(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. [Illustration: _Fig._ 33.] (116.) It was necessary, in the first place, to provide means for admitting and withdrawing the steam at either end of the cylinder. For this purpose let B and B′ (_fig._ 33.) be two steam-boxes, B the upper, and B′ the lower, communicating respectively with the top and bottom of the cylinder by proper passages D D′. Let two valves be placed in B, one, S, above the passage D, and the other, C, below it; and in like manner two other valves in the lower valve-box, B′, one, S′, above the passage D′, and the other, C′, below it. Above the valve S in the upper steam-box is an opening at which the steam-pipe from the boiler enters, and below the valve C is another opening, at which enters the exhausting-pipe leading to the condenser. In like manner, above the valve S′ in the lower steam-box enters a steam-pipe leading from the boiler, and below the valve C′ enters an exhausting-pipe leading to [Pg190] the condenser. It is evident, therefore, that steam can always be admitted above the piston by opening the valve S, and below it by opening the valve S′; and, in like manner, steam can be withdrawn from the cylinder above the piston, and allowed to pass to the condenser, by opening the valve C, and from below it by opening the valve C′. [Illustration: _Fig._ 34.]