Once more the coal-mines of England set invention working on a definite, continuous {8} object. As the shafts were sunk to lower and lower levels, it became impossible to pump the water out of the mines by horse power, and the aid of steam was sought. Just at the close of the seventeenth century Savery devised the first commercial steam-engine, or rather steam fountain, which applied cold water to the outside of the cylinder to condense the steam inside and produce a vacuum; while Papin, one of the Huguenot refugees to whom industrial England owed so much, planned the first cylinder and piston engine. Then in 1705 Newcomen and Cawley, working with Savery, took up Papin's idea, separated boiler from cylinder, and thus produced a vacuum into which atmospheric pressure forced the piston and worked the pump. Next Humphrey Potter, a youngster hired to open and shut the valves of a Newcomen engine, made it self-acting by tying cords to the engine-beam, had his hour for play or idling, and proved that if necessity is the mother of invention, laziness is sometimes its father. Half a century passed without material advance; even as perfected in detail by Smeaton, the Newcomen engine required thirty-five pounds of coal to produce one horse-power per hour, as against one pound {9} to-day. Then James Watt, instrument-maker in Glasgow, seeing that much of the waste of steam was due to the alternate chilling and heating of the cylinder, added a separate condenser in which to do the chilling, and kept the temperature of the cylinder uniform by applying a steam-jacket. Later, by applying steam and a vacuum to each side of the piston alternately, and by other improvements, Watt, with his partner Boulton, brought the reciprocating steam-engine to a high stage of efficiency.