(115.)

Although by these contrivances Watt succeeded in obtaining a continuous circular motion from the reciprocating motion of the steam engine, the machine was still one of intermitting, instead of continuous action. The expedient of a counterweight, elevated during the descending stroke, and giving back the power expended on it in the interval of the returning stroke, did not satisfy the [Pg189] fastidious mechanical taste of Watt. He soon perceived that all which he proposed to accomplish by the application of two cylinders and pistons working alternately, could be attained with greater simplicity and effect by a single cylinder, if he could devise means by which the piston might be impelled by steam upwards as well as downwards. To accomplish this, it was only necessary to throw the lower end of the cylinder into alternate communication with the boiler, while the upper end would be put into communication with the condenser. If, for example, during the descent of the piston, the upper end of the cylinder communicated with the boiler, and the lower end with the condenser; and, on the other hand, during the ascent of the piston, the lower end communicated with the boiler, and the upper end with the condenser; then the piston would be driven continually, whether upwards or downwards, by the power of steam acting against a vacuum. Watt obtained his third patent for this contrivance, on the 12th of March, 1782.

This change in the principle of the machine involved several other changes in the details of its mechanism.

Fig. 33.