In illustration of the application of the Newcomen engine to the drainage of mines, Farey describes a small machine, of which the pump is 8 inches in diameter, and the lift 162 feet. The column of water to be raised weighed 3,535 pounds. The steam-piston was made 2 feet in diameter, giving an area of 452 square inches. The net working-pressure was assumed at 10-3/4 pounds per square inch; the temperature of the water of condensation and of uncondensed vapor after the entrance of the injection-water being usually about 150° Fahr. This gave an excess of pressure on the steam-side of 1,324 pounds, the total pressure on the piston being 4,859 pounds. One-half of this excess is counterweighted by the pump-rods, and by weight on that end of the beam; and the weight, 662 pounds, acting on each side alternately as a surplus, produced the requisite rapidity of movement of the machine. This engine was said to make 15 strokes per minute, giving a speed of piston of 75 feet per minute, and the power exerted usefully was equivalent to 265,125 pounds raised one foot high per minute. As the horse-power is equivalent to 33,000 "foot-pounds" per minute, the engine was of 265125/33000 = 8.034--almost exactly 8 horse-power. It is instructive to contrast this estimate with that made for a Savery engine doing the same work. The latter would have raised the water about 26 feet in its "suction-pipe," and would then have forced it, by the direct pressure of steam, the remaining distance of 136 feet; and the steam-pressure required would have been nearly 60 pounds per square inch. With this high temperature and pressure, the waste of steam by condensation in the forcing-vessels would have been so great that it would have compelled the adoption of two engines of considerable size, each lifting the water one-half the height, and using steam of about 25 pounds pressure. Potter's rude valve-gear was soon improved by Henry Beighton, in an engine which that talented engineer erected at Newcastle-upon-Tyne in 1718, and in which he substituted substantial materials for the cords, as in Fig. 20. In this sketch, _r_ is a plug-tree, plug-rod, or plug-frame, as it is variously called, suspended from the great beam, with which it rises and falls, bringing the pins _p_ and _k_, at the proper moment, in contact with the handles _k k_ and _n n_ of the valves, moving them in the proper direction and to the proper extent. A lever safety-valve is here used, at the suggestion, it is said, of Desaguliers. The piston was packed with leather or with rope, and lubricated with tallow. [Illustration: FIG. 20.--Beighton's Valve-Gear, A. D. 1718.]