An engine worked by steam. The chief parts are the piston,cylinder, and valve gear. The piston works in a cylinder, to which steam is admitted by the action of the valve gear, causing the piston to move backward and forward and communicate motion to the machinery. The atmospheric engine constructed by Newcomen near the beginning of the eighteenth century was the first in which a beam was made to oscillate by the elastic force of steam. It was used for pumping water out of mines. The condensation of the steam in the space below the piston produced a vacuum, and the piston was forced down by the pressure of the atmosphere. One of Watt's first improvements was the introduction of a separate chamber for the condensation of the steam. Another improvement introduced by Watt was double action. By this arrangement the steam was introduced alternately above and below the piston. This consisted in cutting off the steam from the boiler before the piston had reached the top of the cylinder. It need scarcely be said that many later improvements have been introduced in the steam-engine, greatly increasing its powers.