(74.)

(74.)Although Watt was thus attracted by pursuits foreign to his recent investigations respecting the improvement of steam power, he never lost sight of that object. It was not until the year 1768, three years after his great discoveries, that any step was taken to enable him to carry them into effect on a large scale. At that time his friends brought him into communication with Dr. Roebuck, the proprietor of the Carron Iron Works, who rented extensive coal works at Kinneal from the Duchess of Hamilton. Watt was first employed by Roebuck as a civil engineer; but when he made known to him the improvements he had projected in the steam engine, Roebuck proposed to take out a patent for an engine on the principle of the model which had been fitted up at Delft House, and to join Watt in a partnership, for the construction of such engines. Sensible of the advantages to be derived from the influence of Roebuck, and from his command of capital, Watt agreed to cede to him two thirds of the advantages to be derived from the invention. A patent was accordingly taken out on the fifth of January, 1769, nearly four years after the invention had been completed; and an experimental engine on a large scale was constructed by him, and fitted up at Kinneal House. In the first trial this machine more than fulfilled Watt's anticipations. Its[Pg131]success was complete. In the practical details of its construction, however, some difficulties were still encountered, the greatest of which consisted in packing the piston, so as to be steam-tight. The principle of the new engine did not admit of water being kept upon the piston, to prevent leakage, as in the old engines; he was therefore obliged to have his cylinders much more accurately bored, and more truly cylindrical, and to try a great variety of soft substances for packing the piston, which would make it steam-tight without great friction, and maintain it so in a situation perfectly dry, and at the temperature of boiling water.

While Watt was endeavouring to overcome these and other difficulties, in the construction of the machine, his partner, Dr. Roebuck, became embarrassed, by the failure of his undertaking in the Borrowstowness coal and salt works; and he was unable to supply the means of prosecuting with the necessary vigour the projected manufacture of the new engines.

The important results of Watt's labours having happily at this time become more publicly known, Mr. Matthew Boulton, whose establishment at Soho, near Birmingham, was at that time the most complete manufactory for metal-work in England, and conducted with unexampled enterprise and spirit, proposed to purchase Dr. Roebuck's interest in the patent. This arrangement was effected in the year 1773, and in the following year Mr. Watt removed to Soho, where a portion of the establishment was allotted to him, for the erection of a foundery, and other works necessary to realise his inventions on a grand scale.

The patent which had been granted in 1769 was limited to a period of fourteen years, and would consequently expire about the year 1783. From the small progress which had hitherto been made in the construction of engines upon the new principle, and from the many difficulties still to be encountered, and the large expenditure of capital which must obviously be incurred before any return could be obtained, it was apparent that unless an extension of the patent right could be obtained, Boulton and Watt could never expect any advantage adequate to the risk of their great[Pg132]enterprise. In the year 1774 an application was accordingly made to parliament for an extension of the patent, which was supported by the testimony of Dr. Roebuck, Mr. Boulton, and others, as to the merits and probable utility of the invention. An Act was accordingly passed, in 1775, extending the term of the patent until the year 1800.

(75.)The following abstract of this Act may not be uninteresting at this time, when the anticipations expressed in it have been so successfully and extensively realised:—

"An Act for vesting in James Watt, engineer, his executors, administrators, and assigns, the sole use and property of certain steam engines, commonly called fire engines, of his invention, throughout his majesty's dominions, for a limited time:

"And whereas the said James Watt hath employed many years, and a considerable part of his fortune, in making experiments upon steam engines, commonly called fire engines, with a view to improve those very useful machines, by which several very considerable advantages over the common steam engines are acquired; but upon account of the many difficulties which always arise in the execution of such large and complex machines, and of the long time requisite to make the necessary trials, he could not complete his intention before the end of the year 1774, when he finished some large engines as specimens of his construction, which have succeeded, so as to demonstrate the utility of the said invention:

"And whereas, in order to manufacture these engines with the necessary accuracy, and so that they may be sold at moderate prices, a considerable sum of money must be previously expended in erecting mills and other apparatus; and as several years and repeated proofs will be required before any considerable part of the public can be fully convinced of the utility of the invention, and of their interest to adopt the same, the whole term granted by the said letters patent may probably elapse before the said James Watt can receive an advantage adequate to his labour and invention:

"And whereas, by furnishing mechanical power at much less expense, and in more convenient forms, than has hitherto been done, his engines may be of great utility, in facilitating[Pg133]the operations in many great works and manufactures of this kingdom; yet it will not be in the power of the said James Watt to carry his invention into that complete execution which he wishes, and so as to render the same of the highest utility to the public of which it is capable, unless the term granted by the said letters patent be prolonged, and his property in the said invention secured for such time as may enable him to obtain an adequate recompense for his labour, time, and expense:

"To the end, therefore, that the said James Watt may be enabled and encouraged to prosecute and complete his said invention, so that the public may reap all the advantages to be derived therefrom in their fullest extent: it is enacted,

"That from and after the passing of this Act, the sole privilege and advantage of making, constructing, and selling the said engines hereinbefore particularly described, within the kingdom of Great Britain, and his majesty's colonies and plantations abroad, shall be, and are hereby declared to be, vested in the said James Watt, his executors, administrators, and assigns, for and during the term of twenty-five years," &c. &c.

(76.)Thus protected and supported, Watt now directed the whole vigour of his mind to perfect the practical details of his invention, and the result was, the construction on a large scale of the engine which has since been called hisSingle acting Steam Engine.

It is necessary to recollect, that notwithstanding the extensive and various application of steam power in the arts and manufactures, at the time to which our narrative has now reached, the steam engine had never been employed for any other purpose save that of raising water by working pumps. The motion, therefore, which was required was merely an upward force, such as was necessary to elevate the piston of a pump, loaded with the column of water which it raised. The following then is a description of the improved engine of Watt, by which such work was proposed to be performed:—

Fig. 21.

In the cylinder represented atC(fig.21.), the pistonPmoves steam-tight. It is closed at the top, and the piston[Pg134]rod, being accurately turned, runs in a steam-tight collar,B, furnished with a stuffing-box, and is constantly lubricated with melted tallow. A funnel is screwed into the top of the cylinder, through which, by opening a stop-cock, melted[Pg135]tallow is permitted from time to time to fall upon the piston within the cylinder, so as to lubricate it, and keep it steam-tight. Two boxes,A A, called the upper and lower steam boxes, contain valves by which steam from the boiler may be admitted and withdrawn. These steam boxes are connected by a tube of communicationT, and they communicate with the cylinder at the top and bottom by short tubes represented in the figure. The upper steam boxAcontains one valve, by which a communication with the boiler may be opened or closed at pleasure. The lower valve box contains two valves. The lower valveIcommunicates with the tubeT′, leading to the condenserD, which being opened or closed, a communication is made or cut off at pleasure, between the cylinderCand the condenserD. A second valve, or upper valveH, which is represented closed in the figure, may be opened so as to make a free communication between the cylinderCand the tubeT, and by that means between the cylinderC, below the piston and the space above the piston. The condenserDis submerged in a cistern of cold water. At the side there enters it a tube,E, governed by a cock, which being opened or closed to any required extent, a jet of cold water may be allowed to play in the condenser, and may be regulated or stopped, at pleasure. This jet, when playing, throws the water upwards in the condenser towards the mouth of the tubeT′, as water issues from the rose of a watering pot. The tubeSproceeds from the boiler, and terminates in the steam boxA, so that the steam supplied from the boiler constantly fills that box. The valveGis governed by levers, whose pivots are attached to the framing of the engine, and is opened or closed at pleasure, by raising or lowering the leverG′. The valveG, when open, will therefore allow steam to pass from the boiler through the short tube to the top of the piston, and this steam will also fill the tubeT. If the lower valveHbe closed, its circulation beyond that point will be stopped; but if the valveHbe open, the valveIbeing closed, then the steam will circulate equally in the cylinder, above and below the piston. If the valveIbe open, then steam will rush through the tubeT′into the condenser; but this escape of the steam will be[Pg136]stopped, if the valveIbe closed. The valveHis worked by the leverH′, and the valveIby the leverI′.

The valveGis called the upper steam valve,Hthe lower steam valve,Ithe exhausting valve, andEthe condensing valve.

From the bottom of the condenserDproceeds a tube leading to the air-pump, which is also submerged in the cistern of cold water. In this tube is a valveM, which opens outwards from the condenser towards the air-pump. In the piston of the air-pumpNis a valve which opens upwards. The piston-rodQof the air-pump is attached to a beam of wood called a plug frame, which is connected with the working beam by a flexible chain playing on the small arch-head immediately over the air-pump. From the top of the air-pump barrel above the piston proceeds a pipe or passage leading to a small cistern,B, called the hot well. The pipe which leads to this well, is supplied with a valve,K, which opens outwards from the air pump barrel towards the well. From the nature of its construction, the valveMadmits the flow of water from the condenser towards the air-pump, but prevents its return; and, in like manner, the valveKadmits the flow of water from the upper part of the air-pump barrel into the hot wellB, but obstructs its return.

Let us now consider how these valves should be worked in order to move the piston upwards and downwards with the necessary force. It is in the first place necessary that all the air which fills the cylinder, the tubes and the condenser shall be expelled. To accomplish this it is only necessary to open at once the three valvesG,H, andI. The steam then rushing from the boiler through the steam-pipeS, and the open valveGwill pass into the cylinder above the piston, will fill the tubeT, pass through the lower steam valveH, will fill the cylinderCbelow the piston, and will pass through the open valveIinto the condenser. If the valveEbe closed so that no jet shall play in the condenser, the steam rushing into it will be partially condensed by the cold surfaces to which it will be exposed; but if the boiler supply it through the pipeSin sufficient abundance, it will rush with violence through the cylinder and all the passages, and its pressure in the[Pg137]condenserD, combined with that of the heated air with which it is mixed, will open the valveM, and it will rush through mixed with the air into the air-pump barrelN. It will press the valves in the air-pump piston upwards, and, opening them, will rush through, and will collect in the air-pump barrel above the piston. It will then, by its pressure, open the valveK, and will escape into the cisternB.

Throughout this process the steam, which mixed with the air fills the cylinder, condenser and air-pumps will be only partially condensed in the last two, and it will escape mixed with air through the valveK, and this process will continue until all the atmospheric air which at first filled the cylinder, tubes, condenser and air-pump barrel shall be expelled through the valveK, and these various spaces shall be filled with pure steam. When that has happened let us suppose all the valves closed. In closing the valveIthe flow of steam to the condenser will be stopped, and the steam contained in it will speedily be condensed by the cold surface of the condenser, so that a vacuum will be produced in the condenser, the condensed steam falling in the form of water to the bottom. In like manner, and for like reasons, a vacuum will be produced in the air-pump. The valveM, and the valves in the air-pump piston will be closed by their own weight.

By this process, which is calledblowing through, the atmospheric air, and other permanent gases, which filled the cylinder, tubes, condenser and air-pump are expelled, and these spaces will be a vacuum. The engine is then prepared to be started, which is effected in the following manner:—The upper steam valveGis opened, and steam allowed to flow from the boiler through the passage leading to the top of the cylinder. This steam cannot pass to the bottom of the cylinder, since the lower steam valveHis closed. The space in the cylinder below the piston being therefore a vacuum, and the steam pressing above it the piston will be pressed downwards with a corresponding force. When it has arrived at the bottom of the cylinder the steam valveGmust be closed, and at the same time the valveHopened. The valveIleading to the condenser being also closed, the steam[Pg138]which fills the cylinder above the piston is now admitted to circulate through the open valveHbelow the piston, so that the piston is pressed equally upwards and downwards by steam, and there is no force to resist its movement save its friction with the cylinder. The weight of the pump rods on the opposite end of the beam being more than equivalent to overcome this the piston is drawn to the top of the cylinder, and pushes before it the steam which is drawn through the tubeT, and the open valveH, and passes into the cylinderCbelow the piston.

When the piston has thus arrived once more at the top of the cylinder, let the valveHbe closed, and at the same time the valvesGandIopened, and the condensing cockEalso opened, so as to admit the jet to play in the condenser. The steam which fills the cylinderCbelow the piston, will now rush through the open valveIinto the condenser which has been hitherto a vacuum, and there encountering the jet, will be instantly converted into water, and a mixture of condensed steam and injected water will collect in the bottom of the condenser. At the same time, the steam proceeding from the boiler by the steam pipeSto the upper steam boxA, will pass through the open steam valveGto the top of the piston, but cannot pass below it because of the lower steam valveHbeing closed. The piston, thus acted upon above by the pressure of the steam, and the space in the cylinder below it being a vacuum, its downward motion is resisted by no force but the friction, and it is therefore driven to the bottom of the cylinder. During its descent the valvesG,I, andEremained open. At the moment it arrives at the bottom of the cylinder, all these three valves are closed, and the valveHopened. The steam which fills the cylinder above the piston is now permitted to circulate below it, by the open valveH, and the piston being consequently pressed equally upwards and downwards will be drawn upwards as before by the preponderance of the pump rods at the opposite end of the beam. The weight of these rods must also be sufficiently great to draw the air-pump pistonNupwards. As this piston rises in the air-pump, it leaves a vacuum below it into which the water and air collected in the condenser will be drawn through the valveM, which opens outwards. When the[Pg139]air-pump piston has arrived at the top of the barrel, which it will do at the same time that the steam piston arrives at the top of the cylinder, the water and the chief part of the air or other fluids which may have been in the condenser will be drawn into the barrel of the air-pump, and the valveMbeing closed by its own weight, assisted by the pressure of these fluids they cannot return into the condenser. At the moment the steam piston arrives at the top of the cylinder, the valveHis closed, and the three valvesG,I, andEare opened. The effect of this change is the same as was already described in the former case, and the piston will in the same manner and from the same causes be driven downwards. The air-pump piston will at the same time descend by the force of its own weight, aided by the weight of the plug-frame attached to its rod. As it descends, the air below it will be gradually compressed above the surface of the water in the bottom of the barrel, until its pressure becomes sufficiently great to open the valves in the air-pump piston. When this happens, the valves in the air-pump piston, as represented on a large scale infig.22., will be opened, and the air will pass through them above the piston. When the piston comes in contact with the water in the bottom of the barrel, this water will likewise pass through the open valves. When the piston has arrived at the bottom of the air-pump barrel, the valves in it will be closed by the pressure of the fluids above them. The next ascent of the steam piston will draw up the air-pump piston, and with it the fluids in the pump barrel above it. As the air-pump[Pg140]piston approaches the top of its barrel, the air and water above it will be drawn through the valveKinto the hot cisternB. The air will escape in bubbles through the water in that cistern, and the warm water will be deposited in it.

Fig. 22.

The magnitude of the opening in the condensing valveE, must be regulated by the quantity of steam admitted to the cylinder. As much water ought to be supplied through the injection valve as will be sufficient to condense the steam contained in the cylinder, and also to reduce the temperature of the water itself, when mixed with the steam, to a sufficiently low degree to prevent it from producing vapour of a pressure which would injuriously affect the working of the piston. It has been shown, that five and a half cubic inches of ice-cold water mixed with one cubic inch of water in the state of steam would produce six and a half cubic inches of water at the boiling temperature. If then the cylinder contained one cubic inch of water in the state of steam, and only five and a half cubic inches of water were admitted through the condensing jet, supposing this water, when admitted, to be at the temperature of 32°, then the consequence would be that six and a half cubic inches of water at the boiling temperature would be produced in the condenser. Steam would immediately arise from this, and at the same time the temperature of the remaining water would be lowered by the amount of the latent heat taken up by the steam so produced. This vapour would rise through the open exhausting valveI, would fill the cylinder below the piston, and would impair the efficiency of the steam above pressing it down. The result of the inquiries of Watt respecting the pressure of steam at different temperatures, showed, that to give efficiency to the steam acting upon the piston it would always be necessary to reduce the temperature of the water in the condenser to 100°.

Let us then see what quantity of water at the common temperature would be necessary to produce these effects.

If the latent heat of steam be taken at 1000°, a cubic inch of water in the state of steam may be considered for the purposes of this computation, as equivalent to one cubic inch of water at 1212°. Now the question is, how many cubic inches of water at 60° must be mixed with this, in order that the[Pg141]mixture may have the temperature of 100°? This will be easily computed. As the cubic inch of water at 1212° is to be reduced to 100°, it must be deprived of 1112° of its temperature. On the other hand, as many inches of water at 60° as are to be added, must be raised in the same mixture to the temperature of 100°, and therefore each of these must receive 40° of temperature. The number of cubic inches of water necessary to be added will therefore be determined by finding how often 40° are contained in 1112°. If 1112 be divided by 40, the quotient will be 27·8. Hence it appears, that to reduce the water in the condenser to the temperature of 100°, supposing the temperature of the water injected to be 60°, it will be necessary to supply by the injection cock very nearly twenty-eight times as much water as passes through the cylinder in the state of steam; and therefore if it be supposed that all the water evaporated in the boiler passes through the cylinder, it follows that about twenty-eight times as much water must be thrown into the condenser as is evaporated in the boiler.

From these circumstances it will be evident that the cold cistern in which the condenser and air-pump are submerged, must be supplied with a considerable quantity of water. Independently of the quantity drawn from it by the injection valve, as just explained, the water in the cistern itself must be kept down to a temperature of about 60°. The interior of the condenser and air-pump being maintained by the steam condensed in them at a temperature not less than 100°; the outer surfaces of these vessels consequently impart heat to the water in the cold cistern, and have therefore a tendency to raise the temperature of that water. To prevent this, a pump called thecold pump, represented atLinfig.21., is provided. By this pump water is raised from any convenient reservoir, and driven through proper tubes into the cold cistern. This cold pump is wrought by the engine, the rod being attached to the beam. Water being, bulk for bulk, heavier the lower its temperature, it follows that the water supplied by the cold pump to the cistern will have a tendency to sink to the bottom, pressing upwards the warmer water contained in it. A waste-pipe is provided, by which this[Pg142]water is drained off, and the cistern therefore maintained at the necessary temperature.

From what has been stated, it is also evident that the hot wellB, into which the warm water is thrown by the air-pump, will receive considerably more water than is necessary to feed the boiler. A waste-pipe, to carry off this, is also provided; and the quantity necessary to feed the boiler is pumped up by a small pump,O, the rod of which is attached to the beam, as represented infig.21., and which is worked by the engine. The water raised by this pump is conducted to a reservoir from which the boiler is fed, by means which will be hereafter explained.

We shall now explain the manner in which the machine is made to open and close the valves at the proper times. By referring to the explanation already given, it will be perceived that at the moment the piston reaches the top of the cylinder, the upper steam valveGmust be open, to admit the steam to press it down; while the exhausting valveImust be opened, to allow the steam to pass to the condenser; and the condensing valveEmust be opened, to let in the water necessary for the condensation of the steam; and at the same time the lower steam valveHmust be closed, to prevent the passage of the steam which has been admitted throughG. The valvesG,I, andEmust be kept open, and the valveHkept closed, until the piston arrives at the bottom of the cylinder, when it will be necessary to close all the three valves,G,I, andE, and to open the valveH, and the same effects must be produced each time the piston arrives at the top and bottom of the cylinder. All this is accomplished by a system of levers, which are exhibited infig.21.The pivots on which these levers play are represented on the framing of the engine, and the arms of the leversG′,H′, andI′, communicating with the corresponding valvesG,H, andI, are represented opposite a bar attached to the rod of the air-pump, called theplug frame. This bar carries certain pegs and detents, which act upon the arms of the several levers in such a manner that, on the arrival of the beam at the extremities of its play upwards and downwards, the levers are so struck that the valves are opened and closed at the proper[Pg143]times. It is needless to explain all the details of this arrangement. Let it be sufficient, as an example of all, to explain the method of working the upper steam valveG. When the piston reaches the top of the cylinder, a pin strikes the arm of the leverG′, and throws it upwards: this, by means of the system of levers, pulls the arm of the valveGdownwards, by which the upper steam valve is raised out of its seat, and a passage is opened from the steam pipe to the cylinder. The valve is maintained in this state until the piston reaches the bottom of the cylinder, when the armG′is pressed downwards, by which the armGis pressed upwards, and the valve restored to its seat. By similar methods the levers governing the other three valves,H,I, andE, are worked.

Fig. 23.

Fig. 24.

The valves used in these engines were of the kind calledspindle valves. They consisted of a flat circular plate of bell metal,A B,fig.23., with a round spindle passing perpendicularly through its centre, and projecting above and below it. This valve, having a conical form, was fitted very exactly, by grinding into a corresponding circular conical seat,A B C D,fig.24., which forms the passage which it is the office of the valve to open and close. When the valve falls into its seat, it fits the aperture like a plug, so as entirely to stop it. The spindle plays in sockets or holes, one above and the other below the aperture which the valve stops; these holes keep the valve in its proper position, so as to cause it to drop exactly into its place.

In the experimental engine made by Mr. Watt at Kinneal, he used cocks, and sometimes sliding covers, like the regulator described in the old engines; but these he found very soon to become leaky. He was, therefore, obliged to change them for the spindle valves just described, which, being truly[Pg144]ground, and accurately fitted in the first instance, were not so liable to go out of order. These valves are also calledpuppet clacks, orbutton valves.

In the earlier engines constructed by Watt, the condensation was produced by the contact of cold surfaces, without injection. The reason of rejecting the method of condensing by injection was, doubtless, to avoid the injurious effects of the air, which would always enter the condenser, in combination with the water of condensation, and vitiate the vacuum. It was soon found, however, that a condenser acting by cold surfaces without injection, being necessarily composed of narrow pipes or passages, was liable to incrustation from bad water, by which the conducting power of the material of the condenser was diminished; so that, while its outer surface was kept cold by the water of the cold cistern, the inner surface might, nevertheless, be so warm that a very imperfect condensation would be produced.

SOHO, BIRMINGHAM.

FOOTNOTES:[19]Eloge, p. 308.

[19]Eloge, p. 308.

BIRMINGHAM.

[Pg145]TOCINX

CORRESPONDENCE OF WATT WITH SMEATON.—FAILURE OF CONDENSATION BY SURFACE.—IMPROVEMENTS IN CONSTRUCTION OF PISTON.—METHOD OF PACKING.—IMPROVEMENTS IN BORING THE CYLINDERS.—DISADVANTAGES OF THE NEW COMPARED WITH THE OLD ENGINES.—GREATLY INCREASED ECONOMY OF FUEL.—EXPEDIENTS TO FORCE THE NEW ENGINES INTO USE.—CORRESPONDENCE WITH SMEATON.—EFFICIENCY OF FUEL IN THE NEW ENGINES.—DISCOVERY OF THE EXPANSIVE ACTION OF STEAM.—WATT STATES IT IN A LETTER TO DR. SMALL.—ITS PRINCIPLE EXPLAINED.—MECHANICAL EFFECT RESULTING FROM IT.—COMPUTED EFFECT OF CUTTING OFF STEAM AT DIFFERENT PORTIONS OF THE STROKE.—PRODUCES A VARIABLE POWER.—EXPEDIENTS FOR EQUALISING THE POWER.—LIMITATION OF THE EXPANSIVE PRINCIPLE IN WATT'S ENGINES.—ITS MORE EXTENSIVE APPLICATION IN THE CORNISH ENGINES.

(77.)In a letter addressed by Watt to Smeaton, dated April, 1766, Watt refers to some of these practical difficulties which he had to encounter. "I have been," says he, "tormented with exceedingly bad health, resulting from the operation of an anxious mind, the natural consequence of staking everything[Pg146]upon the cast of a die; for in that light I look upon every project which has not received the sanction of repeated success.

"I have made considerable alterations in our engine lately, particularly in the condenser. That which I used at first was liable to be impaired, from incrustations from bad water; therefore we have substituted one which works by an injection. In pursuing this idea I have tried several kinds, and have at last come to one, which I am not inclined to alter. It consists of a jack-head pump, shut at bottom, with a common clack bucket, and a valve in the cover of the pump, to discharge the air and water. The eduction steam pipe, which comes from the cylinder, communicates with this pump both above and below the bucket, and has valves to prevent anything from going back from the pump to the eduction pipe. The bucket descends by its own weight, and is raised by the engine when the great piston descends, being hung to the outer end of the great lever: the injection is made both into the upper part of this pump and into the eduction pipe, and operates beyond my ideas in point of quickness and perfection."

Besides the difficulty arising from incrustation, Watt found the tubulated condensers, and indeed all other expedients for condensing by cold surfaces, subject to a fatal objection. They did not condense instantaneously, and although they were capable of ultimately effecting the condensation, yet that process was not completed until a great part of the stroke of the piston was made. Thus during more or less of the stroke the uncondensed steam resisted the piston, and robbed the moving power of a part of its effect. This objection has ever attended condensation by surface.

Fig. 25.

Fig. 26.

(78.)Another source of difficulty arose from the necessity of constructing the piston and cylinder with greater precision than had been usual in the old engines. To fit the cover to the cylinder so as to be steam-tight; to construct the piston rod so as to move through it without allowing the escape of steam, and yet at the same time without injurious friction; to connect the piston rod with the piston, so as to drive the[Pg147]latter through the cylinder with a perfectly straight and parallel motion; to make such connection perfectly centrical and firm, and yet to allow the piston in its ascent to come nearly into contact with the cover of the cylinder—were all difficulties peculiar to the new engine. In the atmospheric engine the shank of the piston rod was rough and square, and the rod was secured to the piston by two or four branches or stays, as represented infig.25.It is evident that such a construction would be inadmissible in an engine in which the piston in its ascent must be brought nearly into contact with the close cover of the cylinder. Besides this the piston rod of an atmospheric engine might throughout its whole length have any form which was most convenient, and required no other property than the strength necessary to work the beam. In the new engine, on the contrary, it was necessary that it should be accurately turned and finely polished, so as to pass through the hole in the top of the cylinder, and be maintained in it steam-tight. This was effected by a contrivance called astuffing-boxB, represented infig.26.A hole is made in the cover of the cylinder very little greater in magnitude than the diameter of the piston rod. Above this hole is a cup in which, around the piston, is placed a stuffing of hemp or tow, which is saturated with oil or melted tallow. This collar of hemp is pressed down by another piece, also perforated with a hole through which the piston rod plays, and which is screwed down on the said collar of hemp.

(79.)Although the imperfect manner in which the interior of the cylinders was then formed impaired the efficiency of the[Pg148]new engines, yet such imperfections were not so injurious as in the old atmospheric engines. Any imperfection of form of the inner surface of the cylinder would necessarily cause more or less steam or air to escape between the piston and cylinder. In the improved engine this steam passing into the vacuum below the piston would rush into the condenser, and be there condensed, so that its effect in resisting the motion of the piston would necessarily be trifling. But on the other hand, any escape of air between the piston and cylinder of an atmospheric engine would introduce an elastic fluid under the piston, which would injuriously affect the action of the machine.

Fig. 27.

To make the pistons move sufficiently steam-tight in these early imperfect cylinders, Watt contrived a packing formed of a collar of hemp, or tow, as represented infig.27.The bottom of the piston was formed of a circular plate of a diameter nearly, but not altogether equal to the interior diameter of the cylinder. The part of the piston above this was considerably less in diameter, so that the piston was surrounded by a circular groove or channel two inches wide, into which hemp or soft rope, calledgasket, was run, so as to form the packing. The top of the piston was placed over this, having a rim or projecting part, which entered the circular groove and pressed upon the packing, the cover being pressed downwards by screws passing through the piston. The lower part of the groove round the piston was rounded with a curve, so that the pressure on the packing might force the latter against the inner surface of the cylinder. This packing was kept supplied with melted tallow, as already described, from the funnel, screwed into the top of the cylinder. The metallic edges of the piston were by this means prevented from coming into contact with the surface of the cylinder, which was only pressed upon by the stuffing or packing projecting beyond these.

(80.)Improved methods of boring soon, however, relieved[Pg149]the engine from a part of these imperfections, and Watt writes to Mr. Smeaton in the letter above quoted as follows:—

"Mr. Wilkinson has improved the art of boring cylinders; so that I promise, upon a 72 inch cylinder, being not further distant from absolute truth than the thickness of a thin sixpence in the worst part. I am labouring to improve the regulators; my scheme is to make them acute conical valves, shut by a weight, and opened by the force of the steam. They bid fair for success, and will be tried in a few days."

The person here alluded to was Mr. John Wilkinson, of Bersham near Chester, who, about the year 1775, contrived a new machine for accurately boring the insides of cylinders. The cylinder being first obtained from the foundery with a surface as accurate as the process of casting would admit, had its inner surface reduced to still greater accuracy by this machine, which consists of a straight central bar extended along the axis of the cylinder, which was made to revolve slowly round it. During the operation of boring, the borer or cutter was fitted to slide along this bar, which being perfectly straight, served as a sort of ruler to guide the borer or cutter in its progress through the cylinder. In this manner the interior surface of the cylinder was rendered not only true and straight in its longitudinal direction, but also perfectly circular in its cross section.

The grease found to be most eligible for lubrication was the tallow of beef or mutton; but in the earlier cylinders this was soon consumed by reason of the imperfection of the boring, and the piston being left dry ceased to be steam-tight. To prevent this, Watt sought for some substance, which while it would thicken the tallow, and detain it around the piston, would not be subject to decomposition by heat. Black lead dust was used for this purpose, but was soon found to wear the cylinder. In the mean while, however, the improved method of boring supplied cylinders which rendered this expedient unnecessary.

When the inner surface of the cylinder is perfectly true and smooth, the packing of the piston is soon rendered solid and hard, being moulded to the cylinder by working, so as to fit it perfectly. When by wear it became loose, it was[Pg150]only necessary to tighten the screws by which the top and bottom of the piston were held together. The packing being compressed by those means, was forced outwards towards the surface of the cylinder, so as to be rendered steam-tight.

(81.)It was not until about the year 1778, nine years after the date of the patent, and thirteen after the invention of separate condensation, that any impression was produced on the mining interests by the advantages which were presented to them by these vast improvements. This long interval, however, had not elapsed without considerable advantage; for although all the great leading principles of the contrivance were invented so early as the year 1765, yet the details of construction had been in a state of progressive and continued improvement from the time Watt joined Dr. Roebuck, in 1769, to the period now adverted to.

The advantages which the engine offered in the form in which it has been just described, were numerous and important, as compared even with the most improved form of the atmospheric engine; and it should be remembered, that that machine had also gone on progressively improving, and was probably indebted for some of its ameliorations to hints derived from the labours of Watt, and to the adoption of such of his expedients as were applicable to this imperfect machine, and could be adopted without an infraction of his patent.

In the most improved forms to which the atmospheric engine had then attained, the quantity of steam wasted at each stroke of the piston was equal to the contents of the cylinder. Such engines, therefore, consumed twice the fuel which would be requisite, if all sources of waste could have been removed. In Watt's engines, the steam consumed at each stroke of the piston amounted only to11⁄4times the contents of the cylinder. The waste steam, therefore, per stroke, was only a quarter of what was usefully employed. The absolute waste, therefore, of the best atmospheric engines was four times that of the improved engine, and consequently the saving of fuel in the improved engines amounted to about three eighths of all the fuel consumed in atmospheric engines of the same power.[Pg151]

(82.)But independently of this saving of steam, which would otherwise be wasted, the power of Watt's engine, as compared with the atmospheric engine, was so much augmented that the former would work against a resistance of ten pounds on the square inch under the same circumstances in which the latter would not move against more than seven pounds. The cause of this augmentation of power is easily explained. In the atmospheric engine the temperature of the condensed steam could not be reduced below 152° without incurring a greater loss than would be compensated by the advantage to be obtained from any higher degree of condensation. Now steam raised from water at 152° has a pressure of nearly four pounds per square inch. This pressure, therefore, acted below the piston resisting the atmospheric pressure above. In Watt's engine, however, the condenser was kept at a temperature of about 100°, at which temperature steam has a pressure of less than one pound per square inch. A resisting force upon the piston of three pounds per square inch was therefore saved in Watt's engine as compared with the atmospheric engine.

(83.)Besides these direct sources of economy, there were other advantages incidental to Watt's engine. An atmospheric engine possessed very limited power of adaptation to a varying load. The moving power being the atmospheric pressure, was not under control, and, on the other hand, was subject to variations from day to day and from hour to hour, according to the changes of the barometer. In the first construction of such an engine, therefore, its power being necessarily adapted to the greatest load which it would have to move, whenever the load upon its pumps was diminished, the motion of the piston in descending would be rapidly accelerated in consequence of the moving power exceeding the resistance. By this the machinery would be subject to sudden shocks, which were productive of rapid wear, and exposed the machinery to the danger of fracture. To remedy this inconvenience, the following expedient was provided in the atmospheric engine: whenever the load on the engine was materially diminished, the quantity of water admitted through the injection valve to condense the steam was proportionally[Pg152]diminished. An imperfect condensation being therefore produced, vapour remained in the cylinder under the piston, the pressure of which resisted the atmosphere, and mitigated the force of the machine. Besides this, a cock was provided in the bottom of the cylinder, called anair cock, by which atmospheric air could be admitted to resist the piston whenever the motion was too rapid.

These expedients, however, were all attended with a waste of fuel in relation to the work done by the engine; for it is evident that the consumption of steam was necessarily the same, whether the engine was working against its full load or against a reduced resistance.

On the other hand, in the improved engine of Watt, when the load, to work against which the engine exerted its full power, was diminished, a cock or valve was provided in the steam pipe leading from the boiler, which was called athrottle valve, by adjusting which the passage in that pipe could be more or less contracted. By regulating this cock the supply of steam from the boiler was checked, and the quantity transmitted to the cylinder diminished, so that its effect upon the piston might be rendered equal to the amount of the diminished resistance. By this means the quantity of steam transmitted to the cylinder was rendered exactly proportional to the work which the engine had to perform. If, under such circumstances, the boiler was worked to its full power, so as to produce steam as fast as it would when the engine was working at full power, then no saving of fuel would be effected, since the surplus steam produced in the boiler would necessarily escape at the safety valve. But in such case the fireman was directed to limit the fuel of the furnace until the discharge at the safety valve ceased.

By these expedients, the actual consumption of fuel in one of these improved engines was always in the exact proportion of the work which it performed, whether it worked at full power or at any degree under its regular power.

Back to Index Next