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For accurate estimates, the specific heats must also be regarded; these by latest results are, for the vapors named in order, .305, .451, .481, .157, .506, and .156, that of an equal weight of water being 1. Thus far, however, engines driven by vapors other than steam, and also compound-vapor engines, have proved too expensive through waste of the material, and have not attained to a decided success. For the application of hot air as a motor, see ATMOSPHERIC ENGINE. VI. The Production of Steam. The capacity and principle of construction of the boiler or generator of steam must be determined generally with a view to the strength of the materials and the laws of combustion and of heat, and especially with reference to the specific purpose for which the engine is intended. In boilers for marine or locomotive purposes, rapid generation, security, lightness, and compactness should be attained, and if needs be at the cost of some waste of fuel; in those for stationary engines economy is the paramount object. Boilers before the time of Watt were usually in shape of an inverted frustum of a cone, with a hemispherical top and slightly concave bottom, and were set in an arch of brick-work, the fire being admitted to the bottom only, or also about the sides. To make a boiler stronger for its capacity and with greater heating surface, Watt introduced the long rectangular form, with semi-cylindrical top, the ends flat, the bottom and sides slightly concave, and set in a long arch; this was called the wagon boiler. The next transition was to the cylindrical boiler with hemispherical ends, as giving greater strength and heating surface. Subsequently a single straight flue, a single flue bent and returning, or two or more flues, were carried through the interior of the boiler; and the fire being made directly in these, or the heated air of the furnace being made to circulate through them while the water surrounded the flues, a much increased utilization of the heat of combustion was the result. From boilers with flues to those traversed by a large number of small tubes, in which the flame and heated air directly from the furnace shall circulate on its way to the chimney, thus exposing to the fire & maximum of water surface, the transition was a natural one; this form, known as the multitubular or tubular, is best illustrated in the locomotive boiler. The name tubular boiler is more correctly given to those boilers in which the tubes contain water, being surrounded by the flame. Flue and tubular boilers are those now most generally in use; they are either horizontal or upright. For a description of Messrs. Lee and Larned's annular boiler, see

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FIRE ENGINE. The shell of the boiler, which may have a great variety of forms, is the general or outer wall, enclosing a space, part of which, as occupied by water, is the "water room," and the portion above this the "steam room.' The furnace is the chamber which receives the fuel; when within the shell, this is called the fire box. The grate or fire grate is that part of the bottom of the furnace on which lies the fuel; it is constructed of "fire bars” or “grate bars," with spaces for admission of air. The part consisting of a plate, without such openings, is the "dead plate." The chimney affords draught and conveys away the final products of combustion; the flues or tubes for flame sometimes open into a chamber at its lower part, called the smoke box or uptake. The chimney, flues, or tubes may be controlled by dampers. The "tube plates" receive and are pierced by the ends of the tubes; they form either part of the shell, or one side of an internal fire box. The steam chest or dome, upon the upper side of the boiler, is the reservoir for the generated steam, which, as the various valves may allow, is supplied from it to the cylinder; or when its pressure exceeds the load upon the safety valve, directly escapes through this into the air. The height of water in the boiler is known by the water gauge, usually a strong glass tube standing outside the boiler; or by use of "trycocks," three or more at different heights upon the boiler, and opened by hand. A pressure gauge shows the pressure within the boiler. A vacuum valve is a safety valve opening inward, to prevent collapse when the pressure within falls below that of the atmosphere. "Mud holes," at or near the bottom, are for the discharge, when needful, of collections of sediment; but it is very important, especially in marine boilers where sea water is used, to prevent such collections; and for this purpose sediment collectors, and various means for prevention of incrustation by previously precipitating the saline matters from the water, are employed. The apparatus for feeding the boiler with water, especially when the engine is at rest, may require to be driven by a separate and smaller engine; this is then called the "donkey engine." Feed-water heating, previously to admission of the supply to the boiler, may be accomplished in various ways; among them by carrying the water pipe through or about the flues or chimney. Brine pumps may be used for clearing the boiler of brine or sediment, or a large blow-off cock through the bottom of the boiler. Scum is removed by a scum cock or surface-blow at the surface of the water. The importance of keeping the water in the boiler always as near as possible to a certain height, so as to prevent liabilities on the one hand of priming, and on the other of burning or melting its sides or flues, or of explosion, will be very obvious; and a variety of devices, some of them already named, have been applied in order to aid and direct the engineer in this respect. Among the most recent,

and perhaps for safety the most valuable of such devices, are those known as "steam alarms," which are automatic, being caused to open a safety valve and blow a whistle or ring a bell when the water falls below a given level in the boiler. (See further, BOILING POINT, COMBUSTION, EVAPORATION, and FUEL.) The questions of the efficiency of steam boilers, the due proportioning of the fire grate, the flues, the heating surface, &c., to the capacity of the boiler and of the engine, and the duty of engines, are among those of great interest in the practical and commercial aspects of the subject, but for which space can only be allowed in the larger and specific treatises. It may be stated that in large land engines it is customary to allow for each horse power of pressure about 1 square foot of fire surface, 3 cubic feet of furnace room, 10 cubic feet of water, and 10 cubic feet of steam room. To vaporize the water, it is found in England, where coal is more cheaply obtained, and economy in its use not so much an object, that from 10 down to 8 or 7 lbs. of coal per cubic foot of water are consumed; in this country, in cases of good performance, the like effect is often obtained with about 6 lbs. of anthracite coal. VII. Steam Boiler Explosions. These accidents are of different sorts; sometimes the metal is simply rent, steam or water or both escaping; sometimes the boiler or a flue is burst inward, a result known as "collapse;" but in explosions proper, the boiler is not only ruptured and often thrown from its place, but fragments of it are usually hurled with terrible force from the spot, accompanied with escape of steam and scalding water. Of certain peculiar theories proposed to account for explosions, one, to the effect that they are due to an electrical charge within the boiler, has no support whatever in facts, and is wholly visionary; a second, assigning as the cause the occurrence of an explosive mixture of gases within the boiler-hydrogen and oxygen being the only possible gases, and their presence in the needful quantities in any case being extremely questionable is probably not less so. A recent projectile theory of explosions appears to add nothing to our knowledge of the accident; for though projection of the steam and water does occur, it is the effect of the steam pressure, and allowed by relative weakness of the boiler. M. Jobard (1861) has, however, shown that many explosions of stationary boilers are owing first of all to explosive combination of a sort of fire-damp of hydrogen and air, collecting in the chimney or flues during rest of the engine, and ignited on subsequently disturbing the fires, without previous opening of the dampers; the first effect is to throw the boiler out of place, and the flow of the water within it over its upper heated surfaces generates a pressure that next rends the boiler itself. Other causes or conditions which doubtless often lead really to explosions are the following: 1, in a boiler very highly heated, and without fresh feeding

for some time, the body of water may assume the spheroidal condition (see EVAPORATION), and a film of vapor lying between it and the metal, the latter can become unusually heated; on feeding cooler water or disturbing the fires, contact with the metal may be renewed, with sudden generation of steam of extremely high pressure; 2, by long boiling, without renewal, the water may become so far freed of air or other permanent gases that serve to form openings within its mass into which vaporization can begin, that its boiling point shall be raised, and when reached, a considerable body of the water, as has been observed in the experiments of Donny and others, shall burst suddenly into steam; 3, superheating may have taken place unsuspected within the dome, and when by fresh feed or rousing the fires the equilibrium within the boiler is disturbed, water being dashed up into the body of superheated steam, or otherwise, the sudden transfer of heat to the generation of saturated steam may instantly and very greatly increase the pressure. The last named cause could occur at any time during the running of an engine, if the steam became superheated and the boiling afterward more tumultuous; and any of these three causes can readily be supposed to occasion some of the explosions of stationary and steamboat boilers, known so often to occur upon first disturbing the water or fire after a period of rest of the engine. In all three of them, moreover, the production of an enormously great steam pressure may be almost instantaneous; so that the well known principle of blow or shock would come into play, the tenacity of the boiler giving way under the suddenness of the impulsion, before the limit of strength of the metal had been reached. Whatever be the occasion of explosions proper, however, their immediate cause is always the same—a momentary excess of the pressure of steam within the boiler over the strength of some of its parts. In the best arranged modern boilers, there are 2 or 3 safety valves, at least one of which is placed within the boiler, or otherwise beyond the control of the engineer. An advantage of tubular and small-flue boilers is the circulation of the water maintained in them, which retards or prevents formation of scale. Another, with water tubes, is the greatly increased strength to be had even with less thickness of material, owing to the relatively small diameter of the tubes; and further, since the same tube resists rupture from within up to pressures exceeding those that would cause flattening or collapse from without, it follows that, with like size and thickness of tubes, water-tube are stronger and safer than flue-tube boilers. The common impression respecting the greater danger of high pressure than of low pressure boilers, requires some qualification. In itself, and in one way, the high pressure is a source of increased risk; but the boilers generating such steam usually admit of being much smaller, and from this cir

camstance are relatively stronger; beside, they contain (locomotive boilers excepted) a less body of water and of steam to be projected about them in case of explosion. Low pressure boilers, on the other hand, have in them, when working, a large body of steam and also of water; when explosion occurs, this water being suddenly released from the confining pressure, much of the excess above 212° of heat in both the steam and water instantly goes to the generation of a greatly enlarged volume of steam; and this large body of scalding steam and water is projected through a more considerable space about the place of the boiler. Thus, the actual destructiveness and fatality consequent on explosion of the low pressure boiler are likely to be the greater, to say nothing of the fact that, from some of the peculiar conditions named above, even the projectile force given to the fragments and contents of the latter can be, in certain instances, quite as great as with the former. Obviously, most if not all the occasions of boiler explosions are avoidable, through, 1, avoiding the forcing of the fires; 2, keeping the valves in proper condition, and in no case over-weighting them; 3, supplying the feed water regularly, constantly, and in sufficient quantity; 4, in case the plates are discovered, or where they are likely, to be over-heated, abstaining from the sudden introduction of feed water, drawing or extinguishing the fires, and blowing off the steam and water.-STEAM CARRIAGE. For information respecting the invention and early improvement of the locomotive engine, see RAILROAD, and incidentally EVANS, STEPHENSON, and STEVENS. The locomotive engine has so few points of resemblance to any other as to be essentially a new application of the same moving power. The use of steam at very high pressure and with rapid travel of piston, allows of a comparatively diminutive cylinder; while the high steam and great speed sought require a very large boiler and intense fire, to generate such steam with due rapidity. Intended, as its name imports, for locomotion, this engine must carry with it the water and fuel necessary to its action; and being subjected to violent strains and shocks, it should, along with the requisite conditions for furnishing the power, embody great strength and compactness of construction; to secure the latter, the engine and boiler are mounted together upon one carriage, through the wheels of which the tractive power is to be applied. Evahs and others had placed the boiler and engine on one set of wheels; Trevithick and Vivian (1802-'3) separated the traction carriage, or locomotive proper, from those intended to receive the load; and they first discharged the exhaust steam from the cylinder into the chimney, to create draught. George Stephenson, about 1825, applied this principle much more successfully, perhaps reinventing it, and to him it is usually assigned. Seguin in France, and Booth in England, in 1829, furnished the multitubular flue boiler;

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the efficient adhesion of smooth driving wheels had before been discovered; and Stephenson's combination of all these essentials, with his direct connection of the piston rods, one on each side, with the propelling wheels, resulted in the first really successful locomotive, the Rocket, in the renowned "railway year," 1829. Timothy Hackworth contributed, before 1830, the six coupled wheels and the steam chamber over the boiler, placed the cylinders under the boiler, and made other improvements. The size and weight of the locomotive have been since that time much increased, and a construction highly perfected in detail and well adapted to the purposes desired has been attained. Modern English passenger locomotives are of two general types: 1, the "inside cylinder" locomotive, having the cylinders within the framing, under the boiler, with a main driving axle cranked at two points to receive the action of the piston rods; 2, "outside cylinder" locomotives, having the cylinders external to the framing, the axles straight, and the piston rods attaching each to a crank pin fixed between spokes of a driving wheel, the pin turning with the wheel about its axle, and serving as the engine crank. English freight or goods" locomotives are also of two classes: 1, those with 6 wheels of like size, the 3 on either side having their crank pins coupled by horizontal bars called parallel rods, which secure the same crank action upon and movement of all the wheels; 2, those having the fore wheels smaller, the two back pairs only being of like size and coupled. The power of a locomotive, other things being equal, will depend on the amount of the steam pressure upon the piston; but a condition favoring is found also in the use of small driving wheels. As each revolution of the wheel to which the piston rod is attached must correspond to one stroke of the piston, it follows that speed of travel will depend on the speed of the piston, or number of strokes it can make per minute, but that it is also favored by use of large driving wheels. After many fluctuations, American practice has recently tended to the adoption of wheels of somewhat less diameter than those used a few years since. American locomotives are almost invariably outsideconnected, i. e., have their cylinders outside the truck or engine frame, as well as nearly or quite on a level with the axles of the driving wheels. There are but two general types of construction, those for passenger and for freight trains. The former have 8 wheels, 4 in front set in a movable frame, the bogie or truck, turning on a central pivot to allow of running on curves in the road, and 4 larger ones behind, the drivers, or driving wheels, of equal size, and coupled with parallel rods. The freight locomotives are on 10 wheels, the leading 4 in a swivelling truck, and the 6 back wheels, 3 on a side, coupled as drivers. American locomotives are distinguished also by the cab or house at the back end to protect the

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engineer and fireman, with glazed opening in front to afford a view ahead, and by the larger size and form of the chimney or smoke stack, a cylinder of wire net within an inverted cone, with wire net and baffle-plate above to deflect sparks into the intervening space-an arrangement specially adapted to the use of wood as fuel, and called the spark-arrester. Between the cab and the chimney of a locomotive appear, also above the boiler, the smaller dome in front surmounted with the steam whistle, the bell, and the dome proper. The "cowcatcher" or the snow plough may be attached forward, for the purpose of clearing the track; and in front of the chimney is also affixed a lamp with a parabolic reflector, for lighting the track in advance at night. The "tender," which conveys water and fuel, is a smaller carriage next the locomotive, and on 4, 6, or 8 wheels, arranged in two trucks, to facilitate turning. Its back and sides are occupied by the water tank, in horse-shoe form, holding from 800 to 1,000 gallons of water, while in the hollow of this is stowed the fuel, both being intended for some 15 to 30 or more miles run. The water tank communicates through pipes, connected by flexible hosing with two small lift-and-force pumps, worked from the piston rod, and the delivery of which can be regulated so as to feed continuously or at intervals. From the upper part of the dome the steam pipe opens out, descending and running forward within the upper part of the boiler, thence emerging within the chimney, where it divides into two branches, that run down and open each into the valve chest of one of the cylinders, each chest lying at the inner side of its cylinder. Sometimes the dome and commencement of the steam pipe are at the forward end of the boiler. In any case, while working, the chests are kept filled with steam which surrounds the valve-a single slide valve moved by one or two eccentrics and rods from the driving axle-and the steam is thus continually in readiness to flow into either port when uncovered, and to act upon that side of the piston corresponding. (See STEAM ENGINE, II.) The steam pipe may have in it a throttle-valve, under control of the engineer, through a lever handle and links. Thus the governor is rendered unnecessary, as it is also inapplicable. From length of passages, smallness of apertures, failure of the boiler to keep up the supply, or other causes, the pressure the steam can exert at and on the piston is always less than that it has within the boiler; and the necessity and mode of obtaining draught still further deduct from the effective pressure. The double-cased sides of the fire box, a space of 2 to 4 inches within which, directly about the fire, is filled with water, and the small flue tubes traversing the cylindrical portion of the boiler, conveying flame and hot air toward the chimney, and surrounded with water, can generate steam rapidly and of very high pressure. But to this

end, the fire must burn briskly, and the neated gases escape rapidly through the flue tubes, 100 to 200 or more in number, to the chimney. To secure the required draught through these, the exhaust pipe from each valve chest is carried directly into the chimney, and the two are united in a pipe called the blast pipe, which opens at a little height by a contracted mouth. The forcible and rapidly recurring puffs of the waste steam discharged from the cylinders out of this pipe into the chimney, carry with them the surrounding body of air, and create a partial vacuum, which can be supplied only by the consequent rush of fresh air through the grate, the fire, and the flues. Now, the contraction of the blast pipe needful to secure blast unavoidably keeps back somewhat the exhaust steam, and so results in a continued back pressure against the piston, while advancing either way under the last admitted steam charge; and when the speed of the locomotive, and so of the piston, becomes very great, this back pressure is correspondingly increased and rendered well nigh continuous. To accomplish successfully by use of the slide valve the required distribution of steam in the cylinders (see STEAM ENGINE, IV.) has been perhaps the most difficult problem in locomotive construction, and the one on which the greatest amount of ingenuity has been expended.. In the case of a question so highly mathematical and technical as this, nothing beyond an intimation of the objects sought and of the means employed to attain them can be presented; the reader will find the subject fully discussed in Mr. D. K. Clark's "Railway Machinery" (Glasgow and London, 1855). First, the valve should be so moved as to admit and discharge steam when the piston is at or near the beginning or end of the half strokes; secondly, the valve gear should render a variable expansion, or ratio of cutting off, practicable; and thirdly, the valve gear must be capable of reversing at will the action on the piston, so as to reverse the movement of the engine and carriage. Remembering that the diameter of the crank circle equals the length of stroke of the piston; that when the crank pin and piston rod are in the horizontal diameter of the crank circle the piston is at beginning or end of stroke; that when the crank pin is in the vertical positions the piston is about midway in either half stroke; and that in a general way admission and exhaust should begin to occur about the times when the piston begins or ends a half stroke; it will be seen that, to accomplish the first purpose, the valve should begin to open either port to steam and the other to exhaust about the time when the crank pin and rod are in the horizontal positions. Now, remembering further that an eccentric turning on an axle is essentially itself a crank, the distance of the centre of its form from the centre of motion being the length of crank, and double this distance being the diameter of this crank

circle, and also the length of the throw the revolution of the eccentric will impart to its rod; that the valve moving either way from its iniddle position will be just beginning to uncover the ports; and that it will be in such position when the centre of form of the eccentric is in the vertical positions in respect to the axle, it will be seen that, with a single eccentric and rod, the first object is attained by setting the eccentric so that its radius shall be "quartering" or at right angles with the line of the crank pin from the same axle. Thus, the general result secured is, that whenever the crank is in horizontal position, and the piston at beginning or end of stroke has its slowest movement, the eccentric is vertical, and the valve in middle of its throw has the most rapid movement, as required for duly opening the ports; and vice versa. An engine moving at high speed, say 38 miles an hour, or 1,093 yards a minute, the driving wheels 5 ft. 3 in. in diameter, or about 16 ft. 6 in. in circumference, the number of strokes of each piston must be about 200, and of separate or half strokes 400 per minute; if the length of each be 18 inches, this gives the piston an average velocity of 192 yards per minute, or 10 feet per second, i. e., more than three times the usual speed in stationary engines; thus it will appear how important is the proper timing of the steam changes within the cylinder, and how nice must be the adjustments required for economical and perfect working. The third of the

objects above named is, with any method of valve motion, accomplished by means of a reversing handle at the command of the engineer, which by lever and links is made to detach the eccentric rod or reverse its action on the valve, and consequently the direction of movement of the crank and of the wheels. This is employed as an auxiliary means of arresting the movement of the engine and train at high speed, or for the purpose of backing the engine. The second of the three objects named above is that which has presented the chief difficulty. The outer edges of a slide valve determine the times of admission and of suppression; the inner, the times of release and compression. Outside "lap" of valve conspires with

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and throw of the valve, control the distribution of the steam; and any change in either affects the distribution in a definable way. Up to about 1843, a variety of complicated expansion gear for locomotive slide valves had been devised, for varying the travel or length of throw of the valve during stroke, and hence the expansion; and though the link motion then introduced has since come into almost universal use, yet such devices, with single or with complex or cut-off slides, have continued since to be presented. Of the link motion, the simplest and most satisfactory solution of the problem, the construction in one form of shifting link is represented in fig. 9, while the mode of action will be better understood by aid of the diagram, fig. 10. In the figures, vr is the valve

FIG. 10.

rod, which by a stud or pin within the link can be made to stand at different heights within it; 77 is the link; e is the fore and e' the back eccentric, jointly giving movement to the link; their centre of motion is at o, their centres of form, c, c; oc, o c are their radii; f is the fore and b the back eccentric rod, the relative positions appearing reversed in the two figures; O is the crank; o a its radius; ara a half revolution of the crank; v is the slide valve; pp are the steam ports and E the exhaust, to the cylinder; n represents the reversing link, h the reversing lever; but the mode of reversing is better shown in fig. 9, in which k is the supporting link, B a bent lever, with counterpoise W; RR a rod from the lever B to the reversing lever or handle, L, at the hand of the en

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