The beam,' so frequently alluded to, was obviously the readiest mode of connecting the alternating motion of the piston with the pump to be worked, in the atmospheric engine; and owing to the facilities it offers of working the plug-tree and the three pumps necessary in Watt's condensing engines, continued to form a part of the arrangement whether the engine were intended to pump a mine or to drive machinery. The beams of the first engines were made of two or more trees, bolted together to obtain the requisite rigidity, and 'further strengthened by a kind of truss, as is seen in the diagram of Newcomen's engine. But when the art of making heavy iron-castings was perfected, that metal was substituted for wood, to the manifest improvement of the engine in every respect. Watt also removed the cumbrous arched heads, which had been previously employed for the purpose of causing the piston-rod to move up and down in the same right line, though connected with the end of the beam, which necessarily described an arc of a circle, as turning on a fixed centre; this arrangement implied the use of a flexible chain, to suspend the piston, which might wind round, and unwind from, the arch, but a chain could not be used when the piston had to raise the beam, as it had to do in Watt's engine, instead of being raised by it, as in Newcomen's. The object of these arched heads' is attained in modern engines by a system of simple rods or levers, so combined that one point may move in a straight line nearly. There are a variety of combinations by which this may be effected, but that termed the 'parallel motion,' invented by Mr. Watt for the purpose, is the only one which need be here noticed, as being that most commonly used. The geometrical principle of this motion is shown in the diagram, as well as the whole arrangement when the piston is near the top of the cylinder. DD are rods fixed by one end to the frame supporting the beam, while the three other pair of levers being jointed together and to the beam, must obviously, in every position, form a parallelopipedon, whence the name is derived; P is the piston-rod attached to H; Q that of the hot-water pump connected with the parallei motion at I in the centre of that side. cular motion of the latter into a continuous one of the flywheel; this is effected by the rod and crank, a piece of mechanism of such frequent occurrence that it is unnecessary to describe it; the treadle of a lathe is a familiar instance of its application, and for a similar purpose, that of connecting the alternate motion of the turner's foot with the continuous one of the wheel of the lathe; the principle of the treadle, or rod and crank, is in fact the only one by which an alternating can be converted into a continuous circular motion, it must therefore be employed, notwithstanding the variation in the power transmitted by means of it, consequent on that of the angle formed by the rod and crank with each other. Thus, for example, when the rod and crank are in the same direction, which occurs twice at every rotation, no force whatever is transmitted by it, and the primary one is entirely suspended or held in equilibrium by the resistance of the fixed centres on which the crank and rod turn. In the steam-engine the rod and crank are so adjusted that these two neutral positions occur when the piston is at one or the other end of the cylinder, and the valves are so arranged that both steam passages being closed, all communication between the engine and the boiler is cut off, otherwise the steam, which could not under these circumstances move the piston, would exert its force to the detriment of the machine; as soon however as the momentum of the fly-wheel has carried the crank past these positions, the motion reciprocally imparted through it to the piston and valves admits the entrance of the steam from the boiler into the cylinder again. It is one of the important details in the construction of the engine, that the piston should be in that point of its course when the steam exerts its maximum of effect on it, at the time when, the rod and crank being at right angles to each other, the maximum of force may be exerted to turn the fly-wheel. Since the diameter of the circle described by the crank must be equal to the length of the stroke, or to the distance through which the piston moves, it might be thought advantageous to increase the length of the stroke as admitting of a longer crank; but there are limits to this length determined by a variety of circumstances, some of which will be hereafter explained. When Watt substituted the elastic force of steam for the pressure of the atmosphere, he introduced a source of power which might be increased to an indefinite extent, provided it were found advantageous to employ it; and the question naturally suggests itself, what is the elastic force or pressure at which the maximum of useful effect can be produced with a minimum expenditure of fuel? Unfortunately no direct answer can be given; in mathematical language, the unknown quantity is a function of too many variables to be capable of determination, except by repeated experiments for every specific engine, this quantity varying with the principle of its construction, even to details. The results of such experiments seem to show that generally it is more advantageous to employ steam of a comparatively high elastic force; accordingly the pressure was increased, in engines constructed by Watt, from 4 to 8 or even 12 lbs. on the inch, the apprehension of danger from the explosion of boilers in which steam of high pressure was generated constituting the chief limit to a further extension of the practice. The nature of those improvements in the construction of boilers will be briefly explained hereafter, by which steam of 100 lbs. on the inch may be generated, if requisite, with nearly as much security as that of 4lbs. in the earlier boilers; but at present, simply stating that such is the case, we proceed to explain some important changes which have been consequently made in the principles of the engine. When the engine is employed to drive machinery of any kind, a fly-wheel becomes a necessary adjunct to it. A fly- When the steam is first admitted into the cylinder, the wheel is one in which the principal quantity of the matter total space filled by the steam is immediately augmented is distributed in the periphery; when such a wheel revolves by that through which the piston moves; and if the capacity on an axis perpendicular to its plane, the greatest quantity of the boiler were not several times greater than that of of matter moving with a maximum velocity, the momen- the cylinder, the consequence would be a gradual dimitum of whole is a maximum, while its inertia, if it be nution of the pressure, supposing the total quantity to relarge, causes it to control, or equalize, the motion of the main the same: but the moment the pressure in the boiler machinery through which it receives its own. It is the mo- tends to diminish, an additional quantity of water passes mentum of such a wheel which constitutes the disposable into the state of vapour, of the same tension as that preforce available for the multifarious purposes to which ma-viously generated, provided the temperature be maintained; chinery can be applied; so that in the case of the steam- hence the pressure on the piston may be regarded as senengine, although the elasticity of steam is the original source sibly the same throughout the whole of its stroke, provided of power, the immediate one by which the work is executed that pressure be somewhat greater than that of the atmois the momentum of the fly-wheel. sphere, and the communication with the boiler remain open. It must not however be supposed that the pressure on the piston is the same with that of the steam in the boiler; all It is consequently necessary to adapt some contrivance to the end of the beam, which shall convert the alternating cir that is here asserted is that the pressure on the former will | mS would express the volume of steam generated in earn unit of time under the pressure P: by passing into the be uniform. cylinder this steam assumes the pressure P', and, neglecting the further change produced by the variation in the temperature of steam in changing from pressure P to pressure P', the volume of that quantity of steam would be inversely as the pressures by Mariotte's law; consequently the volume mS, when transferred to the cylinder, would p; and putting v for the velocity of the piston become m S P But if the pressure be considerably greater than that of the atmosphere, the steam, even when separated from the water, while expanding in the enlarging space formed by the motion of the piston, will exert sufficient force to continue that motion, till at last the pressure diminishing inversely as the space increases, and directly as the temperature, according to Mariotte's and Gay-Lussac's laws, that pressure will finally be not in equilibrium with the resistThis is the important ance, and all motion will cease. principle of working engines, originally proposed by Watt, though not employed by him, but which now, from the improvements in boilers above alluded to, is becoming general under the name of that of expansion. In the common engine, if the pressure on the piston continue uniform during the stroke, as it would do if the communication with eliminating P' between equations (A) and (B), we obtain the boiler remained open, the piston would move with an If it be asked how it is advantageous to use half the quantity of steam at twice the pressure, when it takes perhaps twice the quantity of fuel to raise the steam to the double pressure, the answer is, that it can be shown analytically that the total force exerted by steam acting expansively is greater than that which would be exerted by steam of a constant pressure, equal to the mean of those exerted, first, at the moment the steam-valve is closed, and, secondly, when the piston arrives at the end of its stroke; consequently, as less steam may be used to produce the required effect, a saving of fuel is the result, or in other words, the quantity of steam may be much less than half, at double the pressure, or the pressure much less than doubled, to produce the same effect. As long as a continued force of any kind produces a continued motion with a constant velocity in any body, the force must be in equilibrium with the resistance it has to overcome; for if the force were greater than the resistance, it would produce an accelerating motion, which is contrary to the supposition; and if the resistance became greater than the force, the velocity would retard till the equilibrium were produced: as long therefore as a steam-engine is moving with a constant velocity, the pressure on the piston must be equal to the resistance to be overcome, consisting of the net work to be done, together with the friction of the various parts, the resistance of the uncondensed steam, of the air on the opposite side of the piston, and of other sources of resistance, which all concur to produce the gross resistance to be overcome. Putting P' for the pressure of the steam on each unit of surface of the piston, and R for the resistance for the same unit, or for the quotient obtained by dividing the total resistance by the number of units of surface, we have P' R and a for its area, av will be the volume of steam expended P mS P v= a R mSP av avR mP R = S= (B) is the general expression for the steam during its action in the engine, u being the volume, and p the pressure, and n and q constants, determined by experiments, for different kinds of engines.* Let a certain volume of water, S, be converted into steam of the pressure p, and let M represent the volume of steam produced, then that is, the volumes will not be inversely as the pressures simply, according to Mariotte's law; but inversely as the pressures augmented by a constant. It can be shown that the density and relative volume of a vapoer, whether or not in contact with the liquid, may be deduced, if its press and temperature are kuown; and that when in contact with the la In deducing for the temperature varies directly with the pressure. for the steam engine, it is necessary to be able to determine an express for the relative volume of the steam in contact with the water, or the vol of the steam at the maximum of density and pressure at any proposed tes Now this cannot be done from the existing formula for analy rature. reasons, and it becomes necessary to adopt some empirical formula, für de termining this relative volume of steam at its maximum of density, in term of its pressure only; this formula must be tested by its conformity with esper ment. The late M. Navier proposed for this purpose, μ = 1000 in which is the ratio of the volume of steam to an equal weight of water, (A) as the first equation of condition; but since the velocity of the motion must be taken into consideration, when the power or force of the engine is to be determined, we must consider the velocity with which this pressure is applied, or, in other words, the rate at which the steam is applied to the cylinder; and it is obvious that when the engine is moving with a constant velocity, the supply to the piston must be exactly that produced in the same time by the evaporation going on in the boiler. If therefore S expresses the volume of water evaporated in a unit of time and transmitted to the cylinder, and m the being the pressure in pounds on the square foot. These formanla in general ratio of the volume of steam, formed under the pressure P in the boiler, to the volume of water which produced it, με μ 10000 = 0.4227+0 00258 p 10000 1-421+0.6023 p for non-condensing engines; From the above equation we get Let P a Pressure at any instant when acting expan- we obtain = length of the stroke. the length of that part of stroke performed the length of that portion of the stroke per- c = clearage, or space in the cylinder at each end When the piston has performed A of its course under the expansive force of the steam, let d.λ be the differential of this length, then the corresponding force or effect will be rad. and at the same instant the space a (l' + c) occupied by the steam before the expansion will become a (λ+c). Hence from (E)— + log-al=aRi (F) + c n+q P by equating the expenditure to the volume furnished by the boiler, which, as has been above stated, must be the condition when the motion is uniform. Eliminating P' between (F) and (G), we get for the final general equation— S 1 v= an+qR "' + c (II) v= S +log a n + q [(1 + d) r + p +f] (K) it will be seen (C) is the total n + q R space occupied by the steam (in contact with the water) under the pressure R: hence to deduce the velocity v, the volume of steam corresponding to the volume of water S, supposed to be converted into steam under a pressure equal to R, must be calculated; and this volume being divided by a, the area of the piston must be multiplied by k. The equation thus deduced shows the relation between all the quantities, known or sought, that enter into the mechanical theory of the engine in its most general form: it should be observed however that to preserve homogeneity, the dimensions a, l, l', should be expressed in the same unit as the volume S of the water evaporated; and the pressures P, r, and p referred to the same unit as S. When this formula is used for computation, it must be understood that the quantity S expresses the effective evaporation; that is, the volume of water which really passes to the cylinder in the form of steam, and which acts on the piston, and does not allow for any loss by leakage or from any peculiarity in the structure of the engine. If the engine is a condensing one, acting expansively, l' must be made equal the length at which the steam is cut off; if expansion is not employed, l' must be put equal to l, or to the whole length of the stroke, in which case the quantity k becomes and the expression for the ve- it might be supposed that when v = 0, the resistance would be infinite, a paradox which would appear to vitiate the correctness of the formula. But it must be borne in mind that when v=0, S=0; for S is the quantity of steam which passes through the cylinder in each unit of time: and since no quantity of steam, however small, can pass without moving the piston, as long as S has any real value, v will have one too: when therefore v = 0, S=0 also; and then and not ∞ ; that is, the formula becomes indeterminate, but not the less direct, as will appear by considering the other quantities it involves, and the consequences of putting v = 0. By supposing the velocity zero, it is, in the first place, evident that no steam can pass to the cylinder, as has been stated; consequently there can be no expansion, that is, l='. Again the velocity being zero, the piston at rest becomes equivalent to the fixed sides of the boiler, and the pressure it sustains is equal to that in the boiler. + log +c. The working of an engine may be considered under three The resistance expressed by R in this formula is the total pressure on each unit of surface of the opposite side of conditions: first, when it is working with a given pressure the piston, and is composed of three parts. First, of the load, of steam, and with any, whatever load or velocity. Secondly, when it is working with a given pressure, and with that or work to be moved or done, which we will denote by r. Secondly, of the resistance arising from the friction of the en-load or velocity compatible with the production of a maxigine, which may be expressed by for; f being the friction when there is no load, and or the increment due to the And, lastly, additional friction for each unit of the load r. of the pressure on the opposite surface of the piston, which will be the atmospheric pressure in non-condensing engines, or that of the uncondensed steam and residue of air in condensing ones: this we shall call p. All these, r, mum of useful or net force with that pressure: this may be termed the relative maximum of useful effect. And thirdly, when the pressure having been determined to furnish the force most consonant with the action of steam in any specifie engine, the load is regulated so as to be that most advanta geous for that pressure: this last constitutes the absolute maximum of useful effect for that machine. of fuel. Fourthly, by the weight raised by the evaporation of a cubic foot of water. Fifthly, by the number of pounds of fuel, or of cubic feet of water, for each horse-power. Sixthly, by the number of horses-power which is furnished by each pound of fuel, or by each foot of water. For the various formulæ by which all these problems may be numerically solved for different kinds of engines, and for the investigations by which those formulæ are deduced, we must refer to more comprehensive works; contenting ourselves here with deducing the general equations for the other unknown quantities of evaporation, useful effect, and horse-power, as we have done for velocity. From (K) we obtain as the expression for the evaporation of which an engine must be capable to overcome a given resistance r, with a proposed velocity v, S being the quantity of water which is to be converted into steam and transmitted to the cylinder in each unit of time. The useful or net force of the engine generated in the same unit of time is obviously arv; since v, the velocity, is in fact the space moved through by the piston in that time; by multiplying therefore both sides of (L) by v we obtain Useful Force B A ble-cylinder engine has not become common, probably owing to the complication of its structure, and the mcreased effects of radiation from so large a surface, more than compensating its merits; and the expansive principle, equally applicable to a single cylinder, is now principally employed in engines of the common construc tion. not in themselves economical, we are enabled to reduce so If by the use of steam under conditions by which, though materially the cost price of the engine that the increased cost of fuel is more than balanced by the interest on capital, or if this more costly power were necessary to render the engine available for purposes which could not otherwise be attained, it is clear it would be had recourse to. It is such considerations which finally led to the adoption of the non-condensing engine, first suggested by Leapold, then patented but not used by Watt, and efficiently carried ont by Trevithick and Vivian in 1804; their object being to produce a locomotive engine to draw waggons on a tram-road. In the article RAILWAY the reader will find a comprehensive account of a locomotive engine as connected with the subject of railways; we shall here confine ourselves therefore to a few observations on this form of engine in connection with others which would have been foreign to the subject there considered. To reduce the compass and weight of the engine suffic:ently to render it portable, the cumbrous apparatus of the condenser, and its attendant pumps and cisterns, had to be discarded; and since the principle of condensation was con and if during the unit of time N lbs. of coals are used in sequently renounced, it was necessary to raise the steam to the furnace, U. F. from 1 lb. of fuel = Useful Force We must now return to our general description of the engine and of its modifications. a pressure sufficient to overcome that of the atmosphere on the opposite side of the piston. To allow the steam to act alternately on both sides of the piston, that which had just acted on one side to drive the piston was expelled which would have connected the cylinder with the corinto the open air through an orifice, corresponding to tha: In 1781 an engineer named Hornblower proposed using this orifice were as large as the diameter of the cylinder, denser in an engine of the usual construction; but unless the expansive principle by means of a double cylinder, but which obviously it never can be nor nearly, the steam, was prevented from carrying out his plan by the compre-retarded in its escape by the contraction of the passage, hensive and jealously guarded patents of Watt and his partIn 1804 however Woolf brought this principle of the double-cylinder engine into use. The annexed figure will explain the mode of its action with an improved slide ners. valve. The steam enters through the passage above the piston in the smaller cylinder A, at a considerable pressure: while the piston is descending under its influence, the steam from beneath passes through the tube to above the piston in the large cylinder B, which is impelled downwards by its expansion, the steam which was previously under this piston having passed to the condenser by the passage t. When the stroke is completed, the slide is moved downwards by its rod o. The small plugs or pistons and w pass below the openings and t, and the slide below the orifices p and q, and the action is reversed. must diminish, by its resistance to compression, the effective Such is the simple principle, and such the greatest de Since the pumps of the condensing engine are d • The non-condensing engine is frequently termed the high-pressure expia. from the comparatively great clastic force of the steam employed in it; wa in contradistinction the condensing engine for the same reason is termed ike low-pressure one: vague and indeunite denominations, which ought to be aha doned, since high and low are relative. Many condensing engines are withal with steam Wait would never have contemplated using; and Trevithick's es But though possessing considerable advantages, the dougine was a low-pressure one compared with many now in use, pensed with in the non-condensing one, the beam may be so likewise; the piston-rod is made to move in a straight line, by having a cross-piece attached at its top, which slides between guides fixed on each side of the cylinder, the rod which works the crank of the fly-wheel being attached to the end of this cross-piece. A still further simplification is effected by connecting the piston-rod directly with the crank on the shaft of the fly-wheel, the cylinder being mounted so as to oscillate as the wheel revolves on the steam passage, and thus alternately to open and close the communication between the top and bottom of the cylinder. Such engines are termed vibratory, and are successfully used where space must be economised, as with marine engines, but the weight of the cylinder thus moved is so much to be deducted from the power of the engine, and further causes a rapid wear of the centres on which it turns, which consequently cannot be long preserved steam-tight, and require frequent renewal. Steam-engines are properly classed, according to the principle on which the physical properties of the steam are employed in them, into Engines. with a maximum of capacity, we could readily determine that the length should be twice the diameter;* but we find that this proportion is not adhered to by the best makers; it varies from 3:1 to 2:1; but in the marine engine it is usually shorter; in some instances the proportion is 1:125. The diameter of the cylinder of a marine engine is usually greater, in proportion to its length, than in others, in order to obtain, by an increased surface of piston, that power which is unattainable by a long stroke, owing to the limited space which can be appropriated to the engine. Formerly, the apprehension of danger, where so many lives were at stake, prevented the use of steam of more than 4 to 6 lbs. on the inch in marine engines, and the expansion principle consequently could not be had recourse to. At present, the economy of using this principle has outweighed the apprehension in the minds of the owners of vessels, while the public, contented with the information that the engine is a condensing one, seldom inquire further, and conceive that the steam is at a low pressure in all marine engines, although, where the expansion principle is used, which it now extensively is, the pressure in the boiler is at about 20 lbs. on the inch above the pressure of the atmosphere. Engineers have always been induced, by the obvious advantage of a continuous over an alternating motion, to aim at contriving a steam-engine in which the steam should act directly to produce such a motion. It does indeed appear at first sight that, where the object of the engine is to produce a continuous circular motion of a fly-wheel, or of wheels of some kind, it would be desirable that the steam should be applied directly to impel the wheel, instead of having its force transmitted through a series of levers, necessarily increasing the friction and the cost of the engine. Watt accord The form of the engine, the arrangement and construc-ingly patented more than one of such rotatory engines, and tion of its parts, its power, &c., depend entirely on the purpose to which it is to be applied, and may be indefinitely diversified, but those most in use may be artificially classed thus many others since have from time to time brought forward arrangements for the purpose, but none have come into permanent and general use. The fact is, that, as can be easily shown, the employment of steam in this way is productive of a greater waste of power, with a greater increase of friction, than can be compensated by any real advantages. In all rotatory steam-engines hitherto proposed, the principle has been to admit the steam to act on a fan or fans revolving round an axis of the cylinder, and, by ingenious excentric movements, the surface of these fans is made to increase as the steam diminishes in elastic force from the enlargement of the space it occupies. Many such engines have been used for a time, but commonly after a few years' trial they have been abandoned, and the reciprocating principle substituted, thus proving that experience confirmed the deductions from theory. ⚫ Marine engines, or those used for propelling vessels, are in this country generally condensing engines, their situation admitting the abundant use of cold water. The principal peculiarity in the arrangement of the marine engine is the In all mechanical combinations the object to be effected position of the beam, which, for the purpose of economis- necessitates a certain characteristic form of the machine, ing room, is placed lower than the cylinder, and is double, which it retains, whatever improvements may have been there being one on each side; a rod from one end of each successively introduced either in its principle or in the deof these is connected with a cross-piece at the top of the tails of its construction. We can recognise in a modern piston-rod, the rectilinear motion of which is produced either Sussex plough the general form of that used by the subjects by guides, or by a crank-arrangement, analogous in its of the Pharaohs to till the banks of the Nile; and Newcoaction to the parallel motion. The other ends of the double men would acknowledge a marine engine made by Maudbeam are connected by a cross-piece, carrying in its centre slay and Field as a descendant of his atmospheric one: but the 'rod' to work the crank on the shaft of the paddles. he would for some time be at a loss to tell the object of a In all vessels of any magnitude, there are two engines locomotive engine of Stephenson's, if he could see it at rest complete, so arranged that while the rod and crank of one only; and the connection between it and its tender would be are in their neutral position, those of the other are in that beyond his comprehension. The reason of this is that a loof greatest effect. Two engines are necessary to equalize comotive engine is a perfectly new one, having no other and continue the motion of the wheels; for in the marine analogy to an ordinary engine except that steam is the engine, the paddles, instead of performing the part of fly-source of power in both; but all locomotive engines will wheels to continue and control the motion of the piston, ever possess a family likeness. require the whole force of the engines to maintain their own, owing to the resistance they have to overcome. There is also this further advantage derived from two engines, that if one should be injured, the vessel may be still propelled the concave surface is only gradually brought into contact with the steam, by by the other, and not be entirely dependent on her sails, as she would otherwise be, The principal causes of this novelty of form are, that Let length, the diameter, c = the capacity of the cylinder: since the motion of the pistou, its effects on the temperature may be considered as about half what it would be if the whole surface were at once exposed. Then the whole surface, including the two ends, being Tlr2 and c = Txl+2 4 Tr 4 It has been mentioned that there is a limit to the propor- therefore 2 2c = 2 2 stroke being diminished by the greater radiation of heat It is stated by Mr. Stevenson that the American steamers which navigate the Mississippi are always non-condensing engines. P. C. No. 1419. 2cd Trds = 0 + On the Mississippi the boat-engines are worked with steam of from 100 lbs. to 130 lbs. on the inch; but the latter enormous pressure is rarely exceedod, 'except,' as an American commander said, on extraordinary occasions.' VOL. XXII.-3 Q |