and the pro

steam cylinder would have been 14.6 instead of 11·6; portion borne by the power of the steam engine alone to that of the binary engine would have been

14.6 187 = 77, leaving 1.00

77 = 23

of the whole power of the binary engine, as the real gain due to the æther engine.

The consumption of fuel, according to M. Gouin's report, was either 2.8 or 2.44 lbs. of coal per indicated horse-power per hour, according as certain experiments made under peculiarly adverse circumstances were included or excluded.

The binary engine is not more economical than steam engines designed with due regard to economy of fuel; but by the addition of an æther engine, a wasteful steam engine may be converted into an economical binary engine.

ADDENDUM.

302 A. Explosive Gas-Engine.-In Lenoir's gas-engine, air and coal-gas in proper proportions are introduced into a cylinder; the admission is cut off, and the mixture exploded by electricity; the explosion causes a sudden increase of pressure; the gaseous mixture expands, driving the piston before it till the stroke is completed, and is expelled during the return stroke. The cylinder is prevented from overheating by water circulating in a coil. Best proportion of mixture, eight volumes of air to one volume of coalgas. Absolute pressure immediately after explosion, Pi = about 5 atmospheres, or 10,580 lbs. on the square foot. Let the atmospheric pressure be denoted by Po; then available heat of explosion, per cubic foot of explosive mixture, H1 = 2·5 (P1 - Po) = 21,160 foot-lbs., nearly. (This is about three-eighths of the total heat of the explosion.)

Let r be the ratio of expansion, p2 the final absolute pressure; W the indicated work per cubic foot of explosive mixture; p. the mean effective pressure; then

W = 2·5 (P1 - P2) — 3·5 (r − 1) P2 + (r − 1) (P2 − Po) ;

P. = W ÷ r.

Rate of expansion for greatest efficiency, r1 =

nearly; then p2 = Po; and

W1 = 2.5 (P1 - Pa) 3·5 (r 1) Po

The preceding formulæ include no deductions for losses through increased back-pressure, and through abstraction of heat, from the gas which is in the act of expanding, by the cold-water coil. These losses, chiefly from the last-mentioned cause, are so great as to increase the expenditure of coal-gas per indicated horse-power per hour nearly four-fold, its actual amount being about 140 cubic feet, according to experiments by Tresca.

In Hugon's gas-engine a small jet of water in the state of spray is injected into the cylinder by a pump during each return stroke. This at once diminishes the back-pressure, and lessens the supply of water required for the cold-water coil. The expenditure of coalgas per indicated horse-power per hour, according to experiments by Tresca, is about 85 cubic feet, or about 24 times that given by the preceding formulæ. The explosive mixture is fired by being put into communication with a gas-flame.

In Otto and Langen's gas-engine there is a very tall vertical cylinder containing a piston, whose rod is connected with the flywheel shaft by means of ratchet-work, which acts during the down-stroke only. The explosive mixture is admitted below the piston, and fired by being put in communication with a gas-flame. The piston, being free from connection with the fly-wheel shaft, shoots up with great speed until it is brought to rest by gravity, and by the atmospheric pressure; the burnt gas cools so rapidly by the expansion as to give out very little heat to the cylinder; and it falls at the end of the expansion to a pressure much below the atmospheric pressure. A water-jacket round the lower end of the cylinder only is found sufficient to prevent overheating. The down-stroke is performed by means of the atmospheric pressure, and of gravity, opposed by the back-pressure; which during a great part of the stroke is about atmosphere, and towards the end rises to 1 atmosphere by compression; and then the gas is expelled. The explosive mixture consists of one volume of coal-gas and nine volumes of air; the pressure immediately after explosion is from 4 to 6 atmospheres; the expenditure of coal-gas per indicated horse-power per hour is said to be about 35 cubic feet. (See Verhandlungen des Vereins für Gewerbfleiss in Preussen, 1868.) (See also Section on Gas, Oil, and Air Engines, by Bryan Donkin, M.Inst.C.E., at end of this volume.)

ADDENDUM TO ARTICLE 296, PAGE 430.

Empirical formula for elasticity of steam-gas at the temperature corresponding to the pressure p' and volume v' of saturated steam. Let po 1 atmosphere; then

=

(From Shipbuilding, Theoretical and Practical, page 260.)

449

CHAPTER IV.

OF FURNACES AND BOILERS.

SECTION I.-Of Boilers and Furnaces in general.

303. General Arrangements of Furnace and Boiler.-The usual relative arrangements or positions of the furnace and boiler of a steam engine may be divided into three principal classes; as follows:

I. In the External Furnace Boiler, the furnace or fire-chamber is wholly outside of, and partly in contact with, the water vessel or boiler; so that the boiler forms part of the boundary of the furnace (generally the top). The other boundaries of the furnace are usually built of fire-brick. As to the thickness required to prevent loss by radiation, see Article 228. Examples of this are the old hay-stack boiler and wagon boiler, the plain cylindrical boiler, without internal flues, and some boilers, such as Gurney's, Perkins's, and Craddock's, in which the water and steam are contained in tubes surrounded by the flame.

II. In the Internal-Furnace Boiler, the fire-chamber is enclosed within the boiler. Examples of this are-t e-the boilers now most common in land engines, with one or more furnaces contained in horizontal cylindrical internal flues; most marine boilers; and all locomotive boilers.

III. The Detached Furnace or Oven is a fire-chamber built of brick, in which the combustion is completed before the hot gas comes in contact with any part of the boiler. This has been already referred to in Article 230, page 283.

304. The Principal Parts and Appendages of a Furnace are— I. The furnace proper, or fire-box, being the space where the solid constituents of the fuel, and the whole or part of its gaseous constituents, are burned.

II. The grate, being that part of the bottom of the furnace proper which is composed of alternate bars and spaces, to support the fuel and admit air.

III. The hearth is a floor of fire-brick, on which, instead of on a grate, the fuel is burned in some furnaces.

IV. The dead plate, or dumb plate, being that part of the bottom of the furnace proper which consists of an iron plate, without bars and spaces.

V. The mouth-piece, being the passage through which fuel is introduced, and sometimes also air. The bottom of the mouthpiece is a dead plate. In many furnaces there is a mere doorway, and no mouth-piece.

VI. The fire-door, which closes the mouth-piece or doorway, and which may or may not have openings and valves in it to admit air. Sometimes the duty of a fire door is performed by a heap of dross closing up the mouth-piece.

VII. The furnace-front, above and on either side of the fire door.

VIII. The ash-pit, being the space below the grate into which the ashes fall, and through which, in most cases, the greater part of the supply of air is admitted.

IX. The ash-pit door, used in some furnaces to regulate the admission of air through the ash-pit.

X. The bridge, being a low vertical partition at one end of the furnace (usually the back) over which the flame passes on its way to the flues or chimney. This is what is meant when "the bridge" is spoken of without qualification; but the word bridge is also applied to any low partition having a passage for flame or hot gas above it. Bridges are usually built of fire-brick; but they are also sometimes made of plate iron, and hollow, so as to contain water within, and form part of the water space of the boiler-they are then called "water bridges." The top of a water bridge ought to slope or curve upwards towards the ends, to admit of the rapid escape of the bubbles of steam which form on its internal surface. Sometimes a water bridge projects downwards from a part of the boiler above the furnace, leaving a passage below for flame-it is then called a "hanging bridge." A water bridge with passages for flame, both above and below, is called a mid-feather."

66

XI. The flame chamber, being the space immediately behind the bridge in which the combustion of the inflammable gases that pass over the bridge is or ought to be completed. It has often a floor of fire-brick, called the flame bed; and is sometimes lined with fire-brick to prevent the cooling and extinction of the flame, and sometimes, for the same purpose, filled with fire clay tiles, made of a horse-shoe form in section, to admit of the circulation of the gases.

XII. Air passages, of various constructions and in various situations, and with or without valves, to admit air for the combustion of the fuel, whether forced in by atmospheric pressure or by a blowing machine.

XIII. Flues, being passages traversed by the hot gas on its way from the fire to the chimney. These are sometimes external, being in contact with the outside of the boiler, and bounded externally by brickwork; and sometimes internal, being contained within,

and forming part of, the boiler. are called tubes.

Internal flues of small diameter

XIV. Bafflers or diffusers, being partitions so placed as to improve the convection of heat, by promoting the completeness of the circulation of the particles of hot gas over the heating surface of the boiler. The various bridges already mentioned fall under this head, and also the spiral blades for boiler tubes recently introduced by various inventors.

XV. The chimney (see Article 233), at the foot of which is sometimes a chamber called the smoke box, or uptake, in which the various flues terminate.

XVI. Blowing apparatus, used in order to produce a draught, whether by forcing air into the furnace by means of a fan, or by driving the gases out of the chimney by means of a blast pipe. See Article 233.

XVII. Dampers, being valves placed in the chimney, flues, tubes, or air passages, to regulate the draught and rate of combustion.

No one furnace possesses all the parts and appendages above enumerated; for some of them are substitutes for others, and some are only employed in furnaces of particular kinds (see page 477). 305. The Principal Parts and Appendages of a Boiler are—

I. The shell, or external boundary of the boiler, for which the usual material is iron or steel; the latter is now mostly employed. The figures usually employed for the shells of boilers are the cylindrical, with internal furnaces, flues, or tubes. The most common figure at the present day is that of a horizontal cylinder, with flat or hemispherical ends. In some peculiar boilers, the shell is a vertical cylinder, or a cluster of vertical tubes connected by means of horizontal tubes (as in Mr. Craddock's boiler); or a set of square tubes or cells (as in Mr. J. M. Rowan's boiler); or a single spiral tube (as in Mr. Perkins's boiler). Tubes which thus contain water internally are called water tubes, to distinguish them from tubes for transmitting furnace gas. In most locomotive boilers, part of the shell is a rectangular box, containing within it another rectangular box, which latter is the fire-box. The shells of ordinary marine boilers are of cylindrical shapes, adapted to the space in the ship which they are to occupy. The circumferential joints are lapped and double or treble riveted; the longitudinal are butt joints with covering plates.

II. The steam chest, or dome, being a part of the shell which usually rises above the level of the rest of the boiler, so as to provide a space in which the steam, before being conducted to the engine, may deposit any particles of spray that it may have carried up from the water. It is usually cylindrical, with a hemispherical or segmental top; but its form is often varied, especially in marine boilers.

It