To keep the pressure in the boiler within the limit of safety, and adjusted to the work being performed, is an important part of the engineer’s duty, and this he could not do without the steam gauge. One of the best known is the Bourdon gauge, shown in Fig. 84, constructed on the principle of the barometer invented by Bourdon of Paris in 1849 and patented in France June, 1849, and in the United States August 3, 1852, No. 9,163. A screw threaded thimble B, with stop cock A, is screwed in the shell of the boiler, and a coiled pipe C communicates at one end with the thimble and is closed at the other end E and connected by a link F, with an arm on an axle, carrying an index hand that moves over a graduated scale. The coiled pipe C is in the nature of a flattened tube, as shown in the enlarged cross section, and is enclosed in a case. When the steam pressure varies in this flat tube its coil expands or contracts, and in moving the index hand over the scale indicates the degree of pressure. [Illustration: FIG. 84.--BOURDON’S PRESSURE GAUGE.] In line with the development of the steam engine must be considered the efforts to economize fuel. These may be divided into the following classes: Increased steam generating surface in boiler construction; surface condensers for exhaust steam; devices for promoting the combustion of fuel and burning the smoke, and feed water heaters. Even before the Nineteenth Century Smeaton devised the cylindrical boiler traversed by a flue, but the multitubular steam boiler of to-day represents a very important Nineteenth Century adjunct to the steam engine. Our locomotives, fire engines, and torpedo boat engines would be of no value without it. Sectional steam boilers made in detachable portions fastened together by packed or screw joints also represent an important development. These permit of the removal and replacement of any one section that may become defective, and are also capable of being built up section by section to any size needed. For promoting the combustion of fuel the draft is energized by blasts of air or steam, or both, either through hollow grate bars, jet pipes in the fire box, or by discharging the exhaust steam in the smoke pipe. Surface condensers pass the exhaust steam over the great surface area of a multitubular construction having cold water flowing through it. Feed water heaters utilize the waste heat escaping in the smoke flue to heat the water that is being fed to the boiler, so that it is warm when it is injected into the boiler, and the furnace is relieved of that much work. [Illustration: FIG. 85.--BRANCA’S STEAM TURBINE, 1629.] [Illustration: FIG. 86.--SECTION OF PARSONS TURBINE OF 1891.]