In Newcomen's engine the valves (_f_ and _d_) at first were opened and shut (at each stroke of the piston) by an attendant, usually a boy. In 1713 a boy named Humphrey Potter, in order to get some time for play, by means of strings and latches, caused the beam in its motion to open and shut the valves without human aid. We must not despise Humphrey because his purpose was to gain time for play. The purpose of almost all inventions is to save human labor so that men may have more time for amusement and rest. Humphrey Potter ought to be remembered not as a lazy boy but as a great inventor. His strings and latches improved the engine wonderfully (Fig. 8). Before his invention the piston made only six or eight strokes a minute; after the valves were made to open and shut by the motion of the beam, it made fifteen or sixteen strokes a minute and the engine did more than twice as much work. [Illustration: FIG. 8.--HUMPHREY POTTER'S LATCHES AND STRINGS.] Newcomen's engine as improved by Potter and others grew rapidly into favor. It was used most commonly to pump water out of the mines but it was put to other uses. In and about London it was used to supply water to large houses and in 1752 a flour mill near Bristol was driven by a steam-engine. In Holland Newcomen's engines were used to assist the wind-mills in draining lakes. [Illustration: FIG. 9.--JAMES WATT STRIVING TO IMPROVE NEWCOMEN'S ENGINE.] For nearly seventy-five years engines were everywhere built after the Newcomen pattern. Improvements in a small way were added now and then but no very important change was made until the latter part of the eighteenth century, when the steam-engine was made by James Watt practically what it is to-day. This great inventor spent years in making improvements upon Newcomen's engine (Fig. 9) and when his labors were finished he had done more for the steam-engine than any man who ever lived. We must try to learn _what_ he did. We can learn what Watt did by studying Figure 10. Here P is a piston working in a cylinder A _closed at both ends_. By the side of the cylinder is a _valve-chest_ C into which steam passes from the pipe T. Connecting C with the cylinder there are _two_ openings, one at the top of the cylinder and the other at the bottom. The valve-chest is provided with valves which are worked by means of the rod F, which moves up and down with the beam B, thanks to Humphrey Potter for the hint. The valves are so arranged that when steam enters the opening at the top of the cylinder it is shut off from the opening at the bottom, and when it enters the opening at the bottom it is shut off from the opening at the top. When the opening at the bottom is closed the steam will rush in at the upper opening and push the piston downward; when the piston has nearly reached the bottom of the cylinder the upper opening will be closed and steam will rush in at the bottom of the steam chest and push the piston upwards. Here was _one_