over the fire), so that a store of heat is accumulated-and, secondly, the sudden contact of such overheated plates with water, so that the heat stored up is suddenly expended in the production of a large quantity of steam at a high pressure. Some engineers hold, that no portion of the plates can thus become overheated, unless the level of the surface of the water sinks so low as to leave that portion of the plates above it, and uncovered; others maintain, with M. Boutigny, that when a metallic surface is heated above a certain elevated temperature, water is prevented from actually touching it either by a direct repulsion, or by a film or layer of very dense vapour; and that when this has once taken place, the plate, being left dry, may go on accumulating heat and rising in temperature for an indefinite time, until some agitation, or the introduction of cold water, shall produce contact between the water and the plate, and bring about an explosion. All authorities, however, are agreed, that explosions of this class are to be prevented by the following means:-1. By avoiding the forcing of the fires, which makes the boiler produce steam faster than the rate suited to its size and surface. 2. By a regular, constant, and sufficient supply of feed water, whether regulated by a self-acting apparatus, or by the attention of the engineman to the water gauge; and 3, Should the plates have actually become overheated, by abstaining from the sudden introduction of feed water (which would inevitably produce an explosion), and by drawing or extinguishing the fires, and blowing off both the steam and the water from the boiler. (See page 557.) 321. Internal Deposits.-Boilers are liable to become encrusted inside with a hard deposit of the minerals contained in the water, which, by resisting the conduction of heat, impairs at once the evaporative power of the boiler, its durability, and its safety. The deposition of carbonate of lime can be prevented by dissolving salammoniac in the water; for that salt and the carbonate of lime are mutually decomposed, producing carbonate of ammonia and chloride of calcium, of which both are soluble in water, and the former is volatile. The deposition of sulphate of lime can be prevented by dissolving carbonate of soda in the water; the products being sulphate of soda and carbonate of lime, of which the former is soluble, and the latter falls down in grains, and does not adhere to the boiler. The most effectual means of preventing internal incrustation is the use of water so pure as to yield no deposit, whether such water be obtained from a natural source, or by means of surface condensation. But galvanic action must be prevented. Plates of zinc suspended in the boiler have been used with advantage for this purpose. (See Appendix, page 589a.) A peculiar deposit of an unctuous nature has been found to clog the water spaces of the boilers of some of the engines in which surface condensation has been employed. That deposit consists of the grease or oil used to lubricate the cylinder, partially altered and decomposed. It can be obviated by introducing little or no grease or oil into the cylinder; and to make that practicable, the surface of contact between the packing of the piston and the interior of the cylinder must be lubricated with water. In order that a small quantity of water may remain in the cylinder in the liquid state for that purpose, the heating of the steam, whether by means of a superheating apparatus or of a steam jacket round the cylinder, must not be carried so far as wholly to prevent condensation in the cylinder. On this point, see Article 286, page 396. 322. An External Crust of a carbonaceous kind is often deposited from the flame and smoke of the furnaces in the flues and tubes, and if allowed to accumulate, seriously impairs the economy of fuel. It is removed from time to time by means of scrapers and wire brushes. The accumulation of this crust is the probable cause of the fact, that in some steam-ships the consumption of coal per indicated horse-power per hour goes on gradually increasing, until it reaches one and a-half its original amount, and sometimes more. The following is an example of that increase, from an ocean steamer of great size and power: On trial trip, On 1st day of voyage,.. On 11th day,. On 26th day, On 30th day,.. On 32d day, On 35th day, Coal per I. H.-P., The increase in the consumption of fuel, although not absolutely continuous, and sometimes even reversed to a small extent, is still sufficiently marked to prove a progressive falling off in the efficiency of the furnace and boiler. 323. Nominal Horse-power of Boilers.----Boilers, especially those of stationary engines, are sometimes stated to be of so many horsepower. This is, in fact, a conventional mode of describing the dimensions of the boiler, according to an arbitrary rule. The rules employed for estimating the nominal horse-power of boilers have been various, and most of them vague and indefinite. A perfectly definite rule, however, has been proposed by Mr. Robert Armstrong, as being founded on the best ordinary practice, viz. : Take a mean proportional between the area of the fire grate in square feet, and the area of the effective heating surface in square yards. The nominal horse-power of the boiler is generally much less than the indicated horse-power of the engine, to which it bears no fixed proportion. SECTION 2.-Examples of Furnaces and Boilers. 324. Wagon Boiler.-This form of boiler, at one time used for C, C, C, C, stay-rods; D, the bridge; N, N, flues. The flame or furnace gas proceeds from the furnace over the bridge, and backwards along the flue below the boiler; it returns forwards along one of the lateral flues N, and again proceeds backwards along the other lateral flue to the chimney. This course of the hot gas is called a wheel-draught. In the figure the boiler has no internal flue; sometimes there is a cylindrical internal flue, along which the hot gas returns forwards, and then divides into two currents, which proceed backwards to the chimney along the lateral flues. This is called a split-draught. W and S are water-gauge cocks; M, the man-hole; I, the steam pipe; V, the safety valve; F is the stone float, partially counterpoised, whose rising and falling regulates the valve for the admission of the feed-water. The column of water in the vertical feedpipe in these old low-pressure boilers acts as a pressure gauge, and a float on the surface of that column is seen to be connected by a chain over a pulley with the damper, whose opening it regulates. 325. Cylindrical Egg-Ended Boiler. This boiler consists simply N N of a cylindrical shell with hemispherical ends. Its figure is very favourable to strength and safety, with a high pressure; but it requires great length as compared with other boilers to give sufficient heating surface. In the crosssection, fig. 118, A is the grate, occupying a length which ought not to exceed about six feet under the front end of the boiler; B, the boiler; D, the bridge, made concave at the top so as to be parallel to the bottom of the boiler; N, N, the flues, through which the hot gas forms a wheel-draught, as in Article 324. D A. Fig. 118. This boiler, like the wagon boiler, is sometimes made with an internal flue, by which the deficiency of heating surface compared with capacity is to a certain extent made up. A serious defect of the cylindrical boiler with the furnace below it is, that the bottom of the boiler where sediment collects is the part exposed to the most intense heat. Unless, therefore, the water used is of uncommon purity, the bottom of the boiler is liable to burn. Cylindrical boilers are sometimes made without lateral flues; the hot gas flowing straight along the bottom of the boiler from the furnace to the chimney. This arrangement is called a "flash flue." It requires a greater length for a given heating surface than any other form of boiler. 326. Retort Boiler. This is the name given by Messrs. Dunn & Hattersley to a boiler introduced by them, in order to obtain the strength of the cylindrical egg-ended boiler, without its disadvan tages in point of compactness, economy of fuel, and durability. It consists of a number of small cylindrical egg-ended shells laid side by side, parallel and horizontally, above the furnace and flues; these contain water to about three-quarters of their depth, and in them the boiling takes place; they all communicate upwards with one long cylindrical egg-ended shell which acts as a steam chest, and below with another which serves as a sediment collector. 327. Cylindrical Boiler with Heaters.-This is called in Britain the "French boiler," from being much used in France. In France it is called "chaudière à bouilleurs." Fig. 119 shows a longitudi nal section of the furnace and flues, and side elevation of the boiler; fig. 120 shows a cross-section of the boiler, furnace, and flues. A is the main boiler shell, cylindrical, with hemispherical ends; B, B, the heaters, or "bouilleurs," being horizontal cylindrical shells of smaller diameter than the main shell, having their backward ends hemispherical or segmental, and their forward ends closed by covers, so as to serve as "mud-holes" for the cleansing out of sediment when required; CCC, CCC, are two rows of vertical tubes, which connect the main boiler shell with the heaters. D is a horizontal brick partition, at the level of the |