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.