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been supplied by Mr. Davies, of Birmingham, by whose permission he would take an early opportunity of describing to the Institution the construction and efficacy of the instrument.

It was very important that a self-registering machine should be made, capable of recording the mean steam pressure operating throughout a stroke of the engine, but it was still more important that this registration should be accurate; and he hoped that the remarks which had been made would only urge Professor Moseley to further investigations, and induce him to enlarge as much as possible the useful powers of the instrument.

Professor Moseley observed, that when a body passes from a state of rest, through a state of motion, and into a state of rest again; or from a state of motion, at a given velocity, through a state of motion, at a different velocity, and back to its first velocity again; then is the work, which must be done upon it by the moving power, the same in amount, whatever may have been the velocity thus intervening between the two states of rest or of equal motion of the body, provided that the resistance opposed to its motion, and the space through which that resistance is overcome, be, in all cases, the same.

In Mr. Wicksteed's engine, the resistance thus opposed to the motion of the piston, and the space through which that resistance is overcome at every stroke, are thus constantly the same, (or in other words, the work done upon the resistance is the same at every stroke of the engine,) and the piston passes, at every stroke, from a state of rest to a state of rest again: it follows, therefore, by the above well-known principle of "vis viva," that the work done by the steam, as the moving power upon the piston of the engine, whilst it completes a stroke, is the same, whatever may be the velocity communicated to it, and to the mass which it carries with it, at any period of the stroke.

It is true, that to put the piston, and the mass carried along with it, at first in motion, a pressure greater than the resistance is required, and therefore greater than the mean pressure necessary to complete the stroke: a pressure equal to the resistance would only bring it into the state of rest bordering upon motion; to cause it to pass from this state of rest to a state of motion,

more pressure is required; and the more as the velocity to be acquired, whilst it moves through a given space, is greater-or in other words, in order to communicate any given velocity to a body whilst it moves through any space, there must be an excess of the work done by the driving pressure through that space, over that expended upon the resistance through that space; but all this excess is accumulated, and unless the steam pressure be afterwards made less than the resistance, or unless the steam be afterwards expanded through a distance dependent on the amount of this accumulated work, so that it may expend itself in overcoming the surplus resistance through that space, then the piston will strike upon the cylinder bottom.

This principle may be illustrated by an example: Suppose that the load upon the piston of an engine is 10 lbs. per square inch, and that the steam is admitted at a pressure of 15 lbs., it is evident that, by reason of the excess of 5 lbs. pressure of the steam above the load, the velocity of the piston will be made continually to increase until the steam is cut off, and afterwards, so long as the steam pressure exceeds the load, or until by its expansion the steam pressure is reduced to 10 lbs. per square inch. Up to this point the velocity of the piston and of the mass moving with it, will continually have been increasing, a great momentum will therefore have been acquired by it, and this momentum will carry it on to the completion of the stroke; although, after this position is passed, the steam pressure will be less than the load, and would by itself be insufficient to move it.

In other words, the work done by the steam upon the piston will have continually exceeded that expended on the load up to this period of the stroke, and the surplus will have been accumulated in the moving mass, which surplus work will carry on the piston to the end of the stroke, when a cylinder full of steam will be delivered, of greatly less pressure than the load. If the steam had been worked at full pressure, it is evident, that at every stroke a cylinder full of steam would have been discharged, of the same pressure as the load. In this consists, therefore, the advantage of working expansively.

It is evident, that the piston acquires its maximum velocity at

the point where the steam-pressure becomes equal to the load, and that the engineer, by the manipulation of the steam-valves, produces that adjustment by which the velocity, acquired by the piston at this point, (or the work then accumulated in it,) is caused to be just sufficient to carry on the piston to the end of the stroke, but without striking the cylinder-bottom: it is moreover evident, that the greater this maximum velocity can be made, the farther the piston will be carried beyond the point where the steam-pressure is equal to the load, and the less will be the pressure of the cylinder, full of steam, discharged at the completion of every stroke, or the greater the economy of the steam power.

A second illustration of the same principle may be drawn from the effect produced by a pressure suddenly thrown upon a spring. Suppose a spring which would rest deflected through an inch under a pressure of one pound.-If, when this spring is in an undeflected state, this pressure of one pound be suddenly thrown upon it, it is certain that the spring will, at first, deflect considerably beyond that distance of one inch, in which its deflection will eventually, after many oscillations, terminate. In fact, if it is thrown on with mathematical suddenness, the first deflection will be two inches. To explain this, let the pound weight be supposed to be applied gradually to the spring, by dropping grain after grain of sand slowly upon it. The spring will then evidently be brought to its deflection, without ever passing it. Now let it be observed, that on this supposition, the first grain of sand only will have descended through one inch, the next descending through less than an inch, the next through yet less, and so on. Thus the work done upon the spring, by each succeeding grain, will be less than that done by the preceding. Yet the aggregate work done by these successive small pressures, each working through a different space, is sufficient to deflect the spring one inch. Now let all the grains be placed at once upon the spring. When it has deflected an inch, each grain will then have worked through an inch, and a great deal more work will, on the whole, have been done on the spring than before, indeed twice as much: but the work done before was enough to deflect the spring an inch; more than enough to deflect it has

now therefore been done: that is, more has been done than has been expended. The remainder is accumulated in the moving mass of the sand and the spring, and carries on the deflection greatly beyond the position of equilibrium.

The Indicator was placed upon the engine of the East London Water-works, in the belief that, by the experiments of Mr. Wicksteed, the work actually performed by that engine, was better known than that of any other. All the calculations and enquiries which have since been made, have fully confirmed that opinion. And he had full confidence in that verification of the registration of the Indicator, which is supplied by its agreement with Mr. Wicksteed's estimate of the work of his engine.

In reference to the use of the term "work," Professor Moseley stated, that the various terms used by foreign engineers, to convey the idea attached to that term, appeared at length to have resolved themselves into the single term "travail;" and that of the variety of corresponding terms, used in England, the term "work" was probably the most obvious translation of " travail ;" that it moreover appeared to him the simplest, and the most intelligible; and that on these grounds he had adopted it.

In answer to the observation made by Mr. Parkes, suggesting the construction of an Indicator which would register the work of the machine at the point where it is applied, instead of at the cylinder of the engine,

Professor Moseley stated, that such an instrument would undoubtedly be very valuable, especially if it could be made to register correctly the work transmitted by a rotating shaft: but that for the purpose comtemplated by him it would be entirely useless; this object was to effect, in respect to ordinary engines working under constantly variable pressures, that constant registration of the duty, the introduction and publication of which had led to so remarkable an economy of steam power in the working of the Cornish engines. No registration of the work done at the working points of the machine, driven by the engine, would supply a fair estimate of the duty done by the engine, a greater or less portion of the work done by the engine being lost by reason of friction in its transfer through the machine, from its 2 E

VOL. XXI.

driving to its working points, according as there was a greater or less complication of moving parts and rubbing surfaces intervening.

He repeated, that his object had been to determine the working qualities of the engine itself; and that he had, for this reason, specially sought to eliminate from his estimate those very influences of the friction of the machine driven, by the engine, which Mr. Parkes thought it so important to include in it. It would have been a fault of his Indicator (for the purpose contemplated by it) if it had taken any notice of the effect of that change made in the machinery of Mr. Lucy's mill, which Mr. Parkes had spoken of. He had used the term effective work (not effective power) of the engine, to signify that excess of the work of the steam on one side of the piston, over that opposed to it, by the imperfectly condensed steam on the other, which it was necessary to know, in order to estimate the real duty of the engine. It was solely for the determination of that duty, that the Indicator had been constructed, and the alterations which Mr. Parkes had suggested, would have subjected its registration to influences which, in reference to that purpose, he had specially sought to eliminate.

[To be continued.

List of Patents

That have passed the Great Seal of IRELAND, from the 17th September to the 18th of October, 1842, inclusive.

To Charles Augustus Preller, of 16, East Cheap, in the city of London, merchant, for improvements in machinery for preparing, combing, and drawing wool and goats' hair,-being a communication from a certain foreigner, residing abroad.—Sealed 30th September.

William Geeves, of Old Cavendish-street, in the county of Middlesex, Gent., for certain improvements in machinery for cutting cork.-Sealed 30th September.

William Henry Kempton, of South-street, Pentonville, in the county of Middlesex, Gent., for improvements in the manufacture of candles.-Sealed 30th September.

Alexander Johnston, of Hill House, in the county of Edinburgh,

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