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STEAM ENGINE

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Originally appearing in Volume V25, Page 833 of the 1911 Encyclopedia Britannica.
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STEAM See also:ENGINE  . 1 . A See also:steam See also:engine is a See also:machine for the See also:conversion of See also:heat into See also:mechanical See also:work, in which the working substance is See also:water and water vapour . The working substance may be regarded from two points of view . Thermodynamically it is the vehicle by which heat is conveyed to and through the engine from the hot source (the See also:furnace and See also:boiler) . See also:Part of this heat suffers a transformation into work as it passes through, and the See also:remainder is rejected, still in the See also:form of heat . Mechanically the working substance is a See also:medium capable of exerting pressure, which effects this transformation in doing work by means of the changes of See also:volume which it undergoes it the operation of the machine . Regarded as a thermodynamic See also:device, the See also:function of the engine is to get as much work as possible from a given quantity of heat or, to go a step further back, from the See also:combustion of a given quantity of See also:fuel . Accordingly, a question of See also:primary importance is what is called the efficiency of the engine, which is the ratio of the work done to the heat supplied . Before, however, proceeding to discuss the steam engine in this aspect, or treating of the See also:mechanics of its See also:modern forms, it maybe useful to give a brief See also:historical See also:sketch of its See also:early development as an See also:industrial appliance . In any such sketch the See also:chief See also:share of See also:attention must necessarily be given to the work of See also:James See also:Watt . But a See also:process of See also:evolution had been going on before the See also:time of Watt which prepared the steam engine for the immense improvements it received at his hands .

His labours stand in natural sequence to those of See also:

Thomas See also:Newcomen, and Newcomen's to those of See also:Denis See also:Papin and Thomas Savery . Savery's engine in its turn was the reduction to See also:practical form of a contrivance which had See also:long before been known as a scientific See also:toy . The most modern type of all, the steam See also:turbine of C . A . See also:Parsons, is a new departure which has but little to connect it directly with the past; but even the steam turbine not only profits by the inventions of Watt, but in its characteristic feature finds crude prototypes in <( I apparatus which employed ~Sl the kinetic See also:energy of jets of steam . 2 . One of these, indeed, is mentioned amongst the See also:ear- See also:Hero, 130 liest notices we have of any heat engine . In the See also:Pneumatics of Hero of See also:Alexandria (c . 130 B.C.) there is described the aeolipile, which is a See also:primitive steam reaction tur- bine, consisting of a spherical See also:vessel pivoted on a central See also:axis and supplied with steam through one of the pivots . The steam escapes by See also:bent pipes facing tangentially in opposite directions, at opposite ends of a See also:diameter perpendicular to the axis . The globe revolves by reaction from the escaping steam just as a See also:Barker's See also:mill is driven by escaping water . Another apparatus described by Hero (fig .

1)1 is interesting as the prototype of a class of engines which long afterwards became practically important . A hollow See also:

altar containing See also:air is heated by a See also:fire kindled on it; the air in expanding drives some of the water contained in a spherical vessel beneath the altar into a bucket, which descends. and opens the See also:temple doors above by pulling See also:round a pair of See also:vertical posts to which the doors are fixed . When the fire is extinguished the air cools, the water leaves the bucket, •andthe doors See also:close . In another device a See also:jet of water driven out by expanding air is turned to See also:account as a See also:fountain . 3 . From the time of Hero to the 17th See also:century there is no progress to See also:record, though here and there we find See also:evidence that appliances like those described by Hero were used for trivial purposes, such as See also:organ-blowing and the Der6oltarPa a' turning of spits . The next distinct step was the publication in . 16or of a See also:treatise on pneumatics by Giovanni Battista della Porta, in which he shows an apparatus similar to Hero's fountain, but with steam instead of air as the displacing fluid . Steam generated in a See also:separate vessel passes into a closed chamber containing water, from which a ' See also:pipe (open under the water) leads out . He also points out that the condensation of steam in the closed chamber may be used to See also:pro-duce a vacuum and suck up water from a See also:lower level . In fact, his suggestions anticipate very fully the engine which a century later became in the hands of Savery the earliest commercially successful steam engine . In 1615 See also:Solomon de Caus gives a See also:plan of forcing up water by a steam fountain which differs from Della Porta's only in having one vessel serve both as boiler and as displacement-chamber, the hot water being itself raised .

4 . Another See also:

line of invention was taken by Giovanni Branca (1629), who designed an engine shaped like a water-See also:wheel, to be driven by the impact of a jet of steam on its vanes, and in its turn to drive other mechanism for various useful purposes . But Branca's See also:suggestion was for the time unproductive, and we find the course of invention reverting to the line followed by Della Porta and De Caus . 5 . The next contributor is one whose See also:place is not easily assigned . To See also:Edward See also:Somerset, second See also:marquis of See also:Worcester, appears to be due the See also:credit of proposing, if not Ma, u1s of making, the first useful steam engine . Its See also:object worcestet, was to raise water, and it worked probably like 1663 . Della Porta's See also:model, but with a pair of displacement-See also:chambers, from each of which alternately water was forced by steam from an See also:independent boiler, or perhaps by applying heat to the chamber itself, while the other vessel was allowed to refill . See also:Lord Worcester's description of the engine in See also:art . 68 of his Century of Inventions (1663) is obscure, and no drawings are extant . It is, therefore, difficult to say whether there were any distinctly novel features except the See also:double See also:action; in particular, it is. not clear whether the suction of . a vacuum was used to raise water as well as the See also:direct pressure of steam . 6 .

The steam engine first became commercially successful in the hands of Thomas Savery,2 who, in 1698, obtained a patent for a water-raising engine, shown in fig . 2 . Steam is admitted to one of the See also:

oval vessels A, displacing water, which it drives up through the check-See also:valve r69s . B . When the vessel A is emptied of water the See also:supply of steam is stopped, and the steam already there is condensed by allowing a jet of See also:cold water from a cistern above to stream over the See also:outer See also:surface of the vessel . This produces a vacuum and causes water to be sucked up through the pipe C and the valve D . Meanwhile steam has been displacing water ' From See also:Greenwood's See also:translation of Hero's Pneumatica . 2 Savery was See also:born probably" in 165o and died in 1715 . See See also:Sir E . Duelling See also:Lawrence's presidential address to the Royal Institution of See also:Cornwall (Journ. of the See also:Roy . Inst. of Cornwall, No. li.), republished with a reprint of Savery's Miner's Friend of 1702, in which he discusses the originality of Savery's invention and dismisses. the claims put forward for Lord Worcester . the safety-valve as an See also:adjunct to his " digester," suggested that the condensation of steam should be employed to make a vacuum under a See also:piston previously raised by the expansion of the steam .

Papin's was the earliest See also:

cylinder and piston steam engine, and his plan of using was that which afterwards took practical shape in the spheric engine of Newcomen . But his See also:scheme was unworkable by the fact that he proposed to use but one vessel as both boiler and cylinder . A small quantity of water was placed at the bottom of a cylinder and heat was applied . When the piston had risen the fire was removed, the steam was allowed to cool, and the piston did work in its down-stroke under the pressure of the See also:atmosphere . After See also:hearing of Savery's engine in 1705 Papin turned his attention to improving it, and devised a modified form, shown in fig . 3, in which the displacement- from the other vessel, and is ready to be condensed there . The valves B and D open only upwards . The supplementary boiler and furnace E are for feeding water to the See also:main boiler; E is filled while cold and a fire is lighted under it; it then acts like the vessel of De Gaus in forcing a supply of feed- water into the main boiler F . The See also:gauge cocks G, G are an inter- esting feature in detail . Another form of Savery's engine had only one dis- placement-chamber and worked intermittently . In the use of artificial means to condense the steam, and in the appli- cation of the vacuum so formed to raise water by suction from a level lower than that of the engine, Savery's engine was probably an improvement on Worcester's; in any See also:case it found what Worcester's engine had failed to find—considerable employment in pumping mines And in raising water to supply houses and towns, and even to drive water-wheels . A serious difficulty which prevented its See also:general use in mines was the fact that the height through which it would lift water was limited by the pressure the boiler and vessels could See also:bear .

Pressures as high as 8 or to atmospheres were employed—and that, too, without a safety-valve—but Savery found it no easy See also:

matter to See also:deal with high-pressure steam; he complains that it melted his See also:common See also:solder, and forced him, as Desaguliers tells us, " to be at the pains and See also:charge to have all his See also:joints soldered with spelter." Apart from this See also:drawback, the See also:waste of fuel was enormous, from the condensation of steam which took place on the surface of the water and on the sides of the displacement-chamber at each stroke; the See also:consumption of See also:coal was, in proportion to the work done, some twenty times greater than in a See also:good modern steam engine . In a See also:tract called The Miner's Friend Savery alludes thus to the alternate See also:heating and cooling of the water-vessel: " On the outside of the vessel you may see how the water goes out as well as if the vessel were transparent, for so far as the steam continues within the vessel so far is the vessel dry without and so very hot as scarce to endure the least See also:touch of the See also:hand . But as far as the water is, the said vessel will be cold and wet where any water has fallen on it; which cold and moisture vanishes as fast as the steam in its descent takes the place of the water." Before Savery's engine was entirely displaced by its successor, Newcomen's, it was improved by J . T . Desaguliers, who applied to it the safety valve (invented by Papin), and substituted condensation by a jet of cold water within the vessel for the surface condensation used by Savery . To Savery is ascribed the first use of the See also:term " See also:horse See also:power " as a measure of the performance of an engine . 7 . So early as 1678 the use of a piston and cylinder (long before known as applied to pumps) in a heat-engine had been cylinder suggested by See also:Jean de Hautefeuille, who proposed and to use the See also:explosion of See also:gun-See also:powder either to raise a Piston piston or to force up water, or to produce, by the sub- engine, sequent cooling of the gases, a partial vacuum into which water might be sucked up . Two years later See also:Christian See also:Huygens described an engine in which the explosion of gun-powder in a cylinder expelled part of the gaseous contents, after which the cooling of the remainder caused a piston to descend under atmospheric pressure, and the piston in descending did work by raising a See also:weight . 8 . In 1690 Denis Papin, who ten years before had inventedNewcomen's success by separating the boiler from the cylinder Atmospheric and by using (as Savery had done) artificial means Engine, to condense the steam . This was Thomas New- 1705' comen, who in 1705, with his assistant, See also:John Cawley, gave the steam engine the form shown in fig .

4 . Steam admitted from the boiler to the cylinder allowed the piston to be raised by a heavy counterpoise on the other See also:

side of the See also:beam . Then the steam valve was shut and a jet of cold water entered the cylinder and condensed the steam . The piston was consequently forced down by the pressure of the atmosphere and did work on the See also:pump . The next entry of steam expelled the condensed FIG . 4.-Newcomen's Atmospheric water from the cylinder Engine, 1705 . ' For an account of Papin's inventions see his See also:Life and See also:Correspondence, by Dr E . Gerland (See also:Berlin, 1881) . See also:ing Engine, 1698 . Papin . steam atmomade chamber A was a cylinder, with a floating See also:diaphragm or piston on the See also:top of the water to keep the water and steam from direct contact with one another . The water was delivered into a closed air-vessel B, from which it issued in a continuous stream, against the vanes of a water-wheel .

After the steam had done its work in the displacement-chamber it was allowed to See also:

escape by the stop-See also:cock C instead of being condensed . Papin's engine was, in fact, a non-condensing single-acting steam pump, with steam cylinder and pump cylinder in one . A curious feature of it was the heater D, a hot See also:mass of See also:metal placed in the diaphragm for the purpose of keeping the steam dry . Among the many inventions of Papin was a boiler with an See also:internal fire-See also:box—the earliest example of a construction that is now almost universal.' 9 . While Papin was thus going back from his first notion of a piston engine to Savery's cruder type, a new inventor had appeared who made the piston engine a practical through an escape valve . The piston was kept tight by a layer of water on its upper surface . Condensation was at first effected by cooling the outside of the cylinder, but the accidental leakage of the packing water past the piston showed the See also:advantage of condensing by a jet of injection water, and this plan took the place of surface condensation . The engine used steam whose pressure was little if at all greater than that of the atmosphere; sometimes, indeed, it was worked with the manhole lid off the boiler . to . About 1711 Newcomen's engine began to be introduced for pumping mines . It is doubtful whether the action was originally automatic, or depended on the periodical self-acting turning of taps by an attendant . The common See also:story , Y is that in 1713 a boy named See also:Humphrey See also:Potter, whose See also:duty it was to open and shut the valves of an engine he attended, made the engine self-acting by causing the beam itself to open and close the valves by suitable cords and catches .

This device was simplified in 1718 by See also:

Henry Beighton, who suspended from the beam a See also:rod called the plug-See also:tree, which worked the valves by means of tappets . By 1725 the engine was in common use in collieries, and it held its place without material See also:change for about three-quarters of a century in all . Near the close of its career the atmospheric engine was much improved in its mechanical details by John See also:Smeaton, who built many large engines of this type about the See also:year 1970, just after the See also:great step which was to make Newcomen's engine obsolete had been taken by James Watt . Compared with Savery's engine, Newcomen's had (as a pumping engine) the great advantage that the intensity of pressure in the pumps was not in any way limited by the pressure of the steam . It shared with Savery's, in a scarcely less degree, the defect already pointed out, that steam was wasted by the alternate heating and cooling of the vessel into which it was led . Though obviously capable of more extended uses, it was in fact almost exclusively employed to raise water—in some instances for the purpose of turning water-wheels to drive other machinery . Even contemporary writers complain of its vast " consumption of fuel," which appears to have been scarcely smaller than that of the engine of Savery . 11 . In 1763 James Watt, an See also:instrument maker in See also:Glasgow, while engaged by the university in repairing a model of New- comen's engine, was struck with the waste of steam watt' 1764 to which the alternate chilling and heating of the . cylinder gave rise . He saw that the remedy, in his own words, would See also:lie in keeping the cylinder as hot as the steam that entered it . With this view he added to the engine a new organ—an empty vessel separate from the cylinder, into which the steam should be allowed to escape from the cylinder, to be condensed there by the application of cold water either outside or as a jet .

To preserve the vacuum in his See also:

condenser he added a pump called the air-pump, whose function was to pump from it the condensed steam and water of See also:con- densation, as well as the air which would otherwise accumulate by leak- See also:age or by being brought in with the steam or with the injection 'water . Then, as the cylinder was no longer used as a condenser, he was able to keep it hot by clothing it with non- conducting bodies, and in particular by the use of a steam jacket, or layer of hot steam between the cylinder and an See also:external casing . Further, and still with the same object, he covered in the top of the cylinder, taking the piston- rod out through a steam-tight stuffing-box, and allowed steam instead of air to See also:press upon the piston's upper surface . The See also:idea of using a separate condenser had no sooner occurred to Watt than he put it to the test by constructing the apparatus shown in fig . 5 . There A is the cylinder, B a surface condenser, and C the air-pump . The cylinder was filled with steam above the piston, and a vacuum was formed in the surface condenser B . On opening the stop-cock D the steam rushed over from the cylinder and was condensed, while the piston See also:rose and lifted a weight . After several trials Watt patented his improvements in 1769; they are described in his See also:specification in the following words, which, apart from their immense historical See also:interest, deserve careful study as a statement of principles which to this See also:day See also:guide the scientific development of the steam engine: " My method of lessening the consumption of steam, and consequently fuel, in fire-engines, consists of the following principles: " First, That vessel in which the See also:powers of steam are to be employed to work the engine, which is called the cylinder in common fire-engines, and which I See also:call the steam-vessel, must, during the whole time the engine is at work, be kept as hot as the steam that enters it; first by enclosing it in a case of See also:wood, or any other materials that transmit heat slowly; secondly, by surrounding it with steam or other heated bodies; and, thirdly, by suffering neither water nor any other substance colder than the steam to enter or touch it during that time . " Secondly, In engines that are to be worked wholly or partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam-vessels or cylinders, although occasionally communicating with them; these vessels I call condensers; and, whilst the engines are working, these condensers ought at least to be kept as cold as the air in the neighbourhood of the engines, by application of water or other cold bodies . Thirdly, Whatever air or other elastic vapour is not condensed by the cold of the condenser, and may impede the working of the engine, is to be See also:drawn out of the steam-vessels or condensers byi means of pumps, wrought by the engines themselves, or other-See also:wise . " Fourthly, I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire-engines .

In cases where cold water cannot be had in plenty, the engines may be wrought by this force of steam only, by discharging the steam into the air after it has done its See also:

office . . " Sixthly, I intend in some cases to apply a degree of cold not capable of reducing the steam to water, but of contracting it considerably, so that the engines shall be worked by the alternate expansion and contraction of the steam . Lastly, Instead of using water to render the pistons and other parts of the engine air and steam tight, I employ See also:oils, See also:wax, resinous bodies, See also:fat of animals, quicksilver and other metals in their fluid See also:state." The fifth claim was for a rotary engine, and need not be quoted here . The " common fire engine " alluded to was the steam engine, or, as it was more generally called, the " atmospheric " engine of Newcomen . Enormously important as Watt's first patent was, it resulted for a time in the See also:production of nothing more than a greatly improved engine of the Newcomen type, much less wasteful of fuel, able to make faster strokes, but still only suitable for pumping, still single-acting, with steam admitted during the whole stroke, the piston, as before, pulling the beam by a See also:chain working on a circular arc . The condenser was generally worked by injection, but Watt has See also:left a model of a surface condenser made up of small tubes, in every essential respect like the condensers now used in marine engines .l 12 . Fig . 6 is an example of the Watt pumping engine of this See also:period . It should be noticed that, although the top of the cylinder is closed and steam has See also:access to the upper side of the WatPs piston, this is done only to keep the cylinder and piston pamplag warm . The engine is still single-acting; the steam in Engiene the upper side merely plays the part which was played 1769. in Newcomen's engine by the atmosphere; and it is the lower end of the cylinder alone that is ever put in communication with the condenser . There are three valves: the " steam " valve a, the " See also:equilibrium " valve b, and the " exhaust " valve c . At the beginning of the down-stroke c is opened to produce a vacuum below the piston and a is opened to admit steam above it .

At the end of the down-stroke a and c are shut and b is opened . This puts the two sides in equilibrium and allows the piston to be pulled up by the pump-rod P, which is heavy enough to serve as a counterpoise . C is the condenser, and A the air-pump, which discharges into the hot well H, whence the supply of the feed-pump F is drawn . 13 . In a second patent (1781) Watt describes the " See also:

sun-andplanet" wheels and other methods of making the engine give 1 An interesting detailed narrative of the steps leading to his invention was written by Watt as a See also:note to the See also:article " Steam Engine " in Robison's See also:System of Mechanical See also:Philosophy (1822) . See See also:Ewing, The Steam Engine and other Heat Engines, pp . 15-19 . See also:mental Apparatus . continuous revolving See also:motion to a See also:shaft provided with a flywheel . He had invented the See also:crank and connecting-rod for this purpose, but it had meanwhile been patented Rotative by one Pickard, and Watt, rather than make terms Engine . with Pickard, whom he regarded as a plagiarist of his own ideas, made use of his sun-and-See also:planet motion until the patent on the crank expired . The reciprocating motion of earlier forms had served only for pumping; by this invention Watt opened up for the steam engine a thousand other channels of usefulness .

The engine was still single-acting; the connecting-rod was attached to the far end of the beam, and that carried a counterpoise which served to raise the piston when steam was admitted below it . 14 . In 1782 Watt patented two further improvements of the first importance, both of which he had invented some years other before . One was the use of double action, that is Inventions to say, the application of steam and vacuum to "watt. each side of the piston alternately . The other (invented as early as 1769) was the use of steam expansively, in other words the plan (now used in all engines that aim at See also:

economy of fuel) of stopping the See also:admission of steam when the piston had made only a part of its stroke, and allowing the See also:rest of the stroke to be performed by the expansion of the steam already in the cylinder . To let the piston push as well as pull the end of the beam Watt devised his so-called parallel motion, an arrangement of links connecting the piston-rod See also:head with the beam in such a way as to guide the rod to move in a very nearly straight line . He further added the throttle valve, for regulating the See also:rate of admission of steam, and the centrifugal See also:governor, a double conical pendulum, which controlled the See also:speed by acting on the throttle-valve . The See also:stage of development reached at this time is illustrated by the engine of fig . 7 (from See also:Stuart's See also:History of the Steam Engine), which shows the parallel motion pp, the governor g, the throttle-valve t, and a pair of steam and exhaust valves at each end of the cylinder . Among other inventions of Watt were the " See also:indicator," by which diagrams showing the relation of the steam pressure in the cylinder to the See also:movement of the piston are automatically drawn; a steam tilt-See also:hammer; and also a steam See also:locomotive for See also:ordinary roads—but this invention was not prosecuted . In See also:partnership with See also:Matthew See also:Boulton, Watt carried on in See also:Birmingham the manufacture and See also:sale of his engines with the utmost success, and held the See also:field against all rivals in spite of severe assaults on the validity of his See also:patents . Notwithstanding his accurate knowledge of the advantage to be gained by using steam expansively, he continued to employ only See also:low pressures—seldom more than 7 lb per sq. in. over that of the atmosphere .

His boilers were fed, as Newcomen's had been, through an open pipe which rose high enough to let the See also:

column of water in it See also:balance the pressure of the steam . He gave a definite numerical significance to the term " horse-power" (q.v.) as a mode of rating engines, defining it as the rate at which work is done when 33,000 lb are raised one See also:foot in one See also:minute . 15 . In the See also:fourth claim in Watt's first patent the second See also:sentence describes a non-condensing engine, which would have required steam of a higher pressure . This, how- Non_ ever, was a line of invention which Watt did not condensing follow up, perhaps because so early as 1725 a engine. non-condensing engine had been described by See also:Jacob Leupold in his Theatrum machinarum . Leupold's proposed engine is shown in fig . 8, which makes its action sufficiently clear . Watt's aversion to high - pressure steam was strong, and its See also:influence on steam engine practice long survived the expiry of his patents . So much indeed was this the case that the terms " high-pressure " and " non-condensing " were for many years synonymous in contradistinction to the " low-pressure " or condensing engines of Watt . This nomenclature no longer holds; in modern practice many condensing engines use as high pressures as non-condensing engines, and by doing so are able to take advantage of Watt's great invention of expansive working to a degree which was impossible in his own practice . 16 . The introduction of the non-condensing and, at that time, relatively high-pressure engine was effected in See also:England by See also:Richard Trevithick and in • See also:America by See also:Oliver High-See also:Evans about 1800 .

Both Evans and Trevithick pressure applied their engines to propel carriages on roads, Steam. and both used for boiler a cylindrical vessel with a cylindrical flue inside—the construction now known as the Cornish boiler . In partnership with See also:

William See also:Bull, . Trevithick had previously made direct acting pumping-engines, with an inverted cylinder set over and in line with the pump-rod, thus dispensing with the beam that had been a feature in all earlier forms . But in these " Bull " engines, as they were called, a condenser was used, or, rather, the steam was condensed by a jet of cold water in the exhaust-pipe, and Boulton and Watt successfully opposed them as infringing Watt's patents . To Trevithick belongs the distinguished See also:honour of being the first to use a steam See also:carriage on a railway; in 1804 he built a locomotive in the modern sense, to run on what had formerly been a horse-See also:tramway, in See also:Wales, and it is noteworthy that th6' Engine, 1782 . exhaust steam was discharged into the See also:funnel to force the furnace See also:draught, a device which, twenty-five years later; in the hands of See also:George See also:Stephenson, went far to make the locomotive what it is to-day . In this connexion it may be added that as early as 1769 a steam carriage for roads had been built in See also:France by See also:Nicolas See also:Joseph Cugnot, who used a pair of single-acting high-pressure cylinders to turn a See also:driving See also:axle step by step by means of pawls and ratchet-wheels . To the initiative of Evans may be ascribed the early general use of high-pressure steam in the See also:United States, a feature which for many years distinguished See also:American from See also:English practice . 17 . Amongst the contemporaries of Watt one name deserves See also:special mention . In 1781 See also:Jonathan See also:Carter Hornblower constructed and patented what would now be called See also:Compound engine. a compound engine, with two cylinders of different sizes . Steam was first admitted into the smaller cylinder, and then passed over into the larger, doing work against a piston in each .

In Hornblower's engine the two cylinders were placed side by side, and both pistons worked on the same end of a beam overhead . This was an instance of the use of steam expansively, and as such was earlier than the patent, though not earlier than the invention, of expansive working by Watt . Hornblower was crushed by the Birmingham See also:

firm for infringing their patent in the use of a separate condenser and air-pump . The compound engine was revived in 1804 by See also:Arthur Woolf, with whose name it is often associated . Using steam of fairly high pressure, and cutting off the supply before the end of the stroke in the small cylinder, Woolf See also:expanded the steam to several times its See also:original volume . Mechanically the double-cylinder compound engine has this advantage over an engine in which the same amount of expansion is performed in a single cylinder, that the sum of the forces exerted by the two pistons in the compound engine varies less throughout the action than the force exerted by the. piston of the single-cylinder engine . This advantage may have been clear to Hornblower and Woolf and to other early users of compound expansion . But another and probably a more important merit of the system lies in a fact of which neither they nor for many years their followers in the use of compound engines were aware —the fact that by dividing the whole range of expansion into two parts the cylinders in which these are separately performed are subject to a reduced range of fluctuation in their temperature . This, as will be seen later, limits to a great extent a source of waste which is See also:present in all steam engines, the waste which results from the heating and cooling of the metal by its alternate contact with hot and cooler steam . The system of compound expansion is now used in nearly all large engines that pretend to economy . Its introduction forms the most outstanding improvement which steam engines of the piston and cylinder type have undergone since the time of Watt; and we are ,able to recognize it as a very important step in the direction set forth in his " first principle " that the cylinder should be kept as hot as the steam that enters it . 18 .

Woolf introduced the compound engine somewhat widely about 1814 as a pumping engine in the mines of Cornwall . But here it met a .strong competitor in the. high- Cornish Engine . Pressure single-cylinder engine hnder en See also:

ine of Trevithick, which had the advantage of greater simplicity in construction . Woolf's engine See also:fell into See also: