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well as to its upper side; an equilibrium of pressure is therefore formed, and the counterpoise pulls the piston to the top, the steam above it flowing through the equilibrium-valve. In this form of engine there is alternately steam and a vacuum on the under side of the piston, the steam being always above the piston.

About the year 1780, Watt introduced another modification, having for its object the attainment of a more perfect condensation: this he proposed to effect by having a perpetual vacuum below the piston, while there was alternate vacuum and steam-pressure above it; thus, on the piston having accomplished its loaded stroke, that is, from top to bottom, the vacuum being made, the whole of the time in which the piston was ascending might be occupied in freeing the cylinder from the air and steam. This idea was carried out by aid of the following arrangements.

In fig. 23, aa is the cylinder, b the piston, c the "steam-valve," d the steam-pipe, e the "eduction-valve," ff the

b

a

a

eduction pipe leading to the condenser, g the steam-port leading to the upper side of the piston. On the piston reaching the bottom of its stroke, the steam-valve c was shut, and the eduction-valve e was opened; the steam from the upper side of the piston rushed through g and e, and down the eduction-pipeƒ to the condenser; the vacuum was thus made on both sides of the piston, the counterpoise pulling the piston to the top; the eductionvalve remaining open during its ascent, a longer time was thus given to the formation of the vacuum above the piston. The advantages expected to flow from this ingenious arrangement did not, however, exist: it was found that in practice the condensation was so quickly performed, in fact almost instantaneously, that the longer time produced no better vacuum; and the cylinder approximating so much to the coolness of the condenser, a considerable quantity more of steam was required. Leakage to some extent also resulted from the arrangement. This arrangement of engine was therefore abandoned.

fig. 23.

We now come to notice an important improvement in the working of steam-engines, which the fertile genius of Watt added to the list of his brilliant inventions: this improvement was that of working the steam expansively. The patent for the expansive steam-engine was taken out in 1782; but the attention of Watt had been directed to the principle many years before; in 1769 he wrote to Dr. Small, as to a "method of still doubling the effect of steam, and that tolerably easy." Many matters, however, diverted his attention from this important point; and it was not until the above date that he took steps to introduce an engine in which the principle was carried out. The due understanding of its rationale is so important to the student of the steam-engine, that we propose entering into its consideration at some length.

Where steam is admitted to press on the top of a cylinder, during the whole of its descent the piston will move downwards with an accelerating

velocity, which, if not checked, will be of such amount as materially to damage the mechanism. An able authority supposes that the value of the expansive principle was made known through the result of some trials which were instituted for the purpose of moderating the velocity of the piston, and consequently the shock as the piston reached the bottom of the cylinder. In Newcomen's engine he supposes this to have been effected by shutting the injection-cock earlier; and in Watt's condensing-engine, by shutting "the steam-valve at such a period of the stroke as would prevent the catch-pins from striking." This shutting off the steam-communication from the boiler, at a certain part of the stroke of the piston, allowing the steam to expand as the piston descends, constitutes the principle of the method of working expansively. By the action of the well-known law of pneumatics (see volume on Natural Philosophy in this series), the pressure of the steam on the piston decreases as the space increases into which the steam has liberty to expand itself; thus if the steam is cut off at onefourth of its stroke, the pressure will, at the end of the stroke, exert only a force of one-fourth of its original pressure. By thus decreasing the power, a simple method of equalising the tendency to an accelerated motion was attainable. In addition, however, to this advantage, a still greater one resulted from the adoption of the principle in the economisation of steam, and the consequent saving of fuel. If steam of the temperature of 212° "flows into a cylinder six feet long, until the piston has moved eighteen inches downwards, when this quantity has expanded into double its former volume, and in doing so has pressed the piston to the middle of the cylinder, it will exert a pressure of not more than 7 pounds on each square-inch area of the piston. When the piston has been depressed another eighteen inches, the vapour will have expanded into three times its original bulk, and will then urge the piston downwards with a force of not more than 4 pounds on each square inch; and when it has reached the bottom of the cylinder, and expanded into four times its original bulk, it will not exert a greater energy than about 3 pounds on each square inch. If now we calculate the varying power of the steam from the commencement to the termination of its stroke, beginning with a force of 14 pounds, and ending with 3 pounds, it will have exerted an average pressure of nearly 8 pounds on each square inch of the piston. But if the vapour had been permitted to flow freely into the cylinder as fast as the piston descended, it would have pressed it with a force of 14 pounds during the entire stroke of the piston. We thus see that one foot and a half of steam, acting expansively, has pressed 8 pounds through six feet; while six feet of steam, operating with its energy uniform and unimpaired, has only carried 14 pounds through six feet; thus showing that more than onehalf of the whole steam has been saved by making it act expansively.

"Although the saving of steam is very considerable by making it work expansively, the power of the engine is reduced; thus, where the steam is cut off at one-fourth of the stroke, while the efficacy of the steam is increased four times,—that is, one-fourth the quantity of steam will complete the stroke, the power is diminished nearly one-half. In engines worked expansively, therefore, the size of cylinder must be increased in proportion to the extent to which the expansive principle is carried. But although the engine is made larger to do the same quantity of work, this work will

be done with a less consumption of fuel: this is obvious from the consideration, that at whatever point the steam is cut off, so much steam is saved; and that the steam, although it exerts a gradually decreasing force on the piston, still exerts a power of some extent, which power, whatever may be its amount, is gained without any expenditure of steam. To carry out the system of expansive working most conveniently, it is best to use steam of a pressure considerably higher than that of the atmosphere: unless this pressure is considerable, expansion cannot be carried out to any great extent with advantage; for if steam of a low pressure were used, the ultimate tension would be reduced to a point so nearly approaching that of the vapour in the condenser, that the difference would not suffice to overcome the friction of the piston, and a loss of power would be occasioned by carrying expansion to such an extent. It is clear that in the case of engines which carry expansion very far, a very perfect vacuum in the condenser is more important than it is in other cases. The advantage of applying steam expansively will be seen by an inspection of the following table: if the steam is cut off at one-half of its stroke, the performance of the engine will be multiplied 1:7 times; at

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Watt effected the cutting off of the steam at any desired point by merely altering the position of the tappets or projecting pins in the plugframe, by which the valves were actuated upon at the proper time. As the motion of the piston was necessarily variable when the expansion principle was adopted, Watt contrived several ingenious mechanical combinations, by which the effect of the engine on the work it had to perform was uniform; he, however, did not apply these to any great extent, as he employed steam a little greater in pressure than that of the atmosphere, and cutting off only at one-third or one-fourth, according as circumstances dictated.

The reader desirous of becoming acquainted with these further evidences of Watt's inventive talent, will find several plans figured, by which this uniformity was obtained, in Stuart's Descriptive and Historical Anecdotes of Steam-Engines. We proceed to the consideration of more interesting and important matters in connection with the inventions of Watt.

Under the new arrangements it was a matter of importance to ascertain the state of the vacuum in the condenser and cylinder; for on the perfection of this obviously depended the efficiency of the engine. In order to ascertain this, Watt applied a mercurial barometer, having a connection with the inside of the pipe leading to the condenser; and another barometer was placed in connection with the boiler. The rise and fall of the mercury in the barometer attached to the condenser indicated the degree of exhaustion which had been made in it; and by the same operation in the barometer attached to the boiler, he had a measure of the pressure of steam acting in the piston: from the data thus obtained, he was able to calculate with considerable precision the amount of power given out by the engine. He afterwards, for this purpose, introduced a highly-ingenious

invention which he termed the indicator; a diagram of which we now append, in order to show the nature of its operation. A small cylinder b,

fig. 24, truly bored, is fixed on the cylinder-cover, having connection with its interior: a small piston c works in the cylinder b, the spindle or rod is continued upwards, its head terminated by a pointer, which is placed along a scale d; round the lower part of the spindle of the piston a spiral spring is coiled, one end of which is fixed to the piston, the other to a small bracket. The upper side of the piston is open to the air, the lower is open to the cylinder. The zero point of the scale is so adjusted, that the pointer will point to it when the cylinder is filled with air; and the pressure on both sides of the piston of the indicator is equal. On a vacuum being made in the steam-cylinder, the piston of the indicator is forced downwards; and the spring being thus put in a state of tension, the pointer will indicate the different points in the scale, corresponding to the degree of vacuum in the cylinder. When the cylinder becomes filled with steam, the piston of the indicator rises, and it falls again on the vacuum being made. Thus the power of the engine at any period of its stroke is faithfully transferred to the piston of the indicator, and by this means the power of the engine is estimated. In a future chapter we shall describe the modern indicator, and the means by which it is made to record on paper the working capabilities of the engine to which it is attached.

b

α

fig. 24.

We give a diagram, in fig. 25, illustrative of the arrangements of Watt's single-acting pumping-engine, as adopted at this period. At the present time the principle of this engine is still the same: the modifications in the details, arising from greater perfection in workmanship, although tending to give an appearance of greater elegance to its form, have not been extended to alteration of its principle of action. To such a high state of perfection did Watt bring it, that an eminent authority states that a pumping-engine" made after Watt's primitive type, would, with an equally effectual boiler, and an equal means of clothing and expansion, do about the same amount of duty as the best of the modern construction." We have already detailed the method of action of this engine, so that we deem a further explanation unnecessary; a literal reference to its parts, and a few words as to the method of "working the engine," may, however, be useful. The cylinder is indicated by the letters aa, b the piston, d the piston-rod, attached to the end of the working beam ee by a chain passing round the quadrant (see p. 84, fig. 164, treatise on Mechanics and Mechanism, in this series, companion volume to the present); ff the plug-frame for working the valves by the tappets and levers as shown; g the equilibrium-pipe; h the condenser; i the air-pump; j the cold-water cistern; k the hot-water cistern. In commencing to work the engine, the first operation is expelling the air from the interior of the cylinder a a: this is effected by relieving the catches of the steam, equilibrium, and eduction valves; these being actuated by weights attached to them, are opened, and steam is thus admitted to all the internal parts, as cylinder and equilibrium-pipe. The first effect of this is to condense the steam by coming in contact with the cold surfaces of the engine; after the cylinder a a becomes hot, the steam finally issues through the "snifting-valve" placed at the foot either of the

d

a

h

2

e

fig. 25.

condenser or of the air-pump. The valves are then to be shut, and a vacuum is thus formed in the engine. The first stroke of the engine is then made by opening the steam and eduction valves, and opening the injectioncock, so as to admit a jet of water to the condenser; the piston is then forced down into the vacuum by the steam above the piston. The plugtree ff (fig. 25), in descending, strikes its tappets on the spanner or levers of the eduction and steam valves, shutting them, and opening the equilibrium-valve; the steam above the piston rushes throught his to the under side of the piston; and an equilibrium being thus established on both sides of the piston, it is drawn up to the top of the cylinder by the action of the counterpoise at the pump end of the beam.

The single-acting engine as here described, although admirably adapted for the purposes for which it was introduced, namely, withdrawing water

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