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the bottom of the cylinder of the steam-engine; in that case, each Indicator shews on its own card what the elastic force of the steam is during the plenum, and what it is during the exhaustion, but the required result, (which is the difference between the two,) must be obtained by combining together in the computation, those distinct curved lines which are drawn on two separate cards. Professor Moseley's combined indicator pistons, acting on the same springs, would at once indicate such difference, by the curve which it would trace on the one card.

In answer to a question from Mr. Parkes, as to whether the new instrument had been put to any other test than its apparent agreement with Mr. Wicksteed's estimate of the resistance overcome; and whether the common Indicator had been applied to the engine at the same time,-Professor Moseley said, that he had not compared the instrument with any other, but had subjected Mr. Wicksteed's calculations to a rigid investigation, and felt quite satisfied that they approximated closely to the truth.He relied upon them as corroborations of the accuracy of the instrument.

Mr. Parkes observed, that it would have been more satisfactory to engineers to have been assured that every means had been taken to demonstrate the truth of the results recorded by an instrument which had such important functions in view. He wished to know in what manner the pressures denoted, had been ascertained,-whether by weights or by comparing them with a mercurial column. He had found the latter mode more exact than weights, in verifying the scale of the common Indicator, as the instrument being heated, was then in precisely the same state as when it was in use. He had found that a certain amount of correction was frequently necessary, as both the spring and the amount of piston friction were affected by heat.

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Professor Moseley replied, that the instrument had not been compared with the mercurial column, but that the resistance of the springs, and the friction of the piston and instrument generally, had been ascertained by very accurate experiments, so that he had full confidence in the results.

Mr. Parkes said, that notwithstanding the respect and defer2 D

VOL. XXI.

ence he felt for Professor Moseley's attainments and ingenuity, his past experience would not permit him to place entire confidence in the results afforded by the instrument: indeed he considered them to be altogether fallacious as representing the force acting on the piston of the Old Ford engine. He could not admit that the apparent near identity between Mr. Wicksteed's computations of resistance, and the constant indicator's registration of force, amounted to proof of the instrument's accuracy; for, it seemed to him that Mr. Wicksteed had omitted to take into account one important item of force, without the exertion of which, a piston could not be brought from a state of rest into a state of motion. He referred to the force required to give velocity to the piston. Mr. Wicksteed had weighed the greater part of that resistance which might be called ponderable, and had estimated the remainder, assigning about 12 lbs. per square inch on the piston as the total amount, after deducting the resistance to the piston's descent, arising from uncondensed steam. Thus, an elastic force of 12 lbs. on the piston would counterpoise a resistance of 12 lbs., but motion would not ensue until a superior force were applied. Considering the number of strokes made by this engine per minute, Mr. Parkes could not estimate the velocity of the piston at less than from 300 to 400 feet per minute, which was very great for this enormous mass, and would require the exertion of proportionate power. He regarded the piston of the engine, loaded with 12 lbs. per square inch, as he would a ball of 12 lbs. weight in a gun, balanced by a fluid of a corresponding elastic force; but such ball would remain motionless unless it were propelled by some additional force. This state of things had been denominated by Professor Barlow "the preparation for motion." Now as Mr. Wicksteed's estimate proceeded no further than the production of this state of things, and as Professor Moseley's constant Indicator recorded the mean pressure of the steam in the cylinder as barely equal to it, he could not assent to the accuracy of either method of determining the total resistance overcome by the steam.

Mr. Parkes would cite the experience of others as to the quantity of force actually expended in giving velocity in a Cornish

engine, over and above that necessary to balance the weight at the opposite end of the beam, friction, &c., included. In the fifth part of vol. iii. Trans. Inst. C. E., Mr. Enys has reported some experiments made, at Mr. Parkes' suggestion, on several engines. He would cite those of the Tresavean, as the cylinder was of the same diameter as the one at Old Ford, viz., 85 inches. The water load was equivalent to 12 lbs. per square inch on the steam piston, and when about half the usual velocity was given to the piston in the in-door stroke, a pressure of about 17 lbs. was denoted throughout the stroke, by a mercurial column connected with the cylinder. Mr. Loam had since transmitted to of experiments made on the same "Two indicators were used at the

him the following abstract engine, January 28, 1842.

same time, and an open mercurial gauge. The engine was held indoors until the mercury became stationary, in order to ascertain the minimum quantity of steam power necessary to produce motion. The pressure was 15 lbs. per square inch, when the water load was 12 lbs.

Here, then, a force of 3 lbs. was found requisite to establish an equilibrium between the power and resistance; and a further force of 2 lbs. per square inch was necessary in order to urge the mass at about half of its ordinary velocity. Mr. Enys stated that less pressure was exhibited when the engine was brought in slower, and vice versa; and this was consistent with everyday experience.

Now, though the Old Ford engine had not to overcome so much frictional resistance as a deep mine engine, yet, having a weight to raise, according to Mr. Wicksteed, alone equal to 11.8 lbs. per square inch on the piston, Mr. Parkes was of opinion that this could not be effected at the usual working velocity, with less than 14 or 15 lbs. pressure of steam per square inch. He would suggest to Mr. Wicksteed to repeat Messrs. Enys and Loam's experiments, and also to work his engine with steam reduced nearly to such pressure in the boiler as would barely suffice to bring the piston down. Such experiments would confirm or invalidate the results given by Professor Moseley's instrument, and probably lead to the discovery of its imperfections, should any exist.

It appeared that Mr. Wicksteed conceived a greater amount of elastic force to be required to perform a stroke in proportion to the degree of expansion given to the steam in the cylinder. He would quote the pressures deduced by Mr. Wicksteed under five cases of expansion, as they exhibited some curious anomalies. They were taken from Table VI. of Mr. Wicksteed's Treatise. According to his table, when the steam was stopped at 6 feet of the stroke, the mean force exerted during the stroke of 10 feet, was 13 lbs. per square inch; at 4 feet, 13 lbs.; at 4 feet, 14 lbs. ; at 3 feet, 15 lbs.; and at 3 feet, 15 lbs. It must be observed, that in every one of these cases, the resistance, as appreciated by Mr. Wicksteed, amounting to 13 lbs. per square inch, was a constant quantity; so that for some unexplained reason, an invariable load appeared to require a variable force to overcome it; a sliding scale of power, given as the measure of a constant resistance. It was possible that some small difference might have existed in the velocity of the stroke, in these cases; it was also possible that some error existed in the method used for determining the pressures, or in the evaporative quantities. However this might be, it was clear there was an error somewhere, as it could not be granted that an effect, deemed constant in all these cases, could require a varying cause for its production. As these appreciations differed widely from each other, and still more so from the indications of Professor Moseley's instrument, Mr. Parkes hoped that Mr. Wicksteed would re-consider and verify this part of the subject. In corroboration of his opinion that the acting force recorded by the instrument, is too low, he would draw attention to the circumstance that, during the period of its application (twenty-eight days), the duty performed by the engine for each 94 lbs. of coal, amounted only to about 68 millions, and it would not be unreasonable to expect that at least 150 millions should have been the result under the different circumstances of mine and water-works engines, as 100 millions had been performed for some months by mine engines under a water-load equal to 12 lbs., and a mean steam pressure of 18 lbs. per square inch of the piston. Whereas, if the water-load of 11 lbs. at Old Ford, was overcome by an amount of force little

exceeding 12 lbs. per square inch, a proportionate increase of duty ought to have resulted, but such was not the case.

Mr. Parkes then proceeded to comment on the phrase "effective power," which he understood from Professor Moseley as significant of the force of the steam, or piston pressure, measured by his instrument. He thought that phrase more strictly applicable to the amount of power given off by the engine, when ascertained, as it might be in the case of pumping-engines, by the weight raised; or as it could only be determined on rotative engines, driving machinery, by a dynamometer applied at the extremity of the crank shaft. He would illustrate this by an example:-There was at Birmingham a corn-mill belonging to Mr. Lucy, worked by an excellent engine of forty horses power, made by Messrs. Boulton and Watt. This engine had a fly-wheel weighing 24 tons, and nine pairs of stones were driven, besides dressing machines. Mr. Lucy had taken out a patent for an apparatus as a substitute for the fly-wheel, which had been removed. The engine, so altered, now drives ten pairs of stones, under the same pressure of steam, and with the same consumption of fuel as before.

Thus, what he should denominate the "effective power" of the engine, was increased, by this simple change, eleven per cent. Yet, Professor Moseley's instrument, or any other indicator, would have exhibited, both before and after the alteration, the exertion of a precisely equal force on the piston.

Neither did the Professor's instrument register the absolute, or what Professor Whewell had denominated the " labouring" force of the steam on the piston of an engine, as it made no deduction of the amount of force, whatever it might be, which was necessarily expended in overcoming the resistance opposed by the uncondensed steam. Its construction permitted it only to record the difference of these amounts. For these reasons he could not regard the instrument as likely, even when made trustworthy, to become of that utility to engineers which was the Professor's aim and hope. There was a greater need of an accurate dynamometer, capable of showing the effective power of an engine, whilst in regular work, and he was happy to say, that this desideratum had

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