If the pressure per square inch is known, it is plain that by multiplying that pressure by the number of square inches in the area of the piston of the engine, the total pressure on the piston can be found. The pressure does not rise instantly when the steam is first admitted, nor does it fall quite abruptly when the steam is cut off and communication opened with the condenser. When the steam is worked expansively, the pressure falls gradually from the time the steam is shut off. Now, the amount of work done by any force is reckoned by the pressure it exerts multiplied into the space through which that pressure is exerted. Therefore the work done by the steam is known by multiplying the pressure in pounds on the whole surface of the piston into the length in feet of the piston’s motion through which that pressure is exerted. The trace of the pencil on the paper—i.e., the indicator diagram—shows the pressures, and also the length of the piston’s path through which each pressure is exerted, and therefore it is not difficult to calculate the actual work which is done by the steam at every stroke of the engine. If this be multiplied by the number of strokes per minute, and the product divided by 33,000, we obtain what is termed the indicated horse-power of the engine. The work done per minute is divided by 33,000, because that number is taken to represent the work that a horse can do in a minute: that is, the average work done in one minute by a horse would be equal to the raising of the weight of 1,000 lbs. thirty-three feet high, or the raising of thirty-three pounds 1,000 feet high. The number, 33,000, as expressing the work that could be done by a horse in one minute, was fixed on by Watt, but more recent experiments have shown that he over-estimated the power of horses, and that we should have to reduce this number by about one-third if we desire to express the actual average working power of a horse. But the power of engines having come to be expressed by stating the horse-power on Watt’s standard, engineers have kept to the original number, which is, however, to be considered as a merely artificial unit or term of comparison between one engine and another; for the power of a horse to perform work will vary with the mode in which its strength is exerted. The source of the power which does the work in the steam engine is the combustion of the coal in the furnace under the boiler. The amount of work a steam engine will do depends not only on the quantity of steam which is generated in a given time, but also upon the pressure, and therefore the temperature at which the steam is formed.
Fig. 6.—Section of Giffard’s Injector.
The water constantly evaporating in the boiler of a steam engine is usually renewed by forcing water into the boiler against the pressure of the steam by means of a small pump worked by the engine. In the engraving of Watt’s engine this pump is shown at M. But recently the feed-pump has been to a great extent superseded by a singular apparatus invented by M. Giffard, and known as Giffard’s Injector. In this a jet of steam from the boiler itself supplies the means of propelling a stream of water directly into the boiler. Fig. [6] is a section of this interesting apparatus through its centre, and it clearly shows the manner in which the current of steam is made to operate on the jet of water. The steam from the boiler passes through the pipe A and into the tube B through the holes. The nozzle of this tube is of a conical shape, and its centre is occupied by a rod pointed to fit into the conical nozzle, and provided with a screw at the other end, so that the opening can be regulated by turning the handle, C. At D the jet of steam comes in contact with the water which feeds the boiler, the arrangement being such that the steam is driven into the centre of the stream of water which enters by the pipe E, and is propelled by the steam jet through another cone, F, issuing with such force from the orifice of the latter that it is carried forward through the small opening at G into the chamber H. Here the water presses on the valve K, which it raises against the pressure of the steam and enters the boiler. The water issuing from the cone, F, actually traverses an open space which is exposed to the air, and where the fluid may be seen rushing into the boiler as a clear jet, except a few beads of steam which may be carried forward in the centre, the rest of the steam having been condensed by the cold water. The steam, of course, rushes from the cone, B D, with enormous velocity, which is partly communicated to the water. The pipe, L, is for the water which overflows in starting the apparatus, until the pressure in H becomes great enough to open the valve. The supplies of water and of steam have to be adjusted according to the conditions of pressure in the boiler, and according to the temperature of the feed-water. It is found that when the feed-water is at a temperature above 120° Fahrenheit, the injector will not work: the condensation of the steam is therefore necessary to the 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 thousand 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.