Discoveries and Inventions of the Nineteenth Century

result. For, as the steam is continually condensed by the cold water, it rushes from D with the same velocity as into a vacuum, and the water is urged on by a momentum due to this velocity. We must observe, moreover, that the net result of the operation is a lessening of the pressure in the boiler; for the entrance of the feed-water produces a fall of temperature in the boiler, and the bulk of steam expended is fourteen times the bulk of the water injected: thus, although the apparatus before actual trial would not appear likely to produce the required result, the effect is no more paradoxical than in the case of the feed-pump. The injector has been greatly improved by Mr. Gresham, who has contrived to make some of the adjustments self-acting, and his form of the apparatus is now largely used in this country. The injector is applicable to stationary, locomotive, or marine engines. Steam boilers are now always provided with one of Bourdon’s gauges, for indicating the pressure of the steam. The construction of the instrument will easily be understood by an examination of Fig. 7. The gauge is screwed into some part of the boiler, where it can always be seen by the person in charge. The stop-cock A communicates with the curved metallic tube C, which is the essential part of the contrivance. This tube is of the flattened form shown at D, having its greatest breadth perpendicular to the plane in which the tube is curved, and it is closed at the end E, where it is attached to the rod F, so that any movement of E causes the axle carrying the index-finger, F, to turn, and the index then moves along the graduated arc. The connection is sometimes made by wheelwork, instead of by the simple plan shown in the figure. The front plate is represented as partly broken away, in order to show the internal arrangement, which, of course, is not visible in the real instrument, where only the index-finger and graduated scale are seen, protected by a glass plate. Fig. 7.—Bourdon’s Pressure Gauge. When a curved tube of the shape here described is subjected to a greater pressure on the inside than on the outside, it tends to become straighter, and the end E moves outward; but when the pressure is removed, the tube resumes its former shape. The graduations on the scale are made by marking the position of the index when known pressures are applied. The amounts of pressure, when the gauges are being graduated, are known by the compression produced in air contained in another apparatus. Gauges constructed on Bourdon’s principle are applied to other purposes, and can be made strong enough to measure very great pressures, such as several 13thousand pounds on the square inch; they may also be made so delicate as to measure variations of pressure below that of the atmosphere. The simplicity and small size of these gauges, and the readiness with which they can be attached, render them most convenient instruments wherever the pressure of a gas or liquid is required to be known. Fig. 8.—Steam Generator. A point to which great attention has been directed of late years is the construction of a boiler which shall secure the greatest possible economy in fuel. Of the total heat which the fuel placed in the furnace is capable of supplying by its combustion, part may be wasted by an incomplete burning of the fuel, producing cinders or smoke or unburnt gases, another part is always lost by radiation and conduction, and a third portion is carried off by the hot gases that escape from the boiler-flues. Many contrivances have been adopted to diminish as much as possible this waste of heat, and so obtain the greatest possible proportion of available steam power from a given weight of fuel. Boilers wholly or partially formed of tubes have recently been much in favour. An arrangement for quickly 14generating and superheating steam is shown in Fig. 8, in connection with a high-pressure engine. Steam engines are constructed in a great variety of forms, adapted to the purposes for which they are intended. Distinctions are made according as the engine is fitted with a condenser or not. When steam of a low pressure is employed, the engine always has a condenser, and as in this way a larger quantity of work is obtainable for a given weight of fuel, all marine engines—and all stationary engines, where there is an abundant supply of water and the size is not objectionable—are provided with condensers. High-pressure steam may be used with condensing engines, but is generally employed in non-condensing engines only, as in locomotives and agricultural engines, the steam being allowed to escape into the air when it has driven the piston to the end of the stroke. In such engines the beam is commonly dispensed with, the head of the piston-rod moving between guides and driving the crank directly by means of a connecting-rod. The axis of the cylinder may be either vertical, horizontal, or inclined. A plan often adopted in marine engines, by which space is saved, consists in jointing the piston-rod directly to the crank, and suspending the cylinder on trunnions near the middle of its length. The trunnions are hollow, and are connected by steam-tight joints, one with the steam-pipe from the boiler, and the other with the eduction-pipe. Such engines have fewer parts than any others; they are lighter for the same strength, and are easily repaired. The trunnion joints are easily packed, so that no leakage takes place, and yet there is so little friction that a man can with one hand move a very large cylinder, whereas in another form of marine engine, known as the side-lever engine, constructed with oscillating beams, the friction is often very great.