Chapter I (11)

(145.) The machinery which has been explained in the preceding chapters, consisting of the cylinder with its passages and valves, the piston-rod, parallel motion, beam, connecting-rod and crank, together with the condenser, air-pump, and other appendages, having no source of moving power in themselves, must be regarded as mere instruments by which the mechanical effect developed by the furnace and the boiler is transmitted to the working point and so [Pg252] modified as to be adapted to the uses to which the machine is applied. The boiler is at once a magazine in which the moving power is stored in sufficient quantity to supply the demands of the engine and an apparatus in which that power is fabricated. The mechanical effect evolved in the conversion of water into steam by heat, is the process by which the power of the steam-engine is produced, and space is provided in the boiler, capacious enough to contain as much steam as is necessary for the engine, besides a sufficient quantity of water to continue that supply undiminished, notwithstanding the constant drafts made upon it by the cylinder: even the water itself, from the evaporation of which the mechanical power is produced, ought to be regarded as an instrument by which the effect of the heat of the combustible is rendered mechanically efficient, inasmuch as the same heat, applied not only to other liquids but even to solids, would likewise be productive of mechanical effects. The boiler and its furnace are therefore parts of the steam-engine, the construction and operation of which are entitled to especial attention.

(146.) COAL, the combustible almost universally used in steam-engines, is a substance, the principal constituents of which are _carbon_ and _hydrogen_, occasionally mixed with sulphur in a small proportion, and earthy incombustible matter. In different sorts of coal the proportions of these constituents vary, but in coal of good quality about three quarters of the whole weight of the combustible is carbon.

When carbon is heated to a temperature of about 700° in an atmosphere of pure oxygen, it will combine chemically with that gas, and the product will be the gas called _carbonic acid_. The volume of carbonic acid produced by this combination, will be exactly equal to that of the oxygen combined with the carbon, and therefore the weight of a given volume of the gas will be increased by the weight of carbon which enters the combination. It is found that two parts by weight of oxygen combined with three of carbon, form carbonic acid. The weight of the carbonic acid, therefore, produced in the combustion, will be greater than the weight of the oxygen, bulk for bulk, in the proportion of five to two, the volume being the same and the gases being [Pg253] compared at the same temperatures and under equal pressures. In this combination heat is evolved in very large quantities. This effect arises from the heat previously latent in the carbon and oxygen being rendered sensible in the process of combustion. The carbonic acid proceeding from the combustion is by such means raised to a very high temperature, and the carbon during the process acquires a heat so intense as to become luminous; no flame, however, is produced.

Hydrogen, heated to a temperature of about 1000°, in contact with oxygen will combine with the latter, and a great evolution of heat will attend the process; the gases will be rendered luminous, and flame will be produced. The product of this process will be water, which being exposed to the intense heat of combustion, will be immediately converted into steam. Hydrogen combines with eight times its own weight of oxygen, producing nine times its own weight of water.

Hydrogen gas is, however, not usually disengaged from coal in a simple form, but combined chemically with a certain portion of carbon, the combination being called carburetted hydrogen. Pure hydrogen burns with a very faintly luminous blue flame, but carburetted hydrogen gives that bright flame occasionally having an orange or reddish tinge, which is seen to issue from burning coals: this is the gas used for illumination, being expelled from the coal by the process of coking, and conducted to the various burners through proper pipes.

The sulphur, which in a very small proportion is contained in coals, is also combustible, and combines in the process of combustion with oxygen, forming sulphurous acid: it is also sometimes evolved in combination with hydrogen, forming sulphuretted hydrogen.

Atmospheric air consists of two gases, azote and oxygen, mixed together in the proportion of four to one; five cubic feet of atmospheric air consisting of four cubic feet of azote and one of oxygen. Any combustible will combine with the oxygen contained in atmospheric air, if raised to a temperature somewhat higher than that which is necessary to cause its combustion in an atmosphere of pure oxygen.

If coals, therefore, or other fuel exposed to atmospheric [Pg254] air, be raised to a sufficiently high temperature, their combustible constituents will combine with the oxygen of the atmospheric air, and all the phenomena of combustion will ensue. In order, however, that the combustion should be continued, and should be carried on with quickness and activity, it is necessary that the carbonic acid, and other products, should be removed from the combustible as they are produced, and fresh portions of atmospheric air brought into contact with it; otherwise the combustible would soon be surrounded by an atmosphere composed chiefly of carbonic acid to the exclusion of atmospheric air, and therefore of uncombined oxygen, and consequently the combustion would cease, and the fuel be extinguished. To maintain the combustion, therefore, a current of atmospheric air must be constantly carried through the fuel: the quantity and force of this current must depend on the quantity and quality of the fuel to be consumed. It must be such that it shall supply sufficient oxygen to the fuel to maintain the combustion, and not more than sufficient, since any excess would be attended with the effect of absorbing the heat of combustion, without contributing to the maintenance of that effect.

Heat is communicated from body to body in two ways, by radiation and by contact.

Rays of heat issue from a heated body, and are dispersed through the surrounding space in a manner, and according to laws, similar to those which govern the radiation of light. The heat thus radiated meeting other bodies is imparted to them, and penetrates them with more or less facility according to their physical qualities.

A heated body also brought into contact with another body of lower temperature, communicates heat to that other body, and will continue to do so until the temperature of the two bodies in contact shall be equalised. Heat proceeds from fuel in a state of combustion in both these ways: the heated fuel radiates heat in all directions around it, and the heat thus radiated will be imparted to all parts of the furnace which are exposed to the fuel.

The gases, which are the products of the combustion, escape from the fuel at a very high temperature, and consequently, in acquiring that temperature they absorb a considerable [Pg255] quantity of the heat of combustion. But besides the gases actually formed in the process of combustion, the azote forming four fifths of the air carried through the fuel to support the combustion, absorbs heat from the combustible, and rises into the upper part of the furnace at a high temperature. These various gases, if conducted directly to the chimney, would carry off with them a considerable quantity of the heat. Provision should therefore be made to keep them in contact with the boiler such a length of time as will enable them to impart such a portion of the heat which they have absorbed from the fuel, as will still leave them at a temperature sufficient, and not more than sufficient, to produce the necessary draft in the chimney.

(147.) The forms of boiler which have been proposed as the most convenient for the attainment of all these requisite purposes have been very various. If strength alone were considered, the spherical form would be the best; and the early boilers were very nearly hemispheres, placed on a slightly concave base. The form adopted by Watt, called the waggon boiler, consists of a semi-cylindrical top, flat perpendicular sides, flat ends, and a slightly concave bottom. The steam intended to be used in boilers of this description did not exceed the pressure of the external atmosphere by more than from 3 to 5 lbs. per square inch; and the flat sides and ends, though unfavourable to strength, could be constructed sufficiently strong for this purpose. In a boiler of this sort, the air and smoke passing through the flues that are carried round it, are in contact at one side only with the boiler. The brickwork, or other materials forming the flue, must therefore be non-conductors of heat, that they may not absorb any considerable portion of heat from the air passing in contact with them. A boiler of this form is represented in _fig._ 71.

The grate and a part of the flues are rendered visible by the removal of a portion of the surrounding masonry in which the boiler is set. The interior of the boiler is also shown by cutting off one half of the semi-cylindrical roof. A longitudinal vertical section is shown in _fig._ 72., and a cross section in _fig._ 73. A horizontal section taken above the level of the grate, and below the level of the water in the boiler, showing [Pg256] the course of the flues, is given in _fig._ 74. The corresponding parts in all the figures are marked by the same letters.

The door by which fuel is introduced upon the grate is represented at A, and the door leading to the ash-pit at B. The fire bars at C slope downwards from the front at an angle of about 25°, giving a tendency to the fuel to move from the front towards the back of the grate. The ash-pit D is constructed of such a magnitude, form, and depth, as to admit a current of atmospheric air to the grate-bars, sufficient to sustain the combustion. The form of the ash-pit is usually wide below, contracting towards the top.

The fuel when introduced at the fire-door A, should be laid on that part of the grate nearest to the fire-door, called the dead plates: there it is submitted to the process of coking, by which the gases and volatile matter which it contains are expelled, and being carried by a current of air, admitted [Pg257] through small apertures in the fire-door over the burning fuel in the hinder part of the grate, they are burnt. When the fuel in front of the grate has been thus _coked_, it is pushed back, and a fresh feed introduced in front. The coal thus pushed back soon becomes vividly ignited, and by continuing this process, the fuel spread over the grate is maintained in the most active state of combustion at the hinder part of the grate. By such an arrangement, the smoke produced by the combustion of the fuel may be burnt before it enters the flues. The flame and heated air proceeding from the burning fuel arising from the grate, and rushing towards the back of the furnace, passes over the _fire-bridge_ E, and is carried through the flue F which passes under the boiler. This flue (the cross section of which is shown in _fig._ 73., by the dark shade put under the boiler) is very nearly equal in width to the bottom of the boiler, the space at the bottom of the boiler, near the corners, being only what is sufficient to give the weight of the boiler support on the masonry forming the [Pg258] sides of the flue. The bottom of the boiler being concave, the flame and heated air as they pass along the flue rise to the upper part by the effects of their high temperature, and _lick_ the bottom of the boiler from the fire-bridge at E to the further end G.

At G the flue rises to H, and turning to the side of the boiler at I I, conducts the flame in contact with the side from the back to the front; it then passes through the flue K across the front, and returns to the back by the other side [Pg259] flue L. The side flue is represented, stripped of the masonry, in _fig._ 71., and also appears in the plan in _fig._ 74., and in the cross section in _fig._ 73. The course of the air is represented in _fig._ 74. by the arrows. From the flue L the air is conducted into the chimney at M.

By such an arrangement, the flame and heated air proceeding from the grate are made to circulate round the boiler, and the length and magnitude of the flues through which it is conducted should be such, that when it shall arrive at the chimney its temperature shall be reduced, as nearly as is consistent with the maintenance of draught in the chimney, to the temperature of the water with which it is in contact.

The method of feeding the furnace, which has been described above, is one which, if conducted with skill and care, would produce a much more perfect combustion of the fuel than would attend the common method of filling the grate from the back to the front with fresh fuel, whenever the furnace is fed. This method, however, is rarely observed in the management of the furnace. It requires the constant attention of the stokers (such is the name given to those who feed the furnaces). The fuel must be supplied, not in large quantities, and at distant intervals, but in small quantities and more frequently. On the other hand, the more common practice is to allow the fuel on the grate to be in a great degree burned away, and then to heap on a large quantity of fresh fuel, covering over with it the burning fuel from the back to the front of the grate. When this is done, the heat of the ignited coal acting upon the fresh fuel introduced, expels the gases combined with it and, mixed with these, a quantity of carbon, in a state of minute division, forming an opaque black smoke. This is carried through the flues and drawn up the chimney. The consequence is, that not only a quantity of solid fuel is sent out of the chimney unconsumed, but the hydrogen and other gases also escape unburned, and a proportional waste of the combustible is produced; besides which, the nuisance of an atmosphere filled with smoke ensues. Such effects are visible to all who observe the chimneys of steam-vessels, while the engine is in operation. When the furnaces are thus filled with fresh fuel, a large volume of [Pg260] dense black smoke is observed to issue from the chimney. This gradually subsides as the fuel on the grate is ignited, and does not reappear until a fresh feed is introduced.

This method of feeding, by which the furnace would be made to consume its own smoke, and the combustion of the fuel be rendered complete, is not however free from counteracting effects. In ordinary furnaces the feed can only be introduced by opening the fire-doors, and during the time the fire-doors are opened a volume of cold air rushes in, which passing through the furnace is carried through the flues to the chimney. Such is the effect of this in lowering the temperature of the flues, that in many cases the loss of heat occasioned is greater than any economy of fuel obtained by the complete consumption of smoke. Various methods, however, may be adopted by which fuel may be supplied to the grate without opening the fire-doors, and without disturbing the supply of air to the fire. A hopper built into the front of the furnace, with a moveable bottom or valve, by which coals may be allowed to drop in from time to time upon the front of the grate, would accomplish this.

(148.) In order to secure the combustion of the gases evolved from the coals placed in the front of the grate, it is necessary that a supply of atmospheric air should be admitted with them over the burning fuel. This is effected by small apertures or regulators, provided in the fire-doors, governed by sliding-plates, by which they may be opened or closed to any required extent.

A patent has recently been granted to Mr. Williams, one of the directors of the City of Dublin Steam Navigation Company, for a method of consuming the unburned gases which escape from the grate, and are carried through the flues. This method consists in introducing into the flue tubes placed in a vertical position, the lower ends of which being inserted in the bottom of the flue are made to communicate with the ash-pit, and the upper ends of which are closed. The sides and tops of these tubes are pierced with small holes, through which atmospheric air drawn from the ash-pit issues in jets. The oxygen supplied by this air immediately combines with the carburetted hydrogen, which [Pg261] having escaped from the furnace unburned is carried through the flues at a sufficient temperature to enter into combination with the oxygen admitted through holes in the tubes. A number of jets of flame thus proceed from these holes, having an appearance similar to the flame of a gas-lamp.

It is evident that such tubes must be inefficient unless they are placed in the flues so near the furnace, that the temperature of the unburned gases shall be sufficiently high to produce their combustion.

(149.) The magnitude of the grate and ash-pit must be determined by the rate at which the evaporation is required to be conducted in the boiler and the quality of the fuel. It must be a matter of regret, that the proportions of the various parts of steam-engines, with their boilers and furnaces, have not been determined by any exact or satisfactory experiments; and those who project and manufacture the engines themselves, are not less in ignorance on those points than others. With coals of the common quality a certain average proportion must exist between the necessary magnitude of the grate-surface and the quantity of water to be evaporated in a given time in the boiler. But what that proportion is for any given quality of fuel, is at present unascertained. Each engine-maker follows his own rule, and the rule thus followed is in most cases a matter of bare conjecture, unsupported by any experimental evidence. Some engine-makers will allow a square foot of grate-surface for every cubic foot of water per hour, which is expected to be evaporated in the boiler; others allow only half a square foot: and practice varies between these limits. Bituminous coals which melt and cake, and which burn with much flame and smoke, must be spread more thinly on the grate than other descriptions of fuel, otherwise a considerable quantity of combustible gases would be dismissed into the flues unburnt. Such coals therefore, other circumstances being the same, require a larger portion of grate-surface; and the same may be said of coals which produce clinkers in their combustion, and form lumps of vitrified matter on the grate, by which the spaces between the grate-bars are speedily closed up. When such fuel is used, the grate-bars require to be frequently raked [Pg262] out, otherwise the spaces between them being obstructed, the draught would become insufficient for the due combustion of the fuel.

To facilitate the raking out of the grate, the bars are placed with their ends towards the fire-door: they are usually made of cast-iron, from two to two inches and a half wide on the upper surface, with intervals of nearly half an inch between them. The bars taper downwards, their under surfaces being much narrower than their upper, the spaces between them thus widening, to facilitate the fall of the ashes between them. The grate-bars slope downwards from the front to the back. The height of the centre of the bottom of the boiler, above the front of the grate, is usually about two feet, and about three feet above the back of it. The concave bottom of the boiler, however, brings its surfaces at the slide closer to the grate.

(150.) Between the evaporating power of the boiler, and the magnitude of surface it exposes to the action of the furnace, there is a relation which, like that of the grate surface, has never been ascertained by any certain or satisfactory experimental investigation; much less have the different degrees of efficiency attending different parts of the boiler-surface been determined. That part of the surface of the boiler immediately over and around the grate, is exposed to the immediate radiation of the burning fuel, and is therefore probably the most efficient in the production of steam. The tendency of flame and heated air to rise, would naturally bring them in the flues into closer contact with those parts of the boiler-surface which are horizontal in their position, and which form the tops of the flues, than with those which are lateral or vertical in their position, and which form the sides of the flues. In a boiler constructed like that already described, the flue-surface therefore, which would be most efficient, would be the concave bottom of the boiler extending from the fire-bridge to its remote end. In some boilers, especially those in which steam of high pressure is produced, the form is cylindrical, the middle flue being formed into an elliptical tube the greater axis of which is horizontal from end to end of the boiler. It seems doubtful, however, [Pg263] whether in such a boiler the heat produces any useful effect on the water below the flue, the water above being always at a higher temperature, and therefore lighter than that below, and consequently no currents being established between the upper and lower strata of the water.

It was considered by Mr. Watt, but we are not aware on what experimental grounds, that from eight to ten square feet of heating surface were sufficient to produce the evaporation of one cubic foot of water per hour. The practice of engine-makers since that time has been to increase the allowance of heating surface for the same rate of evaporation. Engine-builders have varied very much in this respect, some allowing twelve, fifteen, and even eighteen square feet of surface for the same rate of evaporation. It must, however, still be borne in mind, that whether this increased allowance did or did not produce the actual evaporation imputed to it, has not been, as far as we are informed, ever accurately ascertained. The production of a given rate of evaporation by a moderate heat diffused over a larger surface, rather than by a fiercer temperature confined to a smaller surface, is attended with many practical advantages. The plates of the boiler acted upon by the fire are less exposed to oxydisation, and the boiler will be proportionally more durable.

(151.) Besides presenting to the action of the fire a sufficient surface to produce steam at the required rate, the capacity of the boiler must be proportioned to the quantity of water to be evaporated. The space within the boiler is appropriated to a twofold purpose: 1st, To contain the water to be evaporated; 2dly, To contain a quantity of ready made steam for the supply of the cylinder. If the space appropriated to the steam did not bear a considerable proportion to the magnitude of the cylinder, the momentary expansion of the steam passing to the cylinder from the boiler at each stroke would reduce the pressure of the steam in a great proportion, and unless the pressure in the boiler were considerably greater than that which the steam is intended to have in the cylinder, the pressure in the latter would be reduced below the proper amount. The proportion of the [Pg264] steam space in the boiler to the magnitude of the cylinder has been very variously estimated, nor can it be said that any practical rule of a general kind has been adopted. It is held by some that the steam-space will be sufficient if it contain five times the quantity of steam consumed at each stroke, while others maintain that it should contain at least ten times that quantity, and opinions vary between these limits.

(152.) The proportion of water-space in the boiler to its evaporating power should also be regulated, so that the introduction of the feed at a comparatively low temperature may not unduly chill the water in the boiler. Supposing the feed to be introduced in a low pressure boiler at the temperature of 100°, and that the necessary temperature within the boiler be 225°, the quantity of water it contains should be about five times the quantity evaporated, and therefore also five times the quantity introduced through the feed per hour. For every cubic foot of water per hour therefore, intended to be evaporated by the boiler, water-space for five cubic feet should be provided. It is, however, right to repeat that this (like almost every other so called rule) is the result not of any exact general calculation, but one deduced from the custom which has obtained among the manufacturers of steam-engines.

(153.) The surface of the water in the boiler should always be above the range of the flues. When the heated air in the flues acts upon a part of the boiler within which water is contained, the water within receiving an increased temperature becomes, bulk for bulk, lighter than the strata of water above it, and ascends. It is replaced by the descending strata, which, in their turn receiving increased temperature, rise to the surface; or if the action of the heat convert the water into steam, the bubbles of steam rise to the surface, fresh portions of water continually coming into contact with the boiler-plates on which the heated air or flame acts. By this process the boiler-plates are continually cooled, either by being successively washed by water at a lower temperature, or by the heat taken from them becoming latent in the steam bubbles formed in contact with them. But if the heat act [Pg265] upon a part of the boiler containing steam within it, which steam being a slow recipient of heat, and no currents being established, nor any phenomenon produced in which heat is rendered latent, the heat of the fire communicated to the boiler-plates accumulates in them, and raises their temperature to an injurious degree. The plates may by this means be softened, so as to cause the boiler to burst, or the difference between the expansion of the highly heated plates thus exposed to fire in contact with steam and that of the plates which are cooled by contact with water, may cause the joinings of the boiler-plates to open, and the boiler to leak. By whatever means, therefore, the boiler be fed, care should be taken that the evaporation should not be allowed to reduce the level of the water in it below the highest flue.

(154.) As the water by which the boiler is fed must always have a much lower temperature than that at which the boiler is maintained, the supply of the feed will have a constant tendency to lower the temperature of the water, and this tendency will be determined by the proportion between the magnitude of the feed and the quantity of water in the boiler.

Since it is requisite that the level of the water in the boiler shall not suffer any considerable change, it is evident that the magnitude of the feed must be equal to the quantity of water evaporated. If it were less, the level of the water would continually fall by reason of the excess of the evaporation over the feed; and if it were greater, the level would rise by the accumulation of water in the boiler. If therefore the quantity of water-space allowed in the boiler be five times the volume of water evaporated per hour, the quantity introduced by the feed per hour, whether continuously or at intervals, must be of the same amount. Since the process of evaporation is continuous, the variation of level of water in the boiler will be entirely dependent on the intervals between the successive feeds. If the feed be continuous, and always equal to the evaporation, then the level of the water in the boiler will undergo no change; but if while the evaporation is continuous the feed be made at intervals, then the change of level of water in the boiler as [Pg266] well as its change of temperature, will be subject to a variation proportional to the intervals between the successive feeds. It is manifest, therefore, that the feed should either be uninterrupted or be supplied at short intervals, so that the change of level and temperature of the water in the boiler should not be considerable.

(155.) Different methods have been, from time to time, suggested for indicating the level of the water in the boiler. We have already mentioned the two gauge-pipes used in the earlier steam-engines (31.), and which are still generally continued. There are, however, some other methods which merit our attention.

A weight F (_fig._ 75.), half immersed in the water in the boiler, is supported by a wire, which, passing steam-tight through a small hole in the top, is connected by a flexible string, or chain, passing over a wheel W, with a counterpoise A, which is just sufficient to balance F when half immersed. If F be raised above the water, _A_ being lighter will no longer balance it, and F will descend pulling up A, and turning the wheel W. If, on the other hand, F be plunged deeper in the water, A will more than balance it, and will pull it up, so that the only position in which F and A will balance each other is, when F is half immersed. The wheel W is so adjusted, that when two pins placed on its rim are in the horizontal position, the water is at its proper level. Consequently it follows, that if the water rise above this level, the weight F is lifted and A falls, so that the pins come into another position. If, on the other hand, the level of the water fall, F falls and A rises, so that the pins assume a different position. Thus, in general, the position of the pins becomes an indication of the quantity of water in the boiler.

(156.) Another method is to place a glass tube (_fig._ 76.), with one end T entering the boiler above the proper level, and the other end T′ entering it below the proper level. It must [Pg267] be evident that the water in the tube will always stand at the same level as the water in the boiler, since the lower part has a free communication with that water, while the surface is submitted to the pressure of the same steam as the water in the boiler. This and the last-mentioned gauge have the advantage of addressing the eye of the engineer at once, without any adjustment; whereas the gauge-cocks must be both opened, whenever the depth is to be ascertained.

These gauges, however, require the frequent attention of the engine-man; and it becomes desirable either to find some more effectual means of awakening that attention, or to render the supply of the boiler independent of any attention. In order to enforce the attention of the engine-man to replenish the boiler when partially exhausted by evaporation, a tube was sometimes inserted at the lowest level to which it was intended that the water should be permitted to fall. This tube was conducted from the boiler into the engine-house, where it terminated in a mouth-piece or whistle, so that whenever the water fell below the level at which this tube was inserted in the boiler, the steam would rush through it, and issuing with great velocity at the mouth-piece, would summon the engineer to his duty with a call that would rouse him even from sleep.

(157.) In the most effectual of these methods, the task of replenishing the boiler should still be executed by the engineer; and the utmost that the boiler itself was made to do, was to give due notice of the necessity for the supply of water. The consequence was, among other inconveniences, that the level of the water was subject to constant variation.

To remedy this a method has been invented, by which [Pg268] the engine is made to feed its own boiler. The pipe G (_fig._ 77.), which leads from the hot water pump, terminates in a small cistern C in which the water is received. In the bottom of this cistern, a valve V is placed, which opens upwards, and communicates with a feed-pipe, which descends into the boiler below the level of the water in it. The stem of the valve V is connected with a lever turning on the centre D, and loaded with a weight F dipped in the water in the boiler in a manner similar to that described in _fig._ 75., and balanced by a counterpoise A in exactly the same way. When the level of the water in the boiler falls, the float F falls with it, and pulling down the arm of the lever raises the valve V, and lets the water descend into the boiler from the cistern C. When the boiler has thus been replenished, and the level raised to its former place, F will again be raised, and the valve V closed by the weight A. In practice, however, the valve V adjusts itself by means of the effect of the water on the weight F, so as to permit the water from the feeding-cistern C to flow in a continued stream, just sufficient in quantity to supply the consumption from evaporation, and to maintain the level of the water in the boiler constantly the same.

By this arrangement the boiler is made to replenish itself, or, more properly speaking, it is made to receive such a supply, as that it never wants replenishing, an effect which no effort of attention on the part of an engine-man could produce. But this is not the only good effect produced by this contrivance. A part of the steam which originally left the boiler, and having discharged its duty in moving the piston, was condensed and reconverted into water, and lodged by the air-pump in the hot well (_fig._ 77.), is here again restored to the source from which it came, bringing back all the unconsumed portion of its heat preparatory to being once more put in circulation through the machine.

The entire quantity of hot water pumped into the cistern C, is not always necessary for the boiler. A waste-pipe may be provided for carrying off the surplus, which may be turned to any purpose for which it may be required; or it may be discharged into a cistern to cool, preparatory to [Pg269] being restored to the cold cistern, in case water for the supply of that cistern be not sufficiently abundant.

(158.) Another method of arranging a self-regulating feeder is shown in _fig._ 78. A is a hollow ball of metal attached to the end of a lever, whose fulcrum is at B. The other arm of the lever C is connected with the stem of a spindle-valve, communicating with a tube which receives water from the feeding-cistern. Thus, when the level of the water in the boiler subsides, the ball A preponderating over the weight of the opposite arm, the lever falls, the arm C rises and opens the valve, and admits the feeding water. This apparatus will evidently act in the same manner and on the same principles as that already described. [Pg270]

The mouth of the tube by which the feed is introduced should be placed at that part of the boiler which is nearest the end of the flues which issue into the chimney. By such means the temperature of the water in contact with those flues will be lowest at the place where the temperature of the heated air intended to act upon it is also lowest. The difference of the temperatures will therefore be greater than it would be if the point of the boiler containing water of a higher temperature was left in contact with this part of the flue.

(159.) It is necessary to have a ready method of ascertaining at all times the pressure of the steam which is used in working the engine. For this purpose a bent tube containing mercury is inserted into some part of the apparatus, which has free communication with the steam. Let A B C (_fig._ 79.) be such a tube. The pressure of the steam forces the mercury down in the leg A B, and up in the leg B C. If the mercury in both legs be at exactly the same level, the pressure of the steam must be exactly equal to that of the atmosphere; because the steam pressure on the mercury in A B balances the atmospheric pressure on the mercury in B C. If, however, the level of the mercury in B C be above the level of the mercury in B A, the pressure of the steam will exceed that of the atmosphere. The excess of its pressure above that of the atmosphere may be found by observing the difference of the level of the mercury in the tubes B C and B A, allowing a pressure of one pound on each square inch for every two inches in the difference of the levels.

If, on the contrary, the level of the mercury in B C should fall below its level in A B, the atmospheric pressure will [Pg271] exceed that of the steam, and the quantity of the excess may be ascertained exactly in the same way.

If the tube be glass, the difference of levels of the mercury would be visible; but it is most commonly made of iron; and in order to ascertain the level, a thin wooden rod with a float is inserted in the open end of B C, so that the portion of the stick within the tube indicates the distance of the level of the mercury from its mouth. A bulb or cistern of mercury might be substituted for the leg A B, as in the common barometer. This instrument is called the _steam-gauge_.

If the steam-gauge be used as a measure of the strength of the steam which presses on the piston, it ought to be on the same side of the throttle-valve (which is regulated by the governor) as the cylinder; for if it were on the same side of the throttle-valve with the boiler, it would not be affected by the changes which the steam may undergo in passing through the throttle-valve, when partially closed by the agency of the governor.

For boilers in which steam of very high pressure is used, as in those of locomotive engines, a steam-gauge, constructed on the above principle, would have inconvenient or impracticable length. In such boilers the pressure of the steam is equal to four or five times that of the atmosphere, to indicate which the column of mercury in the steam-gauge would be four or five feet in height. In such cases a thermometer-gauge may be used with advantage. The principle of this gauge is founded on the fact, that between the pressure and temperature of steam produced in contact with water there is a fixed relation, the same temperature always corresponding to the same pressure. If, therefore, a thermometer be immersed in the boiler which shall show the temperature of the steam, a scale may be attached to it, on which shall be engraved the corresponding pressures. Such gauges are now very generally used on locomotive engines.

(160.) The force with which the piston is pressed depends on two things, 1st, the actual strength of the steam which presses on it; and, 2dly, on the actual strength of the vapour which resists it. For although the vacuum produced by the method of separate condensation be much more perfect than [Pg272] what had been produced in the atmospheric engines, yet still some vapour of a small degree of elasticity is found to be raised from the hot water in the bottom of the condenser before it can be extracted by the air-pump. One of these pressures is indicated by the steam-gauge already described; but still, before we can estimate the force with which the piston descends, it is necessary to ascertain the force of the vapour which remains uncondensed, and resists the motion of the piston. Another gauge, called the barometer-gauge, is provided for this purpose. A glass tube A B (_fig._ 80.), more than thirty inches long and open at both ends, is placed in an upright or vertical position, having the lower end B immersed in a cistern of mercury C. To the upper end is attached a metal tube, which communicates with the condenser, in which a constant vacuum, or rather high degree of rarefaction, is sustained. The same vacuum must therefore exist in the tube A B, above the level of the mercury, and the atmospheric pressure on the surface of the mercury in the cistern C will force the mercury up in the tube A B, until the column which is suspended in it is equal to the difference between the atmospheric pressure and the pressure of the uncondensed steam. The difference between the column of mercury sustained in this instrument and in the common barometer, will determine the strength of the uncondensed steam, allowing a force proportional to one pound per square inch for every two inches of mercury in the difference of the two columns. In a well-constructed engine which is in good order, there is very little difference between the altitude in the barometer-gauge and the common barometer.

To compute the force with which the piston descends, thus becomes a very simple arithmetical process. First, ascertain the difference of the levels of the mercury in the steam-gauge; this gives the excess of the steam pressure above the atmospheric pressure. Then find the height of the mercury in the barometer-gauge; this gives the excess of the atmospheric pressure above the uncondensed steam. Hence, if these two heights be added together, we shall obtain the [Pg273] excess of the impelling force of the steam from the boiler, on the one side of the piston, above the resistance of the uncondensed steam on the other side: this will give the effective impelling force. Now, if one pound be allowed for every two inches of mercury in the two columns just mentioned, we shall have the number of pounds of impelling pressure on every square inch of the piston. Then, if the number of square inches in the section of the piston be found, and multiplied by the number of pounds on each square inch, the force with which it moves will be obtained.

From what we have stated it appears that, in order to estimate the force with which the piston is urged, it is necessary to refer to both the barometer and the steam-gauge. This double computation may be obviated by making one gauge serve both purposes. If the end C of the steam-gauge (_fig._ 79.), instead of communicating with the atmosphere were continued to the condenser, we should have the pressure of the steam acting upon the mercury in the tube B A, and the pressure of the uncondensed vapour which resists the piston acting on the mercury in the tube B C. Hence the difference of the levels of the mercury in the tubes would at once indicate the difference between the force of the steam and that of the uncondensed vapour, which is the effective force with which the piston is urged.

(161.) But these methods of determining the effective force by which the piston is urged, can only be regarded as approximations, and not very perfect ones. If the condensation of steam on one side of the piston were instantaneously effected, or the uncondensed vapour were of the same tension during the whole stroke; and if, besides this, the pressure of steam on the piston were of uniform intensity from the beginning to the end of the stroke, then the steam and barometer gauges taken together would become an accurate index of the effective force of steam on the piston: but such is not the case. When the steam is first admitted through the steam-valve it acts on the piston with a pressure which is first slightly diminished, and afterwards a little increased, until it arrives at that part of the stroke at which the steam-valve is closed, after which the pressure is diminished. The [Pg274] pressure, therefore, urging the piston is subject to variation; but the pressure of the uncondensed vapour on the other side of the piston is subject to still greater change. At the moment the exhausting-valve is opened, the piston is relieved from the pressure upon it by the commencement of the condensation; but this process during the descent of the piston is gradual, and the vacuum is rendered more and more perfect, until the piston has nearly attained the limit of its play. These variations, both as well of the force urging the piston as of the force resisting it, are such as not to be capable of being accurately measured by a mercurial column, since they would produce oscillations in such a column, which would render any observations of its mean height impracticable.

To measure the mean efficient force of the piston, taking into account these circumstances, Mr. Watt invented an instrument, which, like all his mechanical inventions, has answered its purpose perfectly, and is still in general use. This instrument, called an _indicator_, consists of a cylinder of about 1-3/4 inch in diameter, and 8 inches in length. It is bored with great accuracy, and fitted with a solid piston moving steam-tight in it with very little friction. The rod of this piston is guided in the direction of the axis of the cylinder through a collar in the top, so as not to be subject to friction in any part of its play. At the bottom of the cylinder is a pipe governed by a stop-cock and turned in a screw, by which the instrument may be screwed on the top of the steam-cylinder of the engine. In this position, if the stop-cock of the indicator be opened, a free communication will be made between the cylinder of the indicator and that of the engine. The piston-rod of the indicator is attached to a spiral spring, which is capable of extension and compression, and which by its elasticity is capable of measuring the force which extends or compresses it in the same manner as a spring steel-yard or balance. If a scale be attached to the instrument at any point on the piston-rod to which an index might be attached, then the position of that index upon the scale would be governed by the position of the indicator-piston in its cylinder. If any force pressed the indicator-piston upwards, so as to compress the spring, [Pg275] the index would rise upon the scale; and if, on the other hand, a force pressed the indicator-piston downwards, then the spiral spring would be extended, and the index on the piston-rod descend upon the scale. In each case the force of the spring, whether compressed or extended, would be equal to the force urging the indicator-piston, and the scale might be so divided as to show the amount of this force.

Now, let the instrument be supposed to be screwed upon the top of the cylinder of a steam-engine, and the stop-cock opened so as to leave a free communication between the cylinder of the indicator below its piston and the cylinder of the steam-engine above the steam-piston. At the moment the upper steam-valve is opened, the steam rushing in upon the steam-piston will also pass into the indicator, and press the indicator-piston upwards: the index upon its piston-rod will point upon the scale to the amount of pressure thus exerted. As the steam-piston descends, the indicator-piston will vary its position with the varying pressure of the steam in the cylinder, and the index on the piston-rod will play upon the scale, so as to show the pressure of the steam at each point during the descent of the piston.

If it were possible to observe and record the varying position of the index on the piston-rod of the indicator, and to refer each of these varying positions to the corresponding point of the descending stroke, we should then be able to declare the actual pressure of the steam at every point of the stroke. But it is evident that such an observation would not be practicable. A method, however, was contrived by Mr. Southern, an assistant of Messrs. Boulton and Watt, by which this is perfectly effected. A square piece of paper, or card, is stretched upon a board, which slides in grooves formed in a frame. This frame is placed in a vertical position near the indicator, so that the paper may be moved in a horizontal direction backwards and forwards, through a space of fourteen or fifteen inches. Instead of an index a pencil is attached to the indicator of the piston-rod: this pencil is lightly pressed by a spring against the paper above mentioned, and as the paper is moved in a horizontal direction [Pg276] under the pencil, would trace upon the paper a line. If the pencil were stationary this line would be straight and horizontal, but if the pencil were subject to a vertical motion, the line traced on the paper moved under the pencil horizontally would be a curve, the form of which would depend on the vertical motion of the pencil. The board thus supporting the paper is put into connexion by a light cord carried over pulleys with some part of the parallel motion, by which it is alternately moved to the right and to the left. As the piston ascends or descends, the whole play of the board in the horizontal direction will therefore represent the length of the stroke, and every fractional part of that play will correspond to a proportional part of the stroke of the steam-piston.