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EXPANSIVE ENGINE.

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This facility of equalizing the motion, although a point of great value, not only as it regarded the proper application of the power, but as it influenced the durability of the mechanism by preventing concussions which would have shaken it to pieces, was but of minor importance when compared with the saving of fuel that was made, by allowing the expansive force of a portion of vapour to perform what had been hitherto accomplished by the unimpaired elasticity of the entire measure.

If, for example, steam of the temperature of 212° flows into a cylinder six feet long until the piston has been 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 seven pounds on each square inch area of the surface of the piston. When the piston has been depressed another eighteen inches, the vapour will have expanded into three times its original bulk, it will then urge the piston downwards, with a force of not more than four pounds and a half 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 33 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 fourteen 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

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fourteen pounds during the entire stroke of the piston.

We thus see that one and a half feet of steam, acting expansively, has pressed eight and a quarter pounds through six feet; while six feet of steam operating with its energy, uniform and unimpaired, has only carried fourteen pounds through six feet;-shewing that more than half of the whole steam has been saved, by making it act expansively.

A mere alteration of the tappets in the plugframe, so that the supply of steam should be shut off, when the piston had moved through a third or other proportion of its entire stroke, was all that Watt found necessary to introduce this fine discovery into his common mechanism.

It will be apparent that when steam is to be employed expansively, a larger cylinder will be required to develop the same power, than if the vapour were to act with its power undilated throughout; and from the greatest effect being obtained in this mode, when the first portion of steam is small in comparison with its final enlargement, it will be evident that its use, in the most favourable case, will in practice be almost unobtainable, from the cumbrous size of the vessels necessary to ensure its action; and Watt also found, that, in addition to this inconvenience, these large vessels occasioned a loss of power from their exposing a very great surface to the condensing influence of the atmosphere; and he also found this loss bore a much greater proportion to the power that was gained, than he at first anticipated. He usually operated with steam of a force equal to seventeen or eighteen pounds on the inch, or three or four pounds above the atmosphere pressure, and it was seldom allowed

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to expand beyond some fractional part of its volume.

The average pressure on the piston was always nearly equal to the average load, but he found that the acquired momentum of the ponderous masses of the lever-beam prevented any interruption to the motion of the engine from a small excess in the load.

And this tendency to acceleration in the motion of the piston, which has been described as a great defect in his and in the atmospheric engines, and which elicited much ingenuity to render it evenly, he now called to his aid, as a means to produce a uniform movement in a machine whose motion was unequal, from being impelled by steam of varying energy. And so skilfully was this rectification applied, that, in the Shadwell engine, an almost perfectly equable movement was given, by balancing the irregularity produced by the loss of energy in the expanding vapour, by an irregularity arising from an increase of power by the action of gravity.

Watt also proposed some mechanical combinations to produce an equable motion in the piston of an expansive engine.

In figure 1, two wheels, r, s, are connected by a rod or chain, x, fixed to the end of the levers e, i. These wheels are in the position as if the piston, which is attached by a chain to the circumference of r, were at the top of the cylinder. As it descends, the lever, e, is drawn towards the point, c, which increases its effective length, and Watt directed that this increase should be adjusted to compensate for the decreasing force of the steam on the piston, on account of its expansion within the cylinder. And as the load on the pump was

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the same from the beginning to the end of the stroke, the lever, i, decreases in its length as e increases; so that when the steam is exerting the greatest force on the piston, the leverage of e is least, and that of i greatest; and when the steam presses with the least energy on the piston, the power exerted by the lever, b, to raise the lever, i, is greatest.

The second method is by means of a chain wound upon one spiral, a, or yielding lever, and wound off another spiral, b, as the piston descends; the chain, x, (figure 4) passes over b, and the pump rod is fastened at c; as the piston descends it acts to shorten b, and lengthen a.

In the same manner, when the piston fixed to the chain, a, (figure 2) descends through equal spaces, the pump rod attached to c rises through unequal spaces. In figure 3, the centre or point of suspension of the lever beam, c, is made to change its place during its vibration, so that the end, e, to which the piston rod is suspended, is nearer the axis, as the piston begins to fall, than at the termination of its movement; m, is a hollow curve of wood or metal, fixed to the lower side of the working beam; n, friction roller, furnished with teeth or cogs to prevent its sliding as it rolls between the curve and the plane of support, o. This roller is divided into three parts, the two extremities of which roll on the plane, o, and the lever beam is on its centre. When, by the fall of the piston, the end, i, of the working beam is pulled downwards, the roller proceeds towards that part of the curve which is highest and now nearest to e, and thereby shortens the lever, by which the pumps resist the piston, and lengthen the lever, by which the piston acts on the pumps

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as the steam expands in the cylinder, in any ratio that may be required, according to the form which is given to the curve.

The sixth figure shows another mode of obtaining the same result, by means of a heavy cylinder, a, of iron, which rolls in a hollow curve, c, on the back of the working beam d; so that the centre of gravity of the beam, at the beginning of its motion, lies nearer to the pump end, f, of the beam: but, as the piston descends, the cylinder rolls towards the other extremity of the beam, and acts in aid of the decreasing energy of the vapour. A more complicated variety of this contrivance is shown in figure 5, in which the piston is attached to the curved beam, a; this acts on the pump beam, c, by means of a rod, d, attached to a friction wheel, e, which rolls on the pump lever. By the motion of this wheel, at the descent of the piston, the leverage is increased as the steam weakens its pressure.

Another scheme, which will be easily understood without a diagram, was, to cause a quantity of water to oppose the ascent of the piston in the beginning of its stroke, and to assist it in the latter part. Two cylinders, open at top and bottom, each fitted with a piston, have their rods suspended from the opposite ends of the lever beam. At the beginning of the stroke the weight of the water on one of the pistons produces the required resistance: but as the beam rises, it lifts the water, which flows through a pipe upon the piston in the other cylinder, which thus becomes loaded in proportion as the other is relieved. The adjustment shown in the figure 7 has a weight, a, placed considerably above the axis, b, of the beam, c. At the commencement of the

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