulness far exceeded the immediate purpose for which it was produced. Its diagram, to the eye of an expert, gave valuable information in respect of the setting of the valves, the tightness of the piston, the dryness of the steam, the degree of vacuum in the condenser, and, generally, of the state of efficiency of the engine. “It would be difficult to exaggerate the part which this little instrument has played in the evolution of the steam engine. The eminently philosophic notion of an indicator diagram is fundamental in the theory of thermodynamics; the instrument itself is to the steam engineer what the stethoscope is to the physician, and more, for with it he not only diagnoses the ailments of a faulty machine, whether in one or another of its organs, but gauges its power in health.”[79]

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