In the other class of gauges, shown in fig. 79, the steam acts upon a bent metal tube, a b c, usually of a flattened or elliptical section. It may not be known to all readers that if a tube bent, say in the form of the letter U or C, is subjected to the pressure of a liquid or gas on the inside, the force exerted by the pressure has a tendency to straighten out the tube. This is due to the tendency which a tube of an elliptical or flat section has to change the shape of the latter and approximate to a circular form when the inside is subjected to a pressure. Thus let A B, fig. 80, represent a cross section, and a b d c, a longitudinal section of a part of such a tube contained between two radii, O a and O b, drawn from the centre O of the curve in which the tube is bent. If now we subject the inside of A B to a pressure it will have a tendency to assume the form of the circle C D, and would then be represented in the longitudinal section by the dotted lines a′ b′ d′ c′. If now we draw radial lines through a′ c′ and b′ d′, it will be found that they intersect at O′, instead of O, which was the original centre of the curve of the tube. It will be seen that as the section of the tube approximates to the form of a circle, the portion a b which is outside the curve will be moved farther from the centre, while the other side, c d, is moved nearer to it. As indicated by the radial lines, when this occurs either the outside must be lengthened and the inside shortened, to conform to the radial lines a O and b O, or else the tube will be straightened so that the radial lines will assume the position a′ O′ and b′ O′.

The phenomenon of the straightening of the bent tubes of steam gauges is frequently attributed to the difference between the area of the inside and outside of the curve. This error was shared by the writer, until the fallacy of the reasoning which supported it was pointed out to him.

In the gauge represented in fig. 79, (in which the dial-plate is removed), one end, c, of the tube is attached at d to a lever which has a toothed segment, e, at the other end. The end a of the tube is connected with the lever at f. The connection at d, therefore, forms the fulcrum of the lever. It is obvious that as the two ends of the bent tube are forced apart by the steam pressure, the lever and the segment have motion imparted to them. The latter gears into a pinion on the spindle of the index or pointer, r r, which thus indicates on the dial the degree of pressure in the tube. The latter is connected with the boiler by a tube attached at g. Various forms of this kind of steam-gauge are also made, but all act on essentially the same principle.

The position of the steam-gauge on the engine is shown at M, in fig. 71.

Question 147. Why is the pipe which connects the steam-gauge with the boiler bent as shown in fig. 71?

Answer. To prevent the hot steam from coming in contact with the metal plate or tube, as it is found that the heat of the steam affects their elasticity. When a bent tube is used, the steam from the boiler is condensed and fills the bent portion so that when the steam pressure comes on the surface of the water it forces it up the other leg of the tube into the gauge. A cock is attached to this pipe so that the steam can be shut off in case the gauge should get out of order or require to be removed while there is steam in the boiler.

Question 148. How can the accuracy of a steam-gauge be tested?

Answer. When the gauge is in good working order, the index or pointer moves easily with every change of pressure in the boiler, and if the steam is shut off from the gauge, the index should always go back to 0. In order to determine the accuracy of its indications, however, they should be tested with a column of mercury. This consists of a long, vertical tube, terminating at its base in a closed vessel filled with mercury. The gauge is then attached to the top of this vessel and water or oil is forced into the vessel on top of the mercury and into the gauge. A pressure of one pound per square inch will force up the column of the mercury 2.04 inches, so that by graduating the tube into spaces that distance apart, the divisions will indicate the pressure in pounds per square inch. Thus, a pressure of 50 pounds would force up the column of mercury 102 inches, and with 100 pounds pressure the column would rise 204 inches, and therefore, when the mercury reaches these or any other points, the steam-gauge, if it is accurate, should indicate equivalent pressures.

The ordinary steam-gauges are very liable to get out of order, and therefore they should be frequently tested to ascertain whether their indications are correct.

Question 149. What is the steam whistle, and for what purpose is it used?

Answer. The steam-whistle, W, fig. 71, and shown in section on a larger scale in fig. 81, consists of an inverted metal cup or bell, A, made usually of brass. The lower edge of this cup is placed immediately over an annular opening, a a, from which the steam escapes and strikes the edge of the cup or bell, which produces a deep or shrill sound, according to the size or proportions of the whistle. The annular opening a a is formed by the plate or cover, a a, which nearly fills the mouth of the cup B, which is attached to the stem c. The latter is screwed into the top, D, of the dome. Communication with the steam-space of the boiler is either opened or closed by a valve, b, which is attached to a sort of spindle, d, which extends upward inside of the stem c. This spindle does not entirely fill the opening in the stem c, so that the steam which enters when the valve b is opened rises and escapes through the holes e, e, e, into the cup B and out through the annular opening a a. The valve is opened by the lever E, whose fulcrum is at f. The end g of this lever is connected by a rod, h, figs. 81 and 71, with the cab, and by a suitable handle or lever, h′ h′, fig. 71, it can be opened and the whistle be blown at any time by the locomotive runner or fireman to give signals to the trainmen or of the approach of a train to a station, or to warn persons to get off of the track.

Question 150. How is a locomotive boiler emptied and cleaned?

Answer. One or two large cocks, called blow-off cocks, X, fig. 71, are placed near the bottom of the fire-box, either in front or behind, and sometimes on the side. By opening either of these the water in the boiler is blown out, and much of the loose mud and dirt is carried out with the water. The cock X, fig. 71, is opened by a handle, w, which is connected with the cock by a rod.

In order to clean out the mud and scale which are not entirely loose, what are called mud-holes or hand-holes are placed in the corners of the fire-box near the bottom. These are oval-shaped holes, about 4¹⁄₂ inches long and 2¹⁄₂ inches wide, and covered with two metal plates, one of which is put inside the boiler and the other outside, and fastened with a bolt through both. Another hand-hole is sometimes placed in the bottom of the front tube-sheet. When the boiler is emptied of water these hand-holes are uncovered, and as much dirt is removed as can be scraped out of these holes. A hose pipe is then inserted and a strong stream of water is forced in, which washes out nearly all the loose dirt, so as to leave the boiler comparatively clean.

When the water is very impure, what is called a mud-drum, M, fig. 41, is used. Much of the mud and dirt is deposited in this receptacle, from which it can easily be removed by taking off the cast-iron cover on the bottom of the drum. The cover is also provided with a blow-off cock, which is shown in the figure referred to.

Question 151. What other attachments are there to the boiler of a locomotive?

Answer. There are two cocks, a, a, fig. 71, called heater-cocks, which are connected with pipes to the feed-pipes D D, to admit steam to the latter to prevent the water in them from freezing. There is also another cock, b, called a blower-cock, which is connected to the smoke-stack by a pipe b, b. Steam is conducted through this pipe and escapes up the chimney in a jet, thus producing a draft when the engine is not working. This arrangement is called a blower and is used to blow the fire when the engine is standing still. The action of the jet is similar to that of the exhaust steam which escapes up the chimney, excepting that the steam from the jet escapes in a continuous stream instead of distinct “puffs,” as it does when it is liberated alternately from one end of the cylinders and then from the other.

T′ is a handle which is connected by a rod, T′ T, with the feed-cock (not shown in the engraving) in the pipe D. This cock can be opened or closed by the handle, and the supply of water fed into the boiler by the pump can thus be regulated. J is a handle on the other side of the engine, for regulating the working of the pump on that side.

e, e are handles, also connected by rods with the pet-cocks on the pumps. These cocks can thus be opened or closed, and it can then be known whether the pumps are working.

A is the furnace door, which is fastened by a latch. The latter has a chain, Q, attached to it by which it can be conveniently opened or closed. The door also has a circular register with six holes to admit air into the furnace. These holes can be opened or closed by the revolving circular disc shown in the engraving.

Question 152. How are the grates constructed?

Answer. As has already been explained, they are made usually of cast-iron bars,[40] A, A, A, figs. 82 and 83, called grate-bars. Fig. 82 is a plan, and fig. 83 a horizontal section of one form of grate. The bars in this kind of grate are usually cast in pairs, or some times three or more are cast together. They are made wider on the top than on the bottom edges, as shown in the section, fig. 83, so that cinders and ashes will fall through easily, and also to give free access to the air from below. They are usually from ³⁄₄ to 1¹⁄₂ inches wide on the top, and about ³⁄₄ inch on the lower edges. The spaces between the bars are made from ¹⁄₂ to 1¹⁄₄ inches wide. For burning wood the bars are placed comparatively close together and are stationary, but for burning bituminous coal they are usually made so that they can be moved, in order to shake or stir up the fire, just as is necessary in an ordinary stove or grate fire. In the grate we have illustrated the bars, A, A, are cast in pairs, and run crosswise of the fire-box. The ends are made with a sort of journals, b, b, which rest on two supports, B, B, called bearing bars, which have suitable indentations to receive the ends of the grate-bars. The latter have arms, C, C, fig. 83, cast on the under side, to which a bar, D D, is attached. By moving this bar back and forth, the grate-bars have a rocking motion imparted to them, as shown in fig. 84. It is evident that in this way the fire over the whole surface of the grates will be disturbed or shaken. The bar, D D, is moved by a lever, m m, shown in fig. 71. An extension piece, not shown in fig. 71, is used with the lever m m, so as to increase its length; but it is removed after it has been used, so as not to be in the way of the fireman. Grates which have movable bars are called shaking or rocking grates. A great variety of such grates are made and in use, to describe which would require more room than is available here.

For burning anthracite coal what are called water-grates are used. These consist of wrought-iron tubes, 2 inches in diameter outside, which are fastened in the front and back plates of the fire-box and are inclined upward from the front end, so that there will be a continued circulation of water through them to keep them cool and thus prevent them from being burned out by the intense heat of the fire.

Question 153. How is the fire removed from the fire-box when it is necessary to do so?

Answer. In bituminous coal burning engines, what is called a drop-door, E E, figs. 82, 83 and 84, is provided for that purpose. This door is supported partly on journals, d, d, similar to those in the grate-bars, on which it can turn, and is held up or prevented from dropping by arms, e, e, attached to a shaft, F F. This shaft is operated by a lever, f f, fig. 82, outside the fire-box.

When the arms are in the position represented in fig. 83, the drop-door is held up in the place in which it is shown; but when they are turned as in fig. 84, the door falls down so that the burning coal can be taken out of the opening at G, and, by raising up the ash-pan damper, H, fig. 84, can be raked out on the track or into suitable pits usually provided for this purpose. The drop-doors are sometimes perforated so as to admit air to the fuel on top of them.

The grates for burning anthracite coal usually have about four solid wrought-iron bars between that number of tubes. These bars can be withdrawn, and the fire then falls into the ash-pan through the opening left by the withdrawal of the tubes.

Question 154. How are the dampers of the ash-pan operated?

Answer. They are connected by suitable rods and levers with two handles, l, l, fig. 71, which are raised or lowered, thus opening or closing the dampers.