Steam

same time still restricting ourselves to the essential cirEngine cumstances, and avoiding every peculiarity which is to be found in the prodigious varieties which Mr Watt has introduced into the machines which he has erected, every individual of which has been adapted to local circumstances, or diversified by the progress of Mr Watt's improvements.

61

ted.

Fig. 9.

22 Description Let A (fig. 9.) represent the boiler. This has reof the maceived great improvements from his complete acquainchine after these im- tance with the procedure of nature in the production of *provements steam. In some of his engines the fuel has been placed were add in the midst of the water, surrounded by an iron or copper vessel, while the exterior boiler was made of wood, which transmits, and therefore wastes the heat very slowly. In others, the flame not only plays round the whole outside, as in common boilers, but also runs along several flues which are conducted through the midst of the water. By such contrivances the fire is applied to the water in a most extensive surface, and for a long time so as to impart to it the greatest part of its heat. So skilfully was it applied in the Albion mills, that although it was perhaps the largest engine in the kingdom, its unconsumed smoke was inferior to that of a very small brew-house. In this second engine of Mr Watt, the top of the cylinder is shut up by a strong metal plate g h, in the middle of which is a collar or box of leathers k, formed in the usual manner of a jackhead pump, through which the piston rod PD, nicely turned and polished, can move up and down, without allowing any air to pass by its sides. From the dome of the boiler proceeds a large pipe BCIOQ, which, after reaching the cylinder with its horizontal part BC, descends parallel to its side, sending off two branches, viz. IM to the top of the cylinder, and ON to its bottom. At I is a puppet valve opening from below upwards. At L, immediately below this branch, there is a similar valve, also opening from below upwards. The pipe descends to Q, near the bottom of a large cistern c d e f, filled with cold water constantly renewed. The pipe is then continued horizontally along the bottom of this cistern (but not in contact), and terminates at R in a large pump ST. The piston S has clack valves opening upwards, and its rod S s, passing through a collar of leathers at T, is suspended by a chain to a small arch head on the outer arm of the beam. There is a valve R in the bottom of this pump, as usual, which opens when pressed in the direction QR, and shuts against a contrary pressure. This pump delivers its contents into another pump XY, by means of the small pipe t X, which proceeds from its top. This second pump has a valve at X, and a clack in its piston Z as usual, and the piston rod Z z is suspended from another arch head on the outer arm of the beam. The two valves I and L are opened and shut by means of spanners and handles, which are put in motion by a plug frame, in the same manner as in Newcomen's engine.

Lastly, there may be observed a crooked pipe a bo, which enters the upright pipe laterally a little above Q. This has a small jet hole at o; and the other end a, which is considerably under the surface of the water of the condensing cistern, is covered with a puppet valve v, whose long stalk vu rises above the water, and may be raised or lowered by hand or by the plug beam. The valves R and X, and the clacks in the pistons S and Z,

are opened or shut by the pressure to which they are immediately exposed."

This figure is not an exact copy of any of Mr Watt's engines, but has its parts so disposed that all may come distinctly into view, and exactly perform their various functions. It is drawn in its quiescent position, the outer end of the beam preponderating by the counter weight, and the piston P at the top of the eylinder, and the pistons S and Z in their lowest situations.

In this situation let us suppose that a vacuum is (by any means) produced in all the space below the piston, the valve I being shut. It is evident that the valve R will also be shut, as also the valve v. Now let the valve I be opened. The steam from the boiler, as elastic as common air, will rush into the space above the piston, and will exert on it a pressure as great as that of the atmosphere. It will therefore press it down, raise the outer end of the beam, and cause it to perform the same work as an ordinary engine.

When the piston P has reached the bottom of the cylinder, the plug frame shuts the valve I, and opens L. By so doing the communication is open between the top and bottom of the cylinder, and nothing hinders the steam which is above the piston from going along the passage MLON. The piston is now equally affected on both sides by the steam, even though a part of it is continually condensed by a cylinder, and in the pipe 10Q. Nothing therefore hinders the piston from being dragged up by the counter weight, which acts with its whole force, undiminished by any remaining unbalanced elasticity of steam. Here therefore this form of the engine has an advantage (and by no means a small one) over the common engines, in which a great part of the counter weight is expended in overcoming unbalanced atmospheric pressure.

Whenever the piston P arrives at the top of the cylinder, the valve L is shut by the plug frame, and the valves I and v are opened. All the space below the piston is at this time occupied by the steam which came from the upper part of the cylinder. This being a little wasted by condensation, is not quite a balance for the pressure of the atmosphere. Therefore, during the ascent of the piston, the valve R was shut, and it remains so. When therefore, the valve v is opened, the cold water of the cistern must spout up through the hole o, and condense the steam. To this must be added the coldness of the whole pipe OQS. As fast as it is condensed, its place is supplied by steam from the lower part of the cylinder. We have already remarked, that this successive condensation is accomplished with astonishing. rapidity. In the mean time steam from the boiler presses on the upper surface of the piston. It must therefore descend as before, and the engine must perform a second working stroke.

But in the mean time the injection water lies in the bottom of the pipe OQR, heated to a considerable degree by the condensation of the steam; also a quantity of air has been disengaged from it and from the water in the boiler. How is this to be discharged?—This is the office of the pumps ST and XY. The capacity of ST is very great in proportion to the space in which the air and water are lodged. When, therefore, the piston S has got to the top of its course, there must be a vacuum in the barrel of this pump, and the water and air must open the valve R and come into it. When the

Steam- piston S comes down again in the next returning stroke, Engine. this water and air gets through the valve of the piston; and in the next working stroke they are discharged by the piston into the pump XY, and raised by its piston. The air escapes at Y, and as much of the water as is necessary is delivered into the boiler by a small pipe Yg to supply its waste. It is a matter of indifference whether the pistons S and Z rise with the outer or inner end of the beam, but it is rather better that they rise with the inner end. They are otherwise drawn here, in order to detach them from the rest and show them more distinctly.

62

Canser of

rity over

common

Such is Mr Watt's second engine. Let us examine its principles, that we may see the causes of its avowed and great superiority over the common engines.

We have already seen one ground of superiority, the its superio- full operation of the counter weight. We are authorised by careful examination to say, that in the common engines at least one-half of the counter weight is are, the full expended in counteracting an unbalanced pressure of the operation air on the piston during its ascent. In many engines,

engines

of the counter

weight,

63 and great saving of

steam.

which are not the worst, this extends to th of the whole pressure. This is evident from the examination of the engine at Montrelaix by Bossut. This makes a very great counter weight necessary, which exhausts a proportional part of the moving force.

But the great advantage of Mr Watt's form is the almost total annihilation of the waste of steam by condensation in the cylinder. The cylinder is always boiling hot, and therefore perfectly dry. This must be evident to any person who understands the subject. By the time that Mr Watt had completed his improvements, his experiments on the production of steam had given him a pretty accurate knowledge of its density; and he found himself authorised to say, that the quantity of steam employed did not exceed twice as much as would fill the cylinder, so that not above one-half was unavoidably wasted. But before he could bring the engine to this degree of perfection, he had many difficulties to overcome: He inclosed the cylinder in an outer wooden case at a small distance from it. This diminished the expence of heat by communication to surrounding bo dies. Sometimes he allowed the steam from the boiler to occupy this interval. This undoubtedly prevented all dissipation from the inner cylinder; but in its turn it dissipated much heat by the outer case, and a very sensible condensation was observed between them. This has occasioned him to omit this circumstance in some of his best engines. We believe it was omitted in the Albion mills.

The greatest difficulty was to make the great piston tight. The old and effectual method, by water lying on it, was inadmissible. He was therefore obliged to have his cylinders most nicely bored, perfectly cylindrical, and finely polished; and he made numberless trials of different soft substances for packing his piston, which should be tight without enormous friction, and which should long remain so, in a situation perfectly dry, and hot almost to burning.

After all that Mr Watt has done in this respect, he thinks that the greatest part of the waste of steam which he still perceives in his engines arises from the unavoidable escape by the sides of the piston during its descent

of the same dimensions with a common engine, making Steamthe same number of strokes of the same extent, does Engine. not consume above one-fourth part of the fuel that is consumed by the best engines of the common form. It is also a very fortunate circumstance, that the performance of the engine is not immediately destroyed, nor indeed sensibly diminished, by a small want of tightness in the piston. In the common engine, if air get in, in this way, it immediately puts a stop to the work; but although even a considerable quantity of steam get past the piston during its descent, the rapidity of condensation is such, that hardly any diminution of pressure can be observed.

64 Mr Watt's penetration soon discovered another most Another valuable property of this engine. When an engine of valuable the common form is erected, the engineer must make an property accurate estimate of the work to be performed, and of it must proportion his engine accordingly. He must be careful that it be fully able to execute its task; but its power must not exceed its load in any extravagant degree. This would produce a motion which is too rapid, and which, being alternately in opposite directions, would occasion jolts which no building or machinery could withstand. Many engines have been shattered by the pumps drawing air, or a pump rod breaking; by which accidents the steam-piston descends with such rapidity that every thing gives way. But in most operations of mining, the task of the engine increases, and it must be so constructed at first as to be able to bear this addition. It is very difficult to manage an engine that is much superior to its task; and the easiest way is, to have it almost full loaded, and to work it only during a few hours each day, and allow the pit water to accumulate during its repose. This increases the first cost, and wastes fuel during the inaction of the engine.

65

fitted to

But this new engine can at all times be exactly fitted is, that it (at least during the working stroke) to the load of work can always that then happens to be on it. We have only to ad- be exactly minister steam of a proper elasticity. At the first erec- the load tion the engine may be equal to twice its task, if the which hapsteam admitted above the cylinder be equal to that of pens to be common boiling water; but when once the ebullition on it. is fairly commenced, and the whole air expelled from all parts of the apparatus, it is evident, that by damping the fire, steam of half this elasticity may be continually supplied, and the water will continue boiling although its temperature does not exceed 185° of Fahrenheit's thermometer. This appears by inspecting our table of vaporous elasticity, and affords another argument for rendering that table more accurate by new experiments. We hope that Mr Watt will not withhold from the public the knowledge which he has acquired on this subject. It may very possibly result from an accurate investigation, that it would be advisable to work our steamengines with weak steams, and that the diminution of work may be more than compensated by the diminution of fuel. It is more probable indeed, and it is Mr Watt's opinion, that the contrary is the case, and that it is much more economical to employ great heats. At any rate, the decision of this question is of great importance for improving the engine; and we see, in the mean time, that the engine can at all times be fitted so as to perform its task with a moderate and manageable motion, and that as the task increases we can increase

But

Steam

66

One inconvenience

67 remedied

But the method now proposed has a great inconveEngine. nience. While the steam is weaker than the atmosphere, there is an external force tending to squeeze in the sides and bottom of the boiler. This could not be resisted when the difference is considerable, and common air would rush in through every crevice of the boiler and soon choke the engine: it must therefore be given up. But the same effect will be produced by diminishing the passage for the steam into the cylinder. For this purpose, the puppet valve by which the steam enters the cylinder was made in the form of a long taper spigot, and it was lodged in a cone of the same shape; consequently the passage could be enlarged or contracted at pleasure by the distance to which the inner cone was drawn up.

in some degree.

68

But the

tended

In this way several engines were constructed, and the remedy at- general purpose of suiting the power of the engine to its with some task was completely answered: but (as the mathematidifficulties; cal reader will readily perceive) it was extremely difficult to make this adjustment precise and constant. In a great machine like this going by jerks, it was hardly possible that every succesive motion of the valve should be precisely the same. This occasioned very sensible irregularities in the motion of the engine, which increased and became hazardous when the joints worked loose by long use.

removed.

69 which Mr Mr Watt's genius, always fertile in resources, found Watt's fer- out a complete remedy for all these inconveniences. tile genius Making the valve of the ordinary form of a puppet completely clack, he adjusted the button of its stalk or tail so that it should always open full to the same height. He then regulated the pins of the plug-frame, in such a manner that the valve should shut the moment that the piston had descended a certain proportion (suppose one-fourth, one-third, one-half, &c.) of the cylinder. So far the cylinder was occupied by steam as elastic as common

air. In pressing the piston farther down, it behoved

the steam to expand, and its elasticity to diminish. It is plain that this could be done in any degree we please, and that the adjustment can be varied in a minute, according to the exigency of the case, by moving the plug pins.

In the mean time, it must be observed, that the pressure on the piston is continually changing, and consequently the accelerating force. The motion therefore will no longer be uniformly accelerated: it will approach much faster to uniformity; nay, it may be retarded, because although the pressure on the piston at the beginning of the stroke may exceed the resistance of the load, yet when the piston is near the bottom the resistance may exceed the pressure. Whatever may be the law by which the pressure on the piston varies, an ingenious mechanic may contrive the connecting machinery in such a way that the chains or rods at the outer end of the beam shall continually exert the same pressure, or shall vary their pressure according to any law he finds most convenient. It is in this manner that the watchmaker, by the form of the fuzee, produces an equal pressure on the wheel-work by means of a very unequal action of the main-spring. In like manner, by making the outer arch heads portions of a proper spiral instead of a circle, we can regulate the force of the beam at pleasure..

Thus we see how much more manageable an engine is in this form than Newcomen's was, and also more 3.

Steam

easily investigated in respect of its power in its various positions. The knowledge of this last circumstance was Engine. of mighty consequence, and without it no notion could be formed of what it could perform. This suggested to Mr Watt the use of the barometer communicating with the cylinder; and by the knowledge acquired by these means has the machine been so much improved by its ingenious inventor.

We must not omit in this place one deduction made by Mr Watt from his observations, which may be call-、 ed a discovery of great importance in the theory of the engine.

in the

70

Let ABCD (fig. 10.) represent a section of the cy- A discovery linder of a steam-engine, and EF the surface of its pi- of Mr Watt ston. Let us suppose that the steam was admitted of great while EF was in contact with AB, and that as soon as importance it had pressed it down to the situation EF the steam. theory of cock is shut. The steam will continue to press it down, the engine. and as the steam expands its pressure diminishes. We Fig. 10. may express its pressure (exerted all the while the piston moves from the situation AB to the situation EF) by the line EF. If we suppose the elasticity of the steam proportional to its density, as is nearly the case with air, we may express the pressure on the piston in any other position, such as KL or DC, by Kl and Dc, the ordinates of a rectangular hyperbola F/c, of which AE, AB are the assymptotes, and A the centre. The accumulated pressure during the motion of the piston from EF to DC will be expressed by the area EF C DE, and the pressure during the whole motion by the area ABFCDA.

Now it is well known that the area EF C DE is equal to ABFE multiplied by the hyperbolic logarithm AD AD of =L. and the whole area ABF c DA is AE' AE'

Steam

th of P; and we have found this to be nearly the case Engine. in several very good engines.

It must be remarked, that in the preceding investigation we introduced a quantity M to express the resistances to the motion of the engine. This was done in order to avoid a very troublesome investigation. The resistances are of such a nature as to vary with the velocity, and most of them as the square of the velocity. This is the case with the resistance arising from the motion of the water through the pistons of the pumps, and that arising from the friction in the long lift during the working stroke. Had we taken the direct method, which is similar to the determination of the motion through a medium which resists in the duplicate ratio of the velocity, we must have used a very intricate exponential calculus, which few of our readers would have the patience to look at.

But the greatest part of the quantity m supposes a motion already known, and its determination depends on this motion. We must now show how its different . component parts may be computed.

1. What arises from the inertia of the moving parts is by far the most considerable portion of it. To obtain it, we must find a quantity of matter which, when placed at the end of the beam, will have the same moResistance mentum of inertia with that of the whole moving parts to the mo- in their natural places. Therefore (in the returning tion of the stroke) add together the weight of the great piston with engine its rod and chains; the pit pump-rods, chains, and computed. any weight that is attached to them; the arch-heads and iron-work at the ends of the beam, and 4ths of the weight of the beam itself; also the plug-beam with its arch-head and chain, multiplied by the square of its distance from the axis, and divided by the square of half the length of the beam; also the jack-head pump rod, chain, and arch-head, multiplied by the square of its distance from the axis, and divided by the square of the half length of the beam. These articles added into one sum may be called M, and may be supposed to move with the velocity of the end of the beam. Suppose this beam to have made a six-foot stroke in two seconds, with an uniformly accelerated motion. In one second it would have moved I feet, and would have acquired the velocity of three feet per second. But in one second gravity would have produced a velocity of 32 feet in the same mass. Therefore the accelerating force, which has produced the velocity of three feet, is nearly

M.

II

th of the weight. Therefore is the first constituent of m in the above investigation. If the observed velocity is greater or less than three feet per second, this value must be increased or diminished in the same proportion.

The second cause of resistance, viz. the immersion of the pump rods in water, is easily computed, being the weight of the water which they displace.

The third cause, the friction of the pistons, &c. is almost insignificant, and must be discovered by experiment. The fourth cause depends on the structure of the when made of a proper strength, pumps. These pumps, can hardly have the perforation of the piston more than a fourth part of the area of the working-barrel; and the velocity with which the water passes through it is increased at least 4th by the contraction (see PUMP). The velocity of the water is therefore five times greater

than that of the piston. A piston 12 inches diameter, S and moving one foot per second, meets with a resistance Engin equal to 20 pounds; and this increases as the square of —~ the diameter and as the square of the velocity. If the whole depth of the pit be divided into several lifts, this resistance must be multiplied by the number of lifts, be cause it obtains in each pump.

Thus we make up the value of m; and we must ac knowledge that the method is still indirect, because it supposes the velocity to be known.

We may obtain it more easily in another way, but still with this circumstance of being indirect. We found

that p was equal to /Lm, and consequently m= L' Now in any engine L and p can always be had; and unless p deviates greatly from the proportion which we determined to be the best, the value of m thus obtained will not be very erroneous.

cer

prest

It was farther presumed in this investigation, that the O motions both up and down were uniformly accelerated; but this cannot be the case when the resistances increase with the velocity. This circumstance makes very little, change in the working-stroke, and therefore the theo rem which terminates the best relation of P to L may velocity in this case are a mere trifle when compared be confided in. The resistances which vary with the with the moving power y. These resistances are, 1st, The strangling of the water at the entry and at the standing valve of each pump: This is about 37 pounds for a pump 12 inches diameter, and the velocity one foot per second, increasing in the duplicate ratio of the diameter and velocity. And, 2d, The friction of the water along the whole lift: This for a pump of the is only about 24 pounds, amd varies in the simple prosame size and with the same velocity, lifting 20 fathoms, portion of the diameter and the depth, and in the du plicate proportion of the velocity. The resistance arising from inertia is greater than in the returning stroke; because the M in this case must contain the momentum of the water both of the pit-pumps and the jackheadpump but this part of the resistance does not affect the uniform acceleration. We may therefore confide in the propriety of the formula y= y=2 And we may obtain the velocity of this stroke at the end of a second ty communicated by gravity in a second, and the velo with great accuracy as follows. Let 2g be the veloci city at the end of the first second of the steam piston's descent will be somewhat less than

y

L

M 2g; where Mex

¡a circumstance that deserves

But this motion is affected by a circumstance quite affected by unconnected with any thing yet considered, depending on conditions not mechanical, and so uncertain, that we are not yet able to ascertain them with any precision; particular yet they are of the utmost importance to the good perconsidera- formance and improvement of the engine, and therefore deserve a particular consideration.

tion.

The counter weight has not only to push down the pump rods, but also to drag up the great piston. This it cannot do unless the steam be admitted into the cylinder. If the steam be no stronger than common air, it cannot enter the cylinder except in consequence of the piston's being dragged up. If common air were admitted into the cylinder, some force would be required to drag up the piston, in the same manner as it is required to draw up the piston of a common syringe; for the air would rush through the small entry of the cylinder in the same manner as through the small nozzle of the syringe. Some part of the atmospheric pressure is em ployed in driving in the air with sufficient velocity to fill the syringe, and it is only with the remainder that the admitted air presses on the under surface of the syringe. Therefore some of the atmospheric pressure on its upper surface is not balanced. This is felt by the hand which draws it up. The same thing must happen in the steam-engine, and some part of the counter weight is expended in drawing up the steam piston. We could tell how much is thus expended if we knew the density of the steam; for this would tell us the velocity with which its elasticity would cause it to fill the cylinder. If we suppose it 12 times rarer than air, which it certainly is, and the piston rises to the top of the cylinder in two seconds, we can demonstrate that it will enter with a velocity not less than 1400 feet per second, whereas 500 feet is enough to make it maintain a density ths of that of steam in equilibrio with the air. Hence it follows, that its elasticity will not be less than ths of the elasticity of the air, and therefore not more thanth of counter weight will be expended in drawing up the steam piston.

But all this is on the supposition that there is an unbounded supply of steam of undiminished elasticity. This is by no means the case. Immediately before opening the steam-cock, the steam was issuing through the safety-valve and all the crevices in the top of the boiler, and (in good engines) was about th stronger or more elastic than air. This had been gathering during something more than the descent of the piston, viz. in about three seconds. The piston rises to the top in about two seconds; therefore about twice and a half as much steam as fills the dome of the boiler is now shared between the boiler and cylinder. The dome is commonly about six times more capacious than the cylinder. If therefore no steam is condensed in the cylinder, the density of the steam, when the piston has reached the top, must be about ths of its former density, and still more elasticthan air. But as much steam is condensed by the cold ey

Steam

linder, its elasticity must be less than this. We cannot tell how much less, both because we do not know how Engine. much is thus condensed, and because by this diminution of its pressure on the surface of the boiling water, it must be more copiously produced in the boiler; but an attentive observation of the engine will give us some information. The moment the steam-cock is opened we have a strong puff of steam through the snifting valve. At this time, therefore, it is still more elastic than air; but after this, the snifting valve remains shut during the whole rise of the piston, and no steam any longer issues through the safety-valve or crevices; nay, the whole dome of the boiler may be observed to sink.

41

steam du

minished.

These facts give abundant proof that the elasticity of The elastithe steam during the ascent of the piston is greatly di- city of the minished, and therefore much of the counter weight is ring the expended in dragging up the steam piston in opposition ascent of to the unbalanced part of the atmospheric pressure. The the piston motion of the returning stroke is therefore so much de- greatly di ranged by this foreign and inappreciated circumstance, that it would have been quite useless to engage in the intricate exponential investigation, and we must sit down contented with a less perfect adjustment of the counter weight and weight of water.-Any person who attends to the motion of a steam-engine will perceive that the descent of the pump-rods is so far from being accelerated, that it is nearly uniform, and frequently it is sensibly retarded towards the end. We learn by the way, that it is of the utmost importance not only to have a quick production of steam, but also a very capacious dome, or empty space above the water in the boiler. In engines where this space was but four or five times the capacity of the cylinder, we have always observed a very sensible check given to the descent of the pump-rods after having made half their stroke. This obliges us to employ a greater counter weight, which diminishes the column of water, or retards the working stroke; it also obliges us to employ a stronger steam, at the risk of bursting the boiler, and increases the expence of fuel. 42 It would be a most desirable thing to get an exact How to knowledge of the elasticity of the steam in the cylinder; know the and this is by no means difficult. Take a long glass elasticity of tube exactly calibered, and close at the farther end. Put in the cya small drop of some coloured fluid into it so as to stand linder. at the middle nearly.-Let it be placed in a long box filled with water to keep it of a constant temperature. Let the open end communicate with the cylinder, with a cock between. The moment the steam-cock is opened, open the cock of this instrument. The drop will be pushed towards the close end of the tube, while the steam in the cylinder is more elastic than the air, and it will be drawn the other way while it is less elastic, and, by a scale properly adapted to it, the elasticity of the steam corresponding to every position of the piston may be discovered. The same thing may be done more accurately by a barometer properly constructed, so as to prevent the oscillations of the mercury.

3

the steam

43

know the

It is equally necessary to know the state of the cylin- Necessary der during the descent of the steam piston. We have also to hitherto supposed P to be the full pressure of the atmo- state of the sphere on the area of the piston, supposing the vacuum cylinder below it to be complete. But the inspection of our during the table of elasticity shows that this can never be the case, desceut of because the cylinder is always of a temperature far above the piston. 32°. We have made many attempts to discover its temperature.