_The first stationary steam engines were used for pumping water and were of the single acting type_, in which the steam was admitted at one end of the cylinder, the opposite end being open to the atmosphere. The steam acting on the piston forced it to the limit of its stroke when the supply was cut off. The steam then condensed in the cylinder, forming a partial vacuum, and the force of the atmosphere upon the opposite side of the piston forced it back, causing it to complete its stroke before another supply of steam was admitted. This was a slow process, wasteful of steam and attended with many other inconveniences. An improvement on this device was made in an engine built by Watt in 1774. This was a single acting engine but _the condenser was separated from the cylinder_. The valves were so arranged as to admit live steam into the upper end of the cylinder on the top of the piston and at the same time open the lower end of the cylinder to the condenser. The steam followed the piston in its downward stroke in which action it was aided by the partial vacuum formed in the condenser. At the completion of the downward stroke the valves were changed so as to close the ports to the steam supply and the condenser, and at the same time open a communication between the two ends of the cylinder equalizing the pressure above and below the piston. The weight of the pump rod on the beam or lever connection overbalanced the weight of the piston and caused it to complete the return stroke. In 1782 _the double acting steam engine_ was patented by Watt. This was a device in which the live steam acted on each side of the piston alternately, the opposite side of the cylinder being in communication with the condenser. The same patent covered the method of applying _the principle of expansion of steam in the cylinder_; a non-condensing engine was also described. NOTE.—This invention was of great historical importance as it covered all the essential detail of modern practice in steam engine building and constituted the fundamental principle of all steam engines. Improvements have been made in form and construction, necessitated by new adaptations which have been constantly developed. The requirements for higher speed, increased pressure which implies greater power, and the constant desire for greater economy in fuel have produced a variety of changes in detail but have not altered the fundamental idea.