(57.)

(57.)In order to derive all the knowledge from these experiments which they are capable of imparting, it will be necessary to examine very carefully how water comports itself under a variety of different circumstances.

If water be boiled in an open vessel, with a thermometer immersed, on different days, it will be observed that the fixed temperature which it assumes in boiling will be subject to a variation within certain small limits. Thus, at one time, it will be found to boil at the temperature of 210°; while, at others, the thermometer immersed in it will rise to 213°; and, on different occasions, it will fix itself at different points within these limits. It will also be found, if the same experiment be performed at the same time in distant places, that the boiling points will be subject to a like variation. Now, it is natural to inquire what cause produces this variation; and we shall be led to the discovery of the cause, by examining what other physical effects undergo a simultaneous change.[Pg109]

If we observe the height of the barometer at the time of making each experiment, we shall find a very remarkable correspondence between it and the boiling temperature. Invariably, whenever the barometer stands at the same height, the boiling temperature will be the same. Thus, if the barometer stands at 30 inches, the boiling temperature will be 212°. If the barometer fall to291⁄2inches, the thermometer stands at a small fraction above 211°. If the barometer rise to301⁄2inches, the boiling temperature rises to nearly 213°. The variation in the boiling temperature is, then, accompanied by a variation in the pressure of the atmosphere indicated by the barometer; and it is constantly found that the boiling point will remain unchanged, so long as the atmospheric pressure remains unchanged, and that every increase in the one causes a corresponding increase in the other.

(58.)From these facts it must be inferred, that the pressure excited on the surface of the water has a tendency to resist its ebullition, and to make it necessary, before it can boil, that it should receive a higher temperature; and, on the contrary, that every diminution of pressure on the surface of the water will give an increased facility to the process of ebullition, or will cause that process to take place at a lower temperature. As these facts are of the utmost importance in the theory of heat, it may be useful to verify them by direct experiment.

If the variable pressure excited on the surface of the water by the atmosphere be the cause of the change in the boiling temperature, it must happen, that any change of pressure produced by artificial means on the surface of the water must likewise change the boiling point, according to the same law. Thus, if a pressure considerably greater than the atmospheric pressure be excited on a liquid, the boiling point may be expected to rise considerably above 212°; and, on the other hand, if the surface of the water be relieved from the pressure of the atmosphere, and be submitted to a considerably diminished pressure, the water would boil below 212°.

Fig. 17.

LetB(fig.17.) be a strong spherical vessel of brass, supported on a standS, under which is placed a large spirit lampL, or other means of heating it. In the top of this vessel are three apertures, in two of which are screwed a[Pg110]thermometerT, the bulb of which enters the hollow brass sphere, and a stop-cockC, which may be closed or opened at pleasure, to confine the steam, or allow it to escape. In the third aperture at the top, is screwed a long barometer tube, open at both ends. The lower end of this tube extends nearly to the bottom of the spherical vesselB. In the bottom of this vessel is placed a quantity of mercury, the surface of which rises to some height above the lower end of the tubeA. Over the mercury is poured a quantity of water, so as to half fill the vesselB. Matters being thus arranged, the screws are made tight, so as to confine the water, and the lamp is allowed to act on the vessel; the temperature of the water is raised, and steam is produced, which, being confined within the vessel, exerts its pressure on the surface of the water, and resists its ebullition. The pressure of the steam acting on the surface of the water is communicated to the surface of the mercury, and it forces a portion of the mercury into the tubeA, which presently rises above the point where the tube is screwed into the top of the vesselB. As the action of the lamp continues, the thermometerTexhibits a gradually increasing temperature; while the column of mercury inAshows the force with which the steam presses on the surface of the water inB,—this column being balanced by the pressure of the steam. Thus, the temperature and pressure of the steam at the same moment may always be observed by inspecting the thermometerTand the tubeA. When the column in the tubeAhas risen to the height of 30 inches above the level of the mercury in the vesselB, then the pressure of the steam will be equivalent to double the pressure of the atmosphere, because, the tubeAbeing open at the top, the atmosphere presses on the[Pg111]surface of the mercury in it. The thermometerTwill be observed gradually to rise until it attains the temperature of 212°; but it will not stop there, as it would do if immersed in water boiled in an open vessel. It will, on the other hand, continue to rise; and when the column of mercury inAhas attained the height of 30 inches, the thermometerTwill have risen to 251°,—being 39° above the ordinary boiling point.

During the whole of this process, the surface of the water being submitted to a constantly increasing pressure, its ebullition is prevented, and it continues to receive heat without boiling. That it is the increased pressure which resists its ebullition, and causes it to receive a temperature above 212°, may be easily shown. Let the stop-cockCbe opened; immediately the steam inB, having a pressure considerably greater than that of the atmosphere, will rush out, and will continue to issue fromC, until its pressure is balanced by the atmosphere. At the same time the column of mercury inAwill be observed rapidly to fall, and to sink below the orifice by which it is inserted in the vesselB. The thermometerTwill also fall until it attains the temperature of 212°. At that point, however, it will remain stationary; and the water will now be distinctly heard to be in a state of rapid ebullition. If the stop-cockCbe once more closed, the thermometer will begin to rise, and the column of mercury ascending inAwill be again visible.

If, instead of a stop-cock being atC, the aperture were made to communicate with a valve, like the safety-valve of a steam engine, loaded with a certain weight,—say at the rate of 15 lbs. on the square inch,—then the thermometerT, and the mercury in the tubeA, would not rise indefinitely as before. The thermometer would continue to rise till it attained the temperature of 251°; and the mercury in the tubeAwould rise to the height of 30 inches. At this limit the resistance of the valve would be balanced by the pressure of the steam; and as fast as the water would have a tendency to produce steam of a higher pressure, the valve would be raised and the steam suffered to escape; the thermometerTand the column of mercury inAremaining stationary during this process. If the valve were loaded more heavily, the phenomena would be[Pg112]the same, only that the mercury inTandAwould become stationary at certain heights. But, on the other hand, if the valve were loaded at a less pressure than 15 lbs. on the square inch, then the mercury in the two tubes would become stationary at lower points.

(59.)These experiments show that every increase of pressure above the ordinary pressure of the atmosphere causes an increase in the temperature at which water boils. We shall now inquire whether a diminution of pressure will produce a corresponding effect on the boiling point.

This may be easily accomplished by the aid of an air pump. Let water at the temperature of 200° be placed in a glass vessel under the receiver of an air pump, and let the air be gradually withdrawn. After a few strokes of the pump, the water will boil; and if the mercurial gauge of the pump be observed, it will be found that its altitude will be about231⁄2inches. Thus the pressure to which the water is submitted has been reduced from the ordinary pressure of the atmosphere expressed by the column of 30 inches of mercury, to a diminished pressure expressed by231⁄2inches; and we find that the temperature at which the water boils has been lowered from 212° to 200°. Let the same experiment be repeated with water at the temperature of 180°, and it will be found that a further rarefaction of the air is necessary, but the water will at length boil. If the gauge of the pump be now observed, it will be found to stand at about fifteen inches, showing, that at the temperature of 180° water will boil under half the ordinary pressure of the atmosphere. These experiments may be varied and repeated; and it will be always found, that, as the pressure is diminished or increased, the temperature at which the water will boil will be also diminished or increased.

(60.)The same effects may be exhibited in a striking manner without an air pump, by producing a vacuum by the condensation of steam. Let a small quantity of water be placed in a thin glass flask, and let it be boiled by holding it over a spirit lamp. When the steam is observed to issue abundantly from the mouth of the flask, let it be quickly corked and removed from the lamp. The process of boiling will then cease, and the water will become quiescent; but if the flask be plunged[Pg113]in a vessel of cold water, the water it contains will again pass into a state of violent ebullition, thus exhibiting the singular fact of water being boiled by cooling it. This effect is produced by the cold medium in which the flask is immersed, causing the steam above the surface of the water in it to be condensed, and therefore relieving the water from its pressure. The water, under these circumstances, boils at a lower temperature than when submitted to the pressure of the uncondensed vapour.

(61.)There is no limit to the temperature to which water may be raised, if it be submitted to a sufficient pressure to resist its tendency to take the vaporous form. If a strong metallic vessel be nearly filled with water, so as to prevent the liquid from escaping by any force which it can exert, the water thus inclosed may be heated to any temperature whatever without boiling; in fact, it may be made red-hot; and the temperature to which it may be raised will have no limit, except the strength of the vessel containing it, or the point at which the metal of which it is formed may begin to soften or to be fused.

(62.)The following table will show the temperature at which water will boil under different pressures of the atmosphere corresponding to the altitudes of the barometer between 26 and 31 inches.

From this table it appears, that, for every tenth of an inch which the barometric column varies between these limits, the boiling temperature changes by the fraction of a degree expressed by the decimal ·176, or nearly by the vulgar fraction1⁄6.

(63.)In the experiment already described, by which the latent[Pg114]heat of steam was determined, the water was supposed to be boiled under the ordinary pressure of the atmosphere. Having seen, however, that water may boil at different temperatures, under different pressures, the inquiry presents itself, whether the heat absorbed in vaporisation at different temperatures, and under different pressures, is subject to any variation? Experiments of the same nature as those already described, instituted upon water in a state of ebullition at different temperatures, as well below as above 212°, have led to the discovery of a very remarkable fact in the theory of vapour. It has been found that the heat absorbed by vaporisation is always less, the higher the temperature at which the ebullition takes place; and less, by the same amount as the temperature of ebullition is increased. Thus, if water boil at 312°, the heat absorbed in ebullition will be less by 100° than if it boiled at 212°; and again, if water be boiled under a diminished pressure, at 112°, the heat absorbed in vaporisation will be 100° more than the heat absorbed by water boiled at 212°. It follows, therefore, that the actual consumption of heat in the process of vaporisation must be the same, whatever be the temperature at which the vaporisation takes place; for whatever heat is saved in the sensible form, is consumed in the latent form, andvice versâ.

Let us suppose a given weight of water at the temperature of 32° to be exposed to any regular source by which heat may be supplied to it. If it be under the ordinary atmospheric pressure, the first 180° of heat which it receives will raise it to the boiling point, and the next 1000° will convert it into steam. Thus, in addition to the heat which it contains at 32°, the steam at 212° contains 1180° of heat. But if the same water be submitted to a pressure equal to half the atmospheric pressure, then the first 148° of heat which it receives will cause it to boil, and the next 1032° will convert it into vapour. Thus, steam at the temperature of 180° contains a quantity of heat more than the same quantity of water at 32°, by 1032° added to 148°, which gives a sum of 1180°. Steam, therefore, raised under the ordinary pressure of the atmosphere at 212°, and steam raised under half that pressure at 180°, contain the same quantity of heat,—with this difference[Pg115]only—that the one has more latent heat, and less sensible heat, than the other.

From this fact, that the sum of the latent and sensible heats of the vapour of water is constant, it follows that the same quantity of heat is necessary to convert a given weight of water into steam, at whatever temperature, or under whatever pressure, the water may be boiled. It follows, also, that, in the steam engine, equal weights of high-pressure and low-pressure steam are produced by the same consumption of fuel; and that, in general, the consumption of fuel is proportional to the quantity of water vaporised, whatever the pressure of the steam may be.[18]

(64.)Having explained the conditions under which, by supplying heat to water, it is converted into steam, and, by abstracting heat from steam, it may be reconverted into water, let us now consider the mechanical force which is developed in these phenomena.

Fig. 18.

LetA B(fig.18.) be a tube, or cylinder, the base of which is equal to a square inch, and let a pistonPmove in it so as to be steam-tight. Let it be supposed, that under this piston there is, in the bottom of the cylinder, a cubic inch of water between the bottom of the piston and the bottom of the tube; let the piston be counterbalanced by a weightWacting over a pulley, which will be just sufficient to counterpoise the weight of the piston, so as leave no force tending to keep the piston down, except the force of the atmosphere acting above it. Under the circumstances here supposed, the piston being in contact with the water, and all air being excluded, it will be pressed down by the weight of the atmosphere, which we will suppose to be fifteen pounds, the magnitude of the piston being a square inch.[Pg116]

Now let the flame of a lamp be applied at the bottom of the tube; the water under the piston having its temperature thereby gradually raised, and being submitted to no pressure save that of the atmosphere above the piston, it will begin to be converted into steam when it has attained the temperature of 212°. According as it is converted into steam, it will cause the piston to ascend in the tube until all the water has been evaporated. If the tube were constructed of sufficient length, the piston then would be found to have risen to the height of about seventeen hundred inches, or one hundred and forty-two feet; since, as has been already explained, water passing into steam under the ordinary pressure of the atmosphere undergoes an increase of bulk in the proportion of about seventeen hundred to one.

Now in this process, the air above the piston, which presses on it with a force equal to fifteen pounds, has been raised one hundred and forty-two feet. It appears, therefore, that, by the evaporation of a cubic inch of water under a pressure equal to fifteen pounds per square inch, a mechanical force of this amount is developed.

It is evident that fifteen pounds raised one hundred and forty-two feet successively, is equivalent to one hundred and forty-two times fifteen pounds raised one foot. Now, one hundred and forty-two times fifteen is two thousand one hundred and thirty, and therefore the force thus obtained is equal to two thousand one hundred and thirty pounds raised one foot high. This being within about 110 pounds of a ton, it may be stated, in round numbers, that, by the evaporation of a cubic inch of water under these circumstances, a force is obtained equal to that which would raise a ton weight a foot high.

The augmentation of volume which water undergoes in passing into steam under the pressure here supposed, may be easily retained in the memory, from the accidental circumstance that a cubic inch of water is converted into a cubic foot of steam, very nearly. A cubic foot contains one thousand seven hundred and twenty-eight cubic inches,—which is little different from the proportion which steam bears to water, when raised under the atmospheric pressure.[Pg117]

(65.)It will, therefore, be an advantage to retain in memory the following general facts:—

1.A cubic inch of water evaporated under the ordinary atmospheric pressure, is converted into a cubic foot of steam.

2.A cubic inch of water evaporated under the atmospheric pressure, gives a mechanical force equal to what would raise about a ton weight a foot high.

(66.)Let us, again, suppose the pistonP(fig.23.) to be restored to its original position, with the liquid water beneath it; and, in addition to the weight of the atmosphere which before pressed it down, let us suppose another weight of fifteen pounds laid upon it, so that the water below shall be pressed by double the weight of the atmosphere. If the lamp were now applied, and at the same time a thermometer were immersed in the water, it would be found that the water would not begin to be converted into steam until it attained the temperature of about 250°. The piston would then begin, as before, to ascend, and the water to be gradually converted into vapour. The water being completely evaporated, it would be found that the piston would be raised to a height little more than half its former height, or 72 feet. The mechanical effect, therefore, thus obtained, will be equivalent to double the former weight raised half the former height.

In like manner, if the piston were loaded with thirty pounds in addition to the atmosphere, the whole pressure on the water being then three times the pressure first supposed, the piston would be raised to somewhat more than one third of its first height by the evaporation of the water. This would give a mechanical force equivalent to three times the original weight raised a little more than one third of the original height.

In general, as the pressure on the piston is increased, the height to which the piston would be raised by the evaporation of the water will be diminished in a proportion somewhat less than the proportion in which the pressure on the piston is increased. If the temperature at which the water is converted into steam under these different pressures were the same, then the height to which the piston would be raised by the evaporation of the water would be diminished in precisely[Pg118]the same proportion as the pressure on the piston is increased; and, in that case, the whole mechanical force developed by the evaporation of the water would remain exactly the same under whatever pressure the water might be boiled. We shall explain hereafter the extent to which the variation of temperature in the water and steam corresponding to the variation of pressure modifies this law; but, as the effect of the difference of temperatures is not considerable, it will be convenient to register in the memory the following important practical conclusion:—

(67.)A cubic inch of water converted into steam will supply a mechanical force very nearly equal to a ton weight raised a foot high; and this force will not be subject to considerable variation, whatever be the temperature or pressure at which the water may be evaporated.

GLASGOW.

FOOTNOTES:[18]The preceding paragraphs, and some other parts of the present volume on the general properties of Heat, are taken from my Treatise on Heat, in theCabinet Cyclopœdia, to which those who desire more detailed explanation and more copious illustration should refer.

[18]The preceding paragraphs, and some other parts of the present volume on the general properties of Heat, are taken from my Treatise on Heat, in theCabinet Cyclopœdia, to which those who desire more detailed explanation and more copious illustration should refer.

GLASGOW COLLEGE.

[Pg119]TOCINX

WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE WITH A DUE ECONOMY OF FUEL.—CONCEIVES THE NOTION OF CONDENSING OUT OF THE CYLINDER.—DISCOVERS SEPARATE CONDENSATION.—INVENTS THE AIR-PUMP.—SUBSTITUTES STEAM PRESSURE FOR ATMOSPHERIC PRESSURE.—INVENTS THE STEAM CASE, OR JACKET.—HIS FIRST EXPERIMENTS TO REALISE THESE INVENTIONS.—HIS EXPERIMENTAL APPARATUS.—DIFFICULTIES OF BRINGING THE IMPROVED ENGINES INTO USE.—WATT PRACTISES AS A CIVIL ENGINEER.—HIS PARTNERSHIP WITH ROEBUCK.—HIS FIRST PATENT.—DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE.

(68.)At the period to which we have now brought the history of the invention of the steam engine, Watt had obtained, chiefly by his own experiments, a sufficient knowledge of the phenomena which have been just explained, to enable him to arrive at the conclusion that a very small proportion of the whole mechanical effect attending the evaporation was really rendered available by the atmospheric engine; and that,[Pg120]therefore, extensive and injurious sources of waste existed in its machinery.

He perceived that the principal source of this wasteful expenditure of power consisted in the quantity of steam which was condensed at each stroke of the piston, in heating the cylinder previous to the ascent of the piston. Yet, as it was evident that that ascent could not be accomplished in a cold cylinder, it was apparent that this waste of power must be inevitable, unless some expedient could be devised, by whicha vacuum could be maintained in the cylinder, without cooling it. But, to produce such a vacuum, the steam must be condensed; and, to condense the steam, its temperature must be lowered to such a point that the vapour proceeding from it shall have no injurious pressure; yet, if condensed steam be contained in a cylinder at a high temperature, it will return to the temperature of the cylinder, recover its elasticity, and resist the descent of the piston.

Having reflected on these circumstances, it became apparent to Watt, that a vice was inherent in the structure of the atmospheric engine, which rendered a large waste of power inevitable; this vice arising from the fact, that the condensation of the steam was incompatible with the condition of maintaining the elevated temperature of the cylinder in which that condensation took place. It followed, therefore, either that the steam must be imperfectly condensed, or that the condensation could not take place in the cylinder. It was in 1765, that, pondering on these circumstances, the happy idea occurred to him, that the production of a vacuum could be equally effected, thoughthe placewhere the condensation of the steam took place were not the cylinder itself. He saw, that if a vessel in which a vacuum was produced were put into communication with another containing an elastic fluid, the elastic fluid would rush into the vacuum, and diffuse itself through the two vessels; but if, on rushing into such vacuum, this elastic fluid, being vapour, were there condensed, or restored to the liquid form, that then the space within the two vessels would be equally rendered a vacuum;—that, under such circumstances, one of the vessels might be maintained at any temperature, however high, while[Pg121]the other might be kept at any temperature, however low. This felicitous conception formed the first step in that splendid career of invention and discovery which has conferred immortality on the name of Watt. He used to say, that the moment the idea of separate condensation occurred to him,—that is, of condensing, in one vessel kept cold, the steam coming from another vessel kept hot,—all the details of his improved engine rushed into his mind in such rapid succession, that, in the course of a day, his invention was so complete that he proceeded to submit it to experiment.

Fig. 19.

To explain the first conception of this memorable invention; let a tube or pipe,S(fig.19.), be imagined to proceed from the bottom of the cylinderA Bto a vessel,C, having a stop-cock,D, by which the communication between the cylinder and the vesselCmay be opened or closed at pleasure. If we suppose the pistonPat the top of the cylinder, and the space below it filled with steam, the cylinder and steam being at the usual temperature, while the vesselCis a vacuum, and maintained at a low temperature. Then, on opening the cockD, the steam will rush from the cylinderA Bthrough the tubeS, and, passing into the cold vesselC, will be condensed by contact with its cold sides. This process of condensation will be rendered instantaneous if a jet of cold water is allowed to play in the vesselC. When the steam thus rushing intoC, has been destroyed, and the space in the cylinderA Bbecomes a vacuum, then the pressure of the atmosphere being unobstructed, the piston will descend with the force due to the excess of the pressure of the atmosphere above the friction. When it has descended, suppose the stop-cockDclosed, and steam admitted from[Pg122]the boiler through a proper cock or valve below the piston, the cylinder and piston being still at the same temperature as before. The steam on entering the cylinder, not being exposed to contact with any surface below its own temperature, will not be condensed, and therefore will immediately cause the piston to rise, and the piston will have attained the top of the cylinder when as much steam shall have been supplied by the boiler as will fill the cylinder. When this has taken place, suppose the communication with the boiler cut off, and the cockDonce more opened: the steam will again rush through the pipeSinto the vesselC, where encountering the cold surface and the jet of cold water, it will be condensed, and the vacuum, as before, will be produced in the cylinderA B; that cylinder still maintaining its temperature, the piston will again descend, and so the process may be continued.

(69.)Having carried the invention to this point, Watt saw that the vesselCwould gradually become heated by the steam which would be continually condensed in it. To prevent this, as well as to supply a constant jet of cold water, he proposed to keep the vesselCsubmerged in a cistern of cold water, from which a pipe should conduct a jet to play within the vessel, so as to condense the steam as it would pass from the cylinder.

But here a difficulty presented itself, against which it was necessary to provide. The cold water admitted through the jet to condense the steam, mixed with the condensed steam itself, would gradually collect in the vesselC, and at length choke it. To prevent this, Watt proposed to put the vesselCin communication with a pumpF, which might be wrought by the engine itself, and by which the water, which would collect in the bottom of the vesselC, would be constantly drawn off. This pump would be evidently rendered the more necessary, since more or less atmospheric air, always combined with water in its common state, would enter the vesselCby the condensing jet. This air would be disengaged in the vesselCby the heat of the steam condensed therein; and it would rise through the tubeS, and vitiate the vacuum in the cylinder;—an effect which would be rendered the more injurious,[Pg123]inasmuch as, unlike steam, this elastic fluid would be incapable of being condensed by cold. The pumpF, therefore, by which Watt proposed to draw off the water from the vesselC, might also be made to draw off the air, or the principal part of it.

The vesselCwas subsequently called acondenser; and, from the circumstances just adverted to, the pumpFhas been called theair-pump.

These—namely, the cylinder, the condenser, and the air-pump—were the three principal parts in the invention, as it first presented itself to the mind of Watt—and even before it was reduced to a model, or submitted to experiment. But, in addition to these, other two improvements offered themselves in the very first stage of its progress.

In the atmospheric engine, the piston was maintained steam-tight in the cylinder by supplying a stream of cold water above it, by which the small interstices between the piston and cylinder would be stopped. It is evident that the effect of this water as the piston descended would be to cool the cylinder, besides which any portion of it which might pass between the piston and cylinder and which would pass below the piston, would boil the moment it would fall into the cylinder, which itself would be maintained at the boiling temperature. This water, therefore, would produce steam, the pressure of which would resist the descent of the piston.

Watt perceived, that even though this inconvenience were removed by the use of oil or tallow upon the piston, still, that as the piston would descend in the cylinder, the cold atmosphere would follow it; and would, to a certain extent, lower the temperature of the cylinder. On the next ascent of the piston, this temperature would have to be again raised to 212° by the steam coming from the boiler, and would entail upon the machine a proportionate waste of power.

If the atmosphere of the engine-house could be kept heated to the temperature of boiling water, this inconvenience would be removed. The piston would then be pressed down by air as hot as the steam to be subsequently introduced into it. On further consideration, however, it occurred to Watt that it would be still more advantageous if the cylinder itself could be[Pg124]worked in an atmosphere of steam, having only the same pressure as the atmosphere. Such steam would press the piston down as effectually as the air would; and it would have the further advantage over air, that if any portion of it leaked through between the piston and cylinder, it would be condensed, which could not be the case with atmospheric air. He therefore determined on surrounding the cylinder by an external casing, the space between which and the cylinder he proposed to be filled with steam supplied from the boiler. The cylinder would thus be enclosed in an atmosphere of its own, independent of the external air, and the vessel so enclosing it would only require to be a little larger than the cylinder, and to have a close cover at the top, the centre of which might be perforated with a hole to admit the rod of the piston to pass through, the rod being made smooth, and so fitted to the perforation that no steam should escape between them. This method would be attended also with the advantage of keeping the cylinder and piston always heated, not only inside but outside; and Watt saw that it would be further advantageous to employ the pressure of steam to drive the piston in its descent instead of the atmosphere, as its intensity or force would be much more manageable; for, by increasing or diminishing the heat of the steam in which the cylinder was enclosed, its pressure might be regulated at pleasure, and it might be made to urge the piston with any force that might be required. The power of the engine would therefore be completely under control, and independent of all variations in the pressure of the atmosphere.

(70.)This was a step which totally changed the character of the machine, and which rendered it aSTEAM ENGINEinstead of anATMOSPHERIC ENGINE. Not only was the vacuum below the piston now produced by the property of steam, in virtue of which it is reconverted into water by cold; but the pressure which urged the piston into this vacuum was due to the elasticity of steam.

The external cylinder, within which the working cylinder was enclosed, was calledTHE JACKET, and is still very generally used.

Fig. 20.

(71.)The first experiment in which Watt attempted to[Pg125]realise, on a small scale, his conceptions, was made in the following manner. The cylinder of the engine was represented by a brass syringeA B(fig.20.) an inch and a third in diameter, and ten inches in length, to which a top and a bottom of tin plate was fitted. Steam was conveyed by a pipe,S, from a small boiler into the lower end of this syringe, a communication being made with the upper end of the syringe by a branch pipeD. For the greater convenience of the experiment, it was found desirable to invert the position of the cylinder, so that the steam should press the pistonPupwards instead of downwards. The piston-rodRtherefore was presented downwards. An eduction pipeEwas also inserted in the top of the cylinder, which was carried to the condenser. The piston-rod was made hollow, or rather a hole was drilled longitudinally through it, and a valve was fitted at its lower end, to carry off the water produced by the steam, which[Pg126]would be condensed in the cylinder in the commencement of the process. The condenser used in this experiment operated without injection, the steam being condensed by the contact of cold surfaces. It consisted of two thin pipesF, Gof tin, ten or twelve inches in length, and the sixth of an inch in diameter, standing beside each other perpendicularly, and communicating at the top with the eduction pipe, which was provided with a valve opening upwards. At the bottom these two pipes communicated with another tubeIof about an inch in diameter, by a horizontal pipe, having in it a valve,M, opening towardsI, fitted with a pistonK, which served the office of the air-pump, being worked by the hand. This piston,K, had valves in it opening upwards. These condensing pipes and air-pump were immersed in a small cistern, filled with cold water. The steam was conveyed by the steam-pipeSto the bottom of the cylinder, a communication between the top and bottom of the cylinder being occasionally opened by a cock,C, placed in the branch pipe. The eduction pipe leading to the condenser also had a cock,L, by which the communication between the top of the cylinder and the condenser might be opened and closed at pleasure. In the commencement of the operation, the cockNadmitting steam from the boiler, and the cockLopening a communication between the cylinder and the condenser, and the cockCopening a communication between the top and bottom of the cylinder, being all open, steam rushed from the boiler, passing through all the pipes, and filling the cylinder. A current of mixed air and steam was thus produced through the eduction pipeE, through the condensing pipesFandG, and through the air-pumpI, which issued from the valveHin the eduction pipe, and from the valve in the air-pump piston, all of which opened upwards. The steam also in the cylinder passed through the hole drilled in the piston-rod, and escaped, mixed with air, through the valve in the lower end of that rod. This process was continued until all the air in the cylinder, pipes, and condenser, was blown out, and all these spaces filled with pure steam. The cocksL,C, andN, were then closed, and the atmospheric pressure closed the valveHand the valves in the air-pump piston. The cold surfaces condensing the steam in[Pg127]the pipesFandG, and in the lower part of the air-pump, a vacuum was produced in these spaces. The cock C being now closed, and the cocksLandNbeing open, the steam in the upper part of the cylinder rushed through the pipe E into the condenser, where it was reduced to water, so that a vacuum was left in the upper part of the cylinder. The steam from the boiler passing below the piston, pressed it upwards with such force, that it lifted a weight of eighteen pounds hung from the end of the piston-rod. When the piston reached the top of the cylinder, the cocksLandNwere closed, and the cockCopened. All communication between the cylinder and the boiler, as well as between the cylinder and the condenser, were now cut off, and the steam in the cylinder circulated freely above and below the piston, by means of the open tubeD. The piston, being subject to equal forces upwards and downwards, would therefore descend by its own weight, and would reach the bottom of the cylinder. The air-pump piston meanwhile being drawn up, the air and the condensed steam in the tubesFandGwere drawn into the air-pumpI, through the open horizontal tube at the bottom. Its return was stopped by the valveM. By another stroke of the air-pump, this water and air were drawn out through valves in the piston, which opened upwards. The cockCwas now closed, and the cocksLandNopened, preparatory to another stroke of the piston. The steam in the upper part of the cylinder rushed, as before, into the tubesFandG, and was condensed by their cold surfaces, while steam from the boiler coming through the pipeS, pressed the piston upwards. The piston again ascended with the same force as before, and in the same manner the process was continually repeated.

(72.)The quantity of steam expended in this experimental model in the production of a given number of strokes of the piston was inferred from the quantity of water evaporated in the boiler; and on comparing this with the magnitude of the cylinder and the weight raised by the pressure of the steam, the contrivance was proved to affect the economy of steam, as far as the imperfect conditions of such a model could have permitted. A larger model was next constructed, having an outer cylinder, or steam case, surrounding the working cylinder, and[Pg128]the experiments made with it fully realised Watt's expectations, and left no doubt of the great advantages which would attend his invention. The weights raised by the piston proved that the vacuum in the cylinder produced by the condensation was almost perfect; and he found that when he used water in the boiler which by long boiling had been well cleared of air, the weight raised was not much less than the whole amount of the pressure of the steam upon the piston. In this larger model, the cylinder was placed in the usual position, with a working lever and other apparatus similar to that employed in the Atmospheric Engine.

(73.)It was in the beginning of the year 1765, Watt being then in the twenty-ninth year of his age, that he arrived at these great discoveries. The experimental models just described, by which his invention was first reduced to a rude practical test, were fitted up at a place called Delft House, in Glasgow. It will doubtless, at the first view, be a matter of surprise that improvements of such obvious importance in the economy of steam power, and capable of being verified by tests so simple, were not immediately adopted wherever atmospheric engines were used. At the time, however, referred to, Watt was an obscure artisan, in a provincial town, not then arrived at the celebrity to which it has since attained, and the facilities by which inventions and improvements became public were much less than they have since become. It should also be considered that all great and sudden advances in the useful arts are necessarily opposed by the existing interests with which their effects are in conflict. From these causes of opposition, accompanied with the usual influence of prejudice and envy, Watt was not exempt, and was not therefore likely suddenly to revolutionise the arts and manufactures of the country by displacing the moving powers employed in them, and substituting an engine, the efficacy and power of which depended mainly on physical principles, then altogether new and but imperfectly understood.

Not having the command of capital, and finding it impracticable to inspire those who had, with the same confidence in the advantages of his invention which he himself felt, he was[Pg129]unable to take any step towards the construction of engines on a large scale. Soon after this, he gave up his shop in Glasgow, and devoted himself to the business of a Civil Engineer. In this capacity he was engaged to make a survey of the river Clyde, and furnished an elaborate and valuable Report upon its projected improvements. He was also engaged in making a plan of the canal, by which the produce of the Monkland Colliery was intended to be carried to Glasgow, and in superintending the execution of that work. Besides these, several other engineering enterprises occupied his attention, among which may be mentioned, the navigable canal across the isthmus of Crinan, afterwards completed by Rennie; improvements proposed in the ports of Ayr, Glasgow, and Greenock; the construction of the bridges at Hamilton, and at Rutherglen; and the survey of the country through which the celebrated Caledonian canal was intended to be carried.

"If, forgetful of my duties as the organ of this academy," says M. Arago (whose eloquent observations on the delays of this great invention, addressed to the assembled members of the National Institute of France, we cannot forbear to quote), "I could think of making you smile, rather than expressing useful truths, I would find here matter for a ludicrous contrast. I would call to your recollection the authors, who at our weekly sittings demand with all their might and main (à cor et à cris) an opportunity to communicate some little remark—some small reflection—some trifling note, conceived and written the night before; I would represent them to you cursing their fate, when according to your rules, the reading of their communication is postponed to the next meeting, although during this cruel week, they are assured that their important communication is deposited in our archives in a sealed packet. On the other hand, I would point out to you the creator of a machine, destined to form an epoch in the annals of the world, undergoing patiently and without murmur, the stupid contempt of capitalists,—conscious of his exalted genius, yet stooping for eight years to the common labour of laying down plans, taking levels, and all the tedious calculations connected with the routine of common engineering. While in this conduct you cannot fail to recognise the serenity,[Pg130]the moderation, and the true modesty of his character, yet such indifference, however noble may have been its causes, has something in it not altogether blameless. It is not without reason that society visits with severe reprobation those who withdraw gold from circulation and hoard it in their coffers. Is he less culpable who deprives his country, his fellow citizens, his age, of treasures a thousand times more precious than the produce of the mine; who keeps to himself his immortal inventions, sources of the most noble and purest enjoyment of the mind, who abstains from conferring upon labour those powers, by which would be multiplied in an infinite proportion the products of industry, and by which, with advantage to civilisation and human nature, he would smooth away the inequalities of the conditions of man."[19]

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