(28.)

(28.)Let us now return to the proceedings of Papin. How great a power would result from such a machine as he conceived, will be apparent, if it be considered that the unresisted atmosphere exercises a pressure of about fifteen pounds on[Pg045]each square inch of surface exposed to it, and that if the piston in the cylinder imagined by Papin, had a diameter of only one foot, its superficial magnitude would be about 114 square inches. The pressure of the atmosphere upon it, therefore, would be 114 times fifteen pounds, or 1710 pounds. Papin first proposed to produce the vacuum under the piston by means of common air pumps, worked by a water-wheel; and by such means he conceived that the power of a river, stream, or waterfall might be conveyed by pipes to a distance. While he was in England, in 1687, he laid his contrivance before the Royal Society of London, but was met by objections and difficulties, the nature of which he does not explain.

It is, however, apparent, from what has been already explained, that such a method of proceeding would amount to a mere transfer of power, and would not, properly speaking, be itself a moving force: the moving power would, in reality, be the force of the water by which the water-wheel would be driven; and the air-pumps, tubes, together with the piston and cylinder, would be merely means of conveying the power of the water-wheel to the objects to be moved, or the machinery to be driven. Papin states, that, long before this, he had attempted to expel the air from his cylinder by means of gunpowder; but, notwithstanding all the precautions which he could take, there always remained a considerable quantity; so much, indeed, as to deprive the vacuum of more than half its proper force. At length he adopted an expedient for the production of a vacuum which forms a most important step in the progressive invention of the steam engine, and which gives to Papin's name a high place in the history of that machine. This method is explained in the following paragraph of a work published by Papin in 1695, at Cassel, entitled "Recueil de diverses Pièces touchant quelques nouvelles Machines", p. 53.

"I have endeavoured," says he, "to attain this end (viz. the production of a vacuum in the cylinder) in another way. As water has the property of elasticity, when converted into steam by heat, and afterwards of being so completely recondensed by cold, that there does not remain the least[Pg046]appearance of this elasticity, I have thought that it would not be difficult to work machines in which, by means of a moderate heat and at a small cost, water might produce that perfect vacuum which has vainly been sought by means of gunpowder."

This remarkable passage is given in the work just cited, as an extract from the "Leipsic Acts," of August, 1690.

Let us pause here to explain more fully this important discovery.

(29.)We have explained that, in its conversion into vapour, by the application of heat, water, besides acquiring the property of elasticity, undergoes a vast enlargement of bulk, filling, under ordinary circumstances, about 1700 times more space than it occupied in the liquid form. This fact was known generally, though not with numerical accuracy, by Papin, having been the foundation of the machines previously invented and published by De Caus and Lord Worcester; the happy idea of reversing the process occurred to him. If water in its conversion into steam swelled into many hundred times its original bulk, it would necessarily follow, that steam, being reconverted into water, would shrink into its primitive dimensions. Papin therefore saw, that if he could by any means expel the air from his cylinder under the piston, and replace it by the pure vapour of water, he could cause that vapour to be reconverted into a comparatively minute quantity of water by depriving it of the heat which sustained it in the state of steam, and that by accomplishing this, the space in the cylinder under the piston would become a vacuum; that by such means, the pressure of the atmosphere above the piston would take full effect, and would urge the piston down; that by introducing more steam under the piston, it might be again raised by the elastic force of the steam, the destruction of which by cold water would again produce the descent of the piston with the same mechanical force; and that in this way the alternate ascent and descent of the piston might be continued indefinitely.

In accordance with these ideas, Papin constructed a model consisting of a small cylinder, in which was placed a solid piston;[Pg047]and in the bottom of the cylinder under the piston was contained in a small quantity of water. The piston being in immediate contact with this water, so as to exclude the atmospheric air, on applying fire to the bottom of the cylinder, steam was produced, the elastic force of which raised the piston to the top of the cylinder; the fire being then removed, and the cylinder being cooled by the surrounding air, the steam was condensed and reconverted into water, leaving a vacuum in the cylinder into which the piston was pressed by the force of the atmosphere. The fire being applied and subsequently removed, another ascent and descent were accomplished; and in the same manner the alternate motion of the piston might be continued. Papin described no other form of machine by which this property could be rendered available in practice; but he states generally, that the same end may be attained by various forms of machines easy to be imagined.[8]

Thomas Savery, 1698.

(30.)The discovery of the method of making a vacuum by the condensation of steam was reproduced, before 1698, by Captain Thomas Savery, to whom a patent was granted in that year for a steam engine to be applied to the raising of water, &c. Savery proposed to combine the machine described by the Marquis of Worcester with an apparatus for raising water by suction into a vacuum produced by the condensation of steam.

Savery appears to have been ignorant of the publication of Papin, and stated that his discovery of the condensing principle arose from the following circumstance:—

Having drunk a flask of Florence at a tavern, and flung the empty flask on the fire, he called for a basin of water to wash his hands. A small quantity which remained in the flask began to boil, and steam issued from its mouth. It occurred to him to try what effect would be produced by inverting the flask and plunging its mouth in the cold water. Putting on a thick glove to defend his hand from the heat, he seized the[Pg048]flask, and the moment he plunged its mouth in the water the liquid immediately rushed up into the flask and filled it.

Savery stated that this circumstance immediately suggested to him the possibility of giving effect to the atmospheric pressure by creating a vacuum in this manner. He thought that if, instead of exhausting the barrel of a pump by the usual laborious method of a piston and sucker, it was exhausted by first filling it with steam, and then condensing the same steam, the atmospheric pressure would force the water from the well into the pump-barrel, and into any vessel connected with it, provided that vessel were not more than about thirty-four feet above the elevation of the water in the well. He perceived also, that, having lifted the water to this height, he might use the elastic force of steam in the manner described by the Marquis of Worcester to raise the same water to a still greater elevation, and that the same steam which accomplished this mechanical effect would serve, by its subsequent condensation, to reproduce the vacuum, and draw up more water. It was on this principle that Savery constructed the first engine in which steam was ever brought into practical operation.

BRANCA'S ENGINE.

FOOTNOTES:[1]Arago, Eloge historique de James Watt; p. 22.[2]Ibid., p. 21. note.[3]Farey, Treatise on the Steam Engine, p. 93.[4]Arago, sur les Machines à Vapeur, Annuaire, 1829, p. 165[5]Spiritus,breathorair.[6]Exactly 15·68 oz. = 0·98 lb.[7]This experiment with the tubeArequires to be very carefully executed, and the tube should be one of small bore.[8]Recueil de diverses Pièces touchant quelques nouvelles Machines, p. 38.

[1]Arago, Eloge historique de James Watt; p. 22.

[2]Ibid., p. 21. note.

[3]Farey, Treatise on the Steam Engine, p. 93.

[4]Arago, sur les Machines à Vapeur, Annuaire, 1829, p. 165

[5]Spiritus,breathorair.

[6]Exactly 15·68 oz. = 0·98 lb.

[7]This experiment with the tubeArequires to be very carefully executed, and the tube should be one of small bore.

[8]Recueil de diverses Pièces touchant quelques nouvelles Machines, p. 38.

SAVERY'S ENGINE.

ENGINES OF SAVERY AND NEWCOMEN.

[Pg049]TOCINX

SAVERY'S ENGINE.—BOILERS AND THEIR APPENDAGES.—WORKING APPARATUS.—MODE OF OPERATION.—DEFECTS OF THE ENGINE.—NEWCOMEN AND CAWLEY.—ATMOSPHERIC ENGINE.—ACCIDENTAL DISCOVERY OF CONDENSATION BY INJECTION.—HUMPHREY POTTER MAKES THE ENGINE WORK ITSELF.—ADVANTAGES OF THE ATMOSPHERIC ENGINE OVER THAT OF SAVERY.—IT CONTAINED NO NEW PRINCIPLE.—ITS PRACTICAL SUPERIORITY.

(31.)The steam engine contrived by Savery, like every other which has since been constructed, consists of two parts, essentially distinct. The first is that which is employed to[Pg050]generate the steam, which is called the boiler; and the second, that in which the steam is applied as a moving power.

Fig. 11.

The former apparatus in Savery's engine consists of two strong boilers, sections of which are represented atDandEinfig.11.;Dthe greater boiler, andEthe less. The tubesTandT′communicate with the working apparatus, which we shall presently describe. A thin plate of metalR, is applied closely to the top of the great boilerD, turning on a centreC, so that by moving a lever applied to the axisCon the outside of the top, the sliding plateRcan be brought from the mouth of the one tube to the mouth of the other alternately. This sliding valve is called theregulator, since it is by it that the communications between the boiler and two steam vessels (hereafter described) are alternately opened and closed, the lever which effects this being moved at intervals by the hand of the attendant.

Twogauge cocksare represented atG,G′, the use of which is to determine the depth of water in the boiler. One,G, has its lower aperture a little above the proper depth; and the other,G′, a little below it. Cocks are attached to the upper endsG,G′, which can be opened or closed at pleasure. The steam collected in the top of the boiler pressing on the surface of the water, forces it up in the tubesG,G′, if their lower ends be immersed. Upon opening the cocksG,G′, if water be forced from both, there is too much water in the boiler, since the mouth ofGisbelowits level. If steam issue from both, there is too little water in the boiler, since the mouth ofG′isaboveits level. But if steam issue fromG, and water fromG′, the water in the boiler is at its proper level. This ingenious contrivance for determining the level of the water in the boiler is the invention of Savery, and is used in many instances at the present day.

The mouth of the pipeGshould be at a level of a little less[Pg051]than one third of the whole depth, and the mouth ofG′at a level little lower than one third; for it is requisite that about two thirds of the boiler should be kept filled with water. The tubeIforms a communication between the greater boilerDand the lesser or feeding boilerE, descending nearly to the bottom of it. This communication can be opened and closed at pleasure by the cockK. A gauge pipe is inserted similar toG,G′, but extending nearly to the bottom. From this boiler a tubeFextends, which is continued to a cisternC(fig.12.), and a cock is placed atM, which, when opened, allows the water from the cistern to flow into the feeding boilerE, and which is closed when that boiler is filled. The manner in which this cistern is supplied will be described hereafter.

Let us now suppose that the principal boiler is filled to the level between the gauge pipes, and that the subsidiary boiler is nearly full of water, the cockKand the gauge cocksG G′being all closed. The fire being lighted beneathD, and the water boiled, steam is produced, and is transmitted through one or other of the tubesT,T′, to the working apparatus. When evaporation has reduced the water inDbelow the level ofG′, it will be necessary to replenish the boilerD. This is effected thus:—A fire being lighted beneath the feeding boilerE, steam is produced in it above the surface of the water, which, having no escape, presses on the surface so as to force it up in the pipeI. The cockKbeing then opened, the boiling water is forced into the principal boilerD, into which it is allowed to flow until water issues from the gauge cockG′. When this takes place, the cockKis closed, and the fire removed fromEuntil the great boiler again wants replenishing. When the feeding boilerEhas been exhausted, it is replenished from the cisternC(fig.12.), through the pipeF, by opening the cockM.

(32.)We shall now describe the working apparatus in which the steam is used as a moving power.

LetV V′(fig.12.) be two steam vessels communicating by the tubesT T′(marked by the same letters infig.11.) with the greater boilerD.

Fig. 12.

LetSbe a pipe, called thesuction pipe, descending into[Pg052]the well or reservoir from which the water is to be raised, and communicating with each of the steam vessels through tubesD D′, by valvesA A′, which open upwards. LetFbe a pipe continued from the level of the engine to whatever higher level it is intended to elevate the water. The steam vesselsV V′communicate with theforce-pipeFby valvesB B′, which open upwards, through the tubesE E′. Over the steam vessels and on the force-pipe is placed a small cisternC, already mentioned, which is kept filled with cold water from the force-pipe, and from the bottom of which proceeds a pipe terminated with a cockG. This is called thecondensing pipe, and can be brought alternately over each steam vessel. From this cistern another pipe communicates with the feeding boiler (fig.11.), by the cockM.[9]

The communication of the pipesT T′with the boiler can be opened and closed alternately, by the regulatorR(fig.11.), already described.

Now suppose the steam vessels and tubes to be all filled with common atmospheric air, and that the regulator be placed so that the communication between the tubeTand the boiler be opened, the communication between the other tubeT′and the boiler being closed, steam will flow intoVthroughT. At first, while the vesselVis cold, the steam will be condensed, and will fall in drops of water on the bottom and sides of the vessel. The continued supply of steam from the boiler will at length impart such a degree of heat to the vesselV, that it will cease to condense it. Mixed with the heated air[Pg053]contained in the vesselV, it will have an elastic force greater than the atmospheric pressure, and will therefore force open the valveB, through which a mixture of air and steam will be driven until all the air in the vesselVwill have passed out, and it will contain nothing but the pure vapour of water.

When this has taken place, suppose the regulator be moved so as to close the communication between the tubeTand the boiler, and to stop the further supply of steam to the vesselV; and at the same time let the condensing pipeGbe brought over the vesselV, and the cock opened so as to let a stream of cold water flow upon it. This will cool the vesselV, and the steam with which it is filled will be condensed and fall in a few drops of water, leaving the interior of the vessel a vacuum. The valveBwill be kept closed by the atmospheric pressure. But the elastic force of the air between the valveAand the surface of the water in the well, or reservoir, will openA, so that a part of this air will rush in, and occupy the vesselV. The air in the suction pipeS, being thus allowed an increased space, will be proportionally diminished in its elastic force, and its pressure will no longer balance that of the atmosphere acting on the external surface of the water in the reservoir. This pressure will, therefore, force water up in the tubeSuntil its weight, together with the elastic force of the air above it, balances the atmospheric pressure. When this has taken place, the water will cease to ascend.

Let us now suppose that, by shifting the regulator, the communication is opened betweenTand the boiler, so that steam flows again intoV. The condensing cockGbeing removed, the vessel will be again heated as before, the air expelled, and its place filled by the steam. The condensing pipe being again allowed to play upon the vesselV, and the further supply of steam being stopped, a vacuum will be produced inV, and the atmospheric pressure will force the water through the valveAinto the vesselV, which it will nearly fill, a small quantity of air, however, remaining above it.

Thus far the mechanical agency employed in elevating the water is the atmospheric pressure; and the power of steam is no further employed than in the production of a vacuum.[Pg054]But, in order to continue the elevation of the water through the force pipeF, above the level of the steam vessel, it will be necessary to use the elastic pressure of the steam. The vesselVis now nearly filled by the water which has been forced into it by the atmosphere. Let us suppose that, the regulator being shifted again, the communication between the tubeTand the boiler is opened, the condensing cock removed, and that steam flows intoV. At first, coming in contact with the cold surface of the water and that of the vessel, it is condensed; but the vessel is soon heated, and the water formed by the condensed steam collects in a sheet or film upon the surface of the water inV, so as to form a surface as hot as boiling water.[10]The steam then being no longer condensed, presses on the surface of the water with its elastic force; and when that pressure becomes greater than the atmospheric pressure, the valveBis forced open, and the water issuing through it, passes throughEinto the force-pipeF; and this is continued until the steam has forced all the water fromV, and occupies its place.

The further admission of steam throughTis once more stopped by moving the regulator; and the condensing pipe being again allowed to play onV, so as to condense the steam which fills it, produces a vacuum. Into this vacuum, as before, the atmospheric pressure will force the water, and fill the vesselV. The condensing pipe being then closed, and steam admitted throughT, the water inVwill be forced by its pressure through the valveBand tubeEintoF, and so the process is continued.

We have not yet noticed the other steam vesselV′, which, as far as we have described, would have remained filled with common atmospheric air, the pressure of which on the valveA′would have prevented the water raised in the suction pipeSfrom passing through it. However, this is not the case; for, during the entire process which has been described inV, similar effects have been produced inV′, which we have only omitted to notice to avoid the confusion which the two processes might produce. It will be remembered, that after the steam, in the first instance, having flowed from the boiler[Pg055]throughT, has blown the air out ofVthroughB, the communication betweenTand the boiler is closed. Now the same motion of the regulator which closes this, opens the communication betweenT′and the boiler; for the sliding plateR(fig.11.) is moved from the one tube to the other, and at the same time, as we have already stated, the condensing pipe is brought to play onV. While, therefore, a vacuum is being formed inVby condensation, the steam, flowing throughT′, blows out the air throughB′, as already described in the other vesselV; and while the air inSis rushing up throughAintoV, followed by the water raised inSby the atmospheric pressure, the vesselV′ is being filled with steam, and the air is completely expelled from it.

The communication betweenTand the boiler is now again opened, and the communication betweenT′and the boiler closed by moving the regulatorR(fig.11.) from the tubeTtoT′; at the same time the condensing pipe is removed from overV, and brought to play uponV′. While the steam once more expels the air fromVthroughB, a vacuum is formed by condensation inV′, into which the water inSrushes through the valveA′. In the mean timeVis again filled with steam. The communication betweenTand the boiler is now closed, and that betweenT′and the boiler is opened, and the condensing pipe removed fromV′, and brought to play onV. While the steam from the boiler forces the water inV′throughB′into the force-pipeF, a vacuum is being produced inV, into which water is raised by the atmospheric pressure.

Thus each of the vesselsVV′is alternately filled fromS, and the water thence forced intoF. The same steam which forces the water from the vessels intoF, having done its duty, is condensed, and brings up the water fromS, by giving effect to the atmospheric pressure.

During this process, two alternate motions or adjustments must be constantly made; the communication betweenTand the boiler must be opened, and that betweenT′and the boiler closed, which is done by one motion of the regulator. The condensing pipe at the same time must be brought fromVto play onV′, which is done by the lever placed upon it. Again[Pg056]the communication betweenT′and the boiler is to be opened, and that betweenTand the boiler closed; this is done by moving back the regulator. The condensing pipe is brought fromV′toVby moving back the other lever, and so on alternately.

For the clearness and convenience of description, some slight and otherwise unimportant changes have been made in the position of the parts. A perspective view of this engine is represented at the head of this chapter. The different parts already described will easily be recognised.

The engine of Savery was very clearly described in a small work published in London in 1702, entitled,The Miner's Friend, or an Engine to raise Water by Fire described, and the Manner of Fixing it in Mines; with an Account of the several Uses it is applicable unto, and an Answer to the Objection made against it; by Thomas Savery, Gentleman. This volume was dedicated to William III. (to whom the engine had been exhibited at Hampton Court palace), to the Royal Society, and to the mining adventurers of England. The following are the uses to which Savery proposed the engine should be applied:First, to raise water for turning all sorts of mills;second, supplying palaces and houses with water, and supplying means of extinguishing fire therein by the water so raised;third, the supplying cities and towns with water;fourth, draining fens or marshes;fifth, for ships;sixth, the drainage of mines.

Dr. Harris, in hisLexicon Technicum, or Dictionary of Arts and Sciences, mentions a machine of Savery's for propelling a vessel in a calm, by paddle-wheels placed at the side; but it does not appear that Savery contemplated the application of a steam engine to work these wheels.

It is only from scattered passages in publications of the day that it can be ascertained to what extent the engines of Savery were practically applied. In his address to the Royal Society, he speaks of the "difficulties and expense which he encountered in instructing artisans to make engines according to his wish; but that after much experience the workmen had become such masters of the thing, that they bound themselves to deliver the engines 'exactly tight and fit for[Pg057]service, and such as he (Savery) dare warrant them to every one that has occasion for them.'"

In his address to the miners of England he also says, "that the frequent disorders and cumbersomeness of water engines then in use encouraged him to invent engines to work by this new force; that though they were obliged to encounter the oddest and almost insuperable difficulties, yet he spared neither time, pains, nor money, till he had conquered them."

In Bradley'sImprovements of Planting and Gardening, 1718, the author thus speaks of an engine erected by Savery:—

"Supposing the situation of a house or garden to be a considerable height above any pond, river, or spring, and that it has at present no other conveniency of water than what is brought continually by men or horses to it. In this case, the wonderful invention of the late Mr. Savery, F.R.S., for raising water by fire, will not only supply the defect, by flinging up as much water as may be desired, but may be maintained with very little trouble and very small expense.

"It is now about six years since Mr. Savery set up one of them for that curious gentleman Mr. Balle, at Cambden House, Kensington, near London, which has succeeded so well that there has not been any want of water since it has been built; and, with the improvements since made to it, I am apt to believe will be less subject to be out of order than any engine whatever."

It is remarkable that, notwithstanding the high pressure steam necessary for the operation of Savery's engine, he does not appear to have adopted the obvious expedient of a safety valve. The safety valve had been previously known, having been invented about the year 1681, by Papin, for his digester, which was a close boiler, contrived by him for stewing meat and digesting bones, by submitting them to a higher temperature than that of water boiling in an open vessel.

The safety valve which has ever since been used for steam boilers of every kind is a valve which opens outwards, and is fitted to an aperture in the boiler, so as to be steam tight. It is pressed down by a weight, the amount of which is regulated by the maximum pressure to which it is intended the steam[Pg058]shall be limited. Thus, if the magnitude of the valve be a square inch, and the pressure of the steam be limited to 10 lbs. per square inch above the pressure of the atmosphere, then the valve would be loaded with a weight of 10 lbs.; but as it was found necessary to vary from time to time the limiting pressure of the steam, or the load of the safety valve, these valves were usually constructed so as to be held down by the pressure of a lever having a sliding weight upon it. By moving the weight on the arm of the lever, the pressure on the valve could be increased or diminished at the discretion of the engineer. This contrivance was first applied to Savery's engines, by Desaguliers, about the year 1717, before which year Savery died.

It is justly observed by Mr. Farey, in his treatise on the steam engine, that, "when a comparison is made between Captain[11]Savery's engine and those of his predecessors, the result will be in every respect favourable to his character as an inventor, and as a practical engineer; all the details of his invention are made out in a masterly style, and accidents and contingencies are provided for, so as to render it a real working engine; whereas De Caus, the Marquis of Worcester, Sir Samuel Morland, and Papin, though ingenious philosophers, only produced mere outlines, which required great labour and skill of subsequent inventors to fill up, and make them sufficiently complete to be put in execution."

About the year 1718 further improvements were made in the construction of Savery's engine, by Dr. Desaguliers; but it is probable that some of these were suggested by the proceedings of the inventors of the atmospheric engine, which shall presently describe.

(33.)In order duly to appreciate the value of improvements, it is necessary first to perceive the defects which these improvements are designed to remove. Savery's steam engine, considering how little was known of the value and properties of steam, and how low the general standard of mechanical knowledge was in his day, is certainly highly[Pg059]creditable to his genius. Nevertheless it had very considerable defects, and was finally found to be inefficient for the most important purposes to which he proposed applying it.

At the time of this invention, the mines in England had greatly increased in depth, and the process of draining them had become both expensive and difficult; so much so, that it was found in many instances that their produce did not cover the cost of working them. The drainage of these mines was the most important purpose to which Savery proposed to apply his steam engine.

It has been already stated that the pressure of the atmosphere amounts to about fifteen pounds on every square inch. Now, a column of water, whose base is one square inch, and whose height is thirty-four feet, weighs about fifteen pounds. If we suppose that a perfect vacuum were produced in the steam vesselsV V′(fig.12.) by condensation, the atmospheric pressure would fail to force up the water, if the height of the top of these vessels above the water to be raised exceeded thirty-four feet. It is plain, therefore, that the engine cannot be more than thirty-four feet above the water which it is intended to elevate. But in fact it cannot be so much; for the vacuum produced in the steam vesselsV V′is never perfect. Water, when not submitted to the pressure of the atmosphere, will vaporise at a very low temperature, as we shall hereafter explain; and it was found that a vapour possessing a considerable elasticity would, notwithstanding the condensation, remain in the vesselsV V′and the pipeS, and would oppose the ascent of the water. In consequence of this, the engine could never be placed with practical advantage at a greater height than twenty-six feet above the level of the water to be raised.

(34.)When the water is elevated to the engine, and the steam vessels filled, if steam be introduced above the water inV, it must first balance the atmospheric pressure, before it can force the water through the valveB. Here, then, is a mechanical pressure of fifteen pounds per square inch expended, without any water being raised by it. If steam of twice that elastic force be used, it will elevate a column inFof thirty-four feet in height; and if steam of triple the force be used, it will raise a column of sixty-eight feet high,[Pg060]which, added to twenty-six feet raised by the atmosphere, gives a total lift of ninety-four feet.

In effecting this, steam of a pressure equal to three times that of the atmosphere acts on the inner surface of the vesselsV V′. One third of this bursting pressure is balanced by the pressure of the atmosphere on the external surface of the vessels; but an effective pressure of thirty pounds per square inch still remains, tending to burst the vessels. It was found that the apparatus could not be constructed to bear more than this with safety; and, therefore, in practice, the lift of such an engine was limited to about ninety perpendicular feet. In order to raise the water from the bottom of the mine by these engines, therefore, it was necessary to place one at every ninety feet of the depth; so that the water raised by one through the first ninety feet should be received in a reservoir, from which it was to be elevated the next ninety feet by another, and so on.

Besides this, it was found that sufficient strength could not be given to those engines, if constructed upon a large scale.

They were, therefore, necessarily very limited in their dimensions, and were incapable of raising the water with sufficient speed. Hence arose a necessity for several engines at each level, which greatly increased the expense.

(35.)These, however, were not the only defects of Savery's engines. The consumption of fuel was enormous, the proportion of heat wasted being much more than what was used in either forcing up the water, or producing avacuum. This will be very easily understood by attending to the process of working the engine already described.

When the steam is first introduced from the boiler into the steam vesselsV V′, preparatory to the formation of a vacuum, it is necessary that it should heat these vessels up to the temperature of the steam itself; for until then the steam will be condensed the moment it enters the vessel by the cold surface. All this heat, therefore, spent in raising the temperature of the steam vessels is wasted. Again, when the water has ascended and filled the vesselsV V′, and steam is introduced to force this water throughB B′intoF, it is immediately condensed by the cold surface inV V′, and does not[Pg061]begin to act until a quantity of hot water, formed by condensed steam, is collected on the surface of the cold water which fills these vessels. Hence another source of the waste of heat arises.

When the steam begins to act upon the surface of the water inV V′, and to force it down, the cold surface of the vessels is gradually exposed to the steam, and must be heated while the steam continues its action; and when the water has been forced out of the vessel, the vessel itself has been heated to the temperature of the steam which fills it, all which heat is dissipated by the subsequent process of condensation. It must thus be evident that the steam used in forcing up the water inF, and in producing a vacuum, bears a very small proportion indeed to what is consumed in heating the apparatus after condensation.

(36.)There is also another circumstance which increases the consumption of fuel. The water must be forced throughB, not only against the atmospheric pressure, but also against a column of sixty-eight feet of water. Steam is therefore required of a pressure of forty-five pounds on the square inch. Consequently the water in the boiler must be boiled under this pressure. That this should take place, it is necessary that the water should be raised to a temperature considerably above 212°, even so high as 275°; and thus an increased heat must be given to the boiler. Independently of the other defects, this intense heat weakened and gradually destroyed the apparatus.

Savery was the first who suggested the method of expressing the power of an engine with reference to that of horses. In this comparison, however, he supposed each horse to work but eight hours a day, while the engine works for twenty-four hours. This method of expressing the power of steam engines will be explained hereafter.

(37.)The failure of the engines proposed by Captain Savery in the work of drainage, from the causes which have been just mentioned, and the increasing necessity for effecting this object, arising from the large property in mines which became every year unproductive by being flooded, stimulated the ingenuity[Pg062]of mechanics to contrive some means of rendering those powers of steam exhibited in Savery's engine available.

Thomas Newcomen, the reputed inventor of the atmospheric engine, was an ironmonger, or, according to some, a blacksmith, in the town of Dartmouth in Devonshire. From his personal acquaintance and intercourse with Dr. Hooke, the celebrated natural philosopher, it is probable that he was a person of some education, and therefore likely to be above the position of a blacksmith. Being in the habit of visiting the tin mines in Cornwall, Newcomen became acquainted with the engine invented by Savery, and with the causes which led to its inefficiency for the purposes of drainage.

It has been stated that Papin, about the year 1690, proposed the construction of an engine working by the atmospheric pressure acting on one side of a piston against a vacuum produced by the condensation of steam on the other side. Papin was not conscious of the importance of this principle; for, so far from ever having attempted to apply it to practical purposes, he probably never constructed, even on a small scale, any machine illustrating it. On the contrary, he abandoned the project the moment he was informed of the principle and structure of the steam engine of Savery; and he then proposed an engine for raising water, acting by the expansive force of steam similar to Savery's, but abandoning the method of working by a vacuum.

This engine is described by Papin in a work published in 1707.

Fig. 13.

A(fig.13.) is an oval boiler, having a safety-valveB, which limits the pressure of the steam. It is connected with a cylinderC, by a curved pipe having a stop-cock atD. A pipe with a stop-cockGopens from the top of the cylinder into the atmosphere, and a safety-valveFis placed upon the cylinder. A hollow copper pistonHmoves freely in the cylinder, and floats upon the water.Ois a funnel with a valveLin the bottom, opening downwards, through which the cylinderCmay be filled with water to the level of the top of the funnel. A close air-vessel communicates with the cylinderCby the curved tube, and has a valveKopening upwards. The force-pipe through which the water is raised communicates[Pg063]with the air-vesselI. If the cockDbe shut, and the cockGopened, water poured into the funnelOwill rise into the cylinderC, the air which fills the cylinder escaping through the open pipeG. When the cylinder is thus filled with water, let the cockGbe closed, and the cockDopened. The steam from the boiler, after heating the metal of the cylinder, will force the piston downwards, and drive the water through the curved tube into the vesselI, from which its return is prevented by the valveK, which is closed by its weight. The air which filled the vesselIwill then be compressed, and by its elasticity will drive a column of water up the pipeN. After the contents of the cylinder have been thus discharged it may be refilled in the same manner, and the process repeated.

It will be perceived that this project is nothing more than a reproduction of the engine of the Marquis of Worcester. In the preface to the work containing this description, Papin gives an extract from a letter addressed by him to Leibnitz in 1698, from which it appears that he had abandoned his idea of working the piston by the atmospheric pressure acting against a vacuum, considering it to be a contrivance inferior[Pg064]to the engine now described. "We now raise water," he says, "by the force of fire,in a more advantageous manner than that which I had published some years before; for besides the suction, we now also use the pressure which the water exerts upon other bodies in dilating itself by heat; instead of which I before employed the suction only, the effects of which are more limited."

From documents which have been preserved in the Royal Society, it appears that Newcomen was acquainted with Papin's writings, and therefore probably first derived from them the suggestion which he subsequently realised in the atmospheric engine. Among some papers of Dr. Hooke's have been found notes for the use of Newcomen, on Papin's method of transmitting the force of a stream or fall of water to a distance by pipes. Hooke dissuaded Newcomen from attempting any machine on this principle, which, as first proposed by Papin, was impracticable. He exposed the fallacy of Papin's first project in several discourses before the Royal Society, and considered his improved edition of it, though free from fallacy, as impracticable.

Papin's project for producing a vacuum under a piston by condensing the steam having been published in theActæ Eruditorum, in Latin, in 1690, and in French, at Cassel, in 1695, and subsequently, in thePhilosophical Transactions, in England in 1697, cannot be supposed to be unknown to Dr. Hooke; and if known to him, would probably have been communicated to Newcomen. Dr. Hooke died in 1703, some years before the date of Newcomen's invention.

John Cawley, who was the associate of Newcomen in his experiments and inquiries, was a plumber and glazier of the same town. Newcomen and Cawley obtained a patent for the atmospheric engine in 1705, in which Savery was associated, he having previously obtained a patent for the method of producing a vacuum by the condensation of steam, which was essential to Newcomen's contrivance. It was not, however, until about the year 1711 that any engine had been constructed under this patent.

In the latter end of that year, according to Desaguliers, the patentees "made proposals to drain a colliery at Griff, in[Pg065]Warwickshire, in which work five hundred horses were constantly employed. This proposal not being accepted, they contracted, in the following March, to drain water for Mr. Back of Wolverhampton, where, after many laborious attempts, they succeeded in making their engine work; but not being either philosophers to understand the reason, or mathematicians enough to calculate the power and proportions of the parts, they very luckily, by accident, found what they sought for."

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