Subsequently, however, boilers producing steam of much higher pressure were applied, and the steam was cut off when the piston had performed a much smaller part of the whole stroke. The great theatre of these experiments and improvements has been the mining districts in Cornwall, where, instead of working with steam of a pressure not much exceeding that of the atmosphere, it has been found advantageous to use steam whose pressure is at least four times as great as [Pg166] that of the atmosphere; and instead of limiting its expansion to the last half or fourth of the stroke, it is cut off after the piston has performed one fourth part of the stroke or less, all the remainder of the stroke being accomplished by the expansive power of the steam, and by momentum. [Illustration: BRIDGE OVER THE CLYDE AT HAMILTON, DESIGNED BY WATT.] [Pg167] [Illustration: DOUBLE-ACTING ENGINE, ZINC WORKS, CITY ROAD, LONDON.] CHAP. VII. PROPERTIES OF STEAM. — COMMON STEAM. — SUPERHEATED STEAM. — LAW OF DALTON AND GAY LUSSAC. — LAW OF MARIOTTE. — RELATION BETWEEN TEMPERATURE AND PRESSURE OF COMMON STEAM. — EFFECTS OF THE EXPANSION OF COMMON STEAM. — MECHANICAL EFFECTS OF STEAM. — METHOD OF EQUALISING THE EXPANSIVE FORCE. — HORNBLOWER'S ENGINE. — WOOLF'S ENGINE. — WATT'S ATTEMPTS TO EXTEND THE STEAM ENGINE TO MANUFACTURES. — PAPIN'S PROJECTED APPLICATIONS OF THE STEAM ENGINE. — SAVERY'S APPLICATIONS OF THE ENGINE TO MOVE MACHINERY. — JONATHAN HULL'S APPLICATION TO WATER WHEELS. — STEWART'S APPLICATION OF THE ENGINE TO MILL WORK. — WASHBOROUGH'S APPLICATION OF THE FLY WHEEL AND CRANK. — WATT'S SECOND PATENT. — DOUBLE-ACTION VALVES. (93.) Since the application of the expansive action of steam involves the consideration of its properties when it ceases to be in contact with the water from which it was produced, and likewise the variation of its pressure in different states of [Pg168] density and at different temperatures, it is necessary here to explain some of the most important of these properties of vapour. Steam may exist in two states, distinguished from each other by the following circumstances:— 1st. It may be such that the abstraction from it of any portion of heat, however small, will cause its partial condensation. 2d. It may be such as to admit of the abstraction of heat from it without undergoing any other change than that which air would undergo under like circumstances, viz. a diminution of temperature and pressure. (94.) We shall call, for distinction, the former _Common Steam_, and the latter _Superheated Steam_. [Illustration: _Fig._ 29.]