necting-rod, and of all those other bearings common to both sorts of engines, it must be evident that the total amount of the friction due to those parts in which a difference between the engines exists, must be but a small portion of this one-fifth. Taking one-tenth or ten per cent. of the whole power of an engine, as the amount of power required to overcome the friction of the engine itself, which was allowed to be ample, one-fifth of this would be two per cent.: and therefore the degree in which either engine could surpass the other in the amount of friction, could only be, as already stated, a small portion of this two per cent.

In comparing the efficiency of these engines, it would thus appear that neither could be said to possess advantages over the other, as regards friction, in such a degree as to be appreciable in practice, or to render the point of importance, in a choice between the engines; and that if the one kind of engine had advantages over the other, they must arise from other causes than difference in friction.

Having taken this view of the case with a supposed side-lever engine, of the same length of stroke and diameter of cylinder as the direct-action engine, if manufacturers varied in a slight degree from this proportion, it was for the purpose of obtaining a better proportion of stroke and diameter of cylinder, and consequently a better engine than the one supposed to exist for the purpose of making the observations.

Mr. Vignoles looked upon the second proposition assumed by the author, as being overthrown by the results of the experiments of Wood and others, as to the ratio of friction to the area of rubbing surface; and it was well known practically, that the application of various unctuous substances materially altered the amount of the friction. A certain proportion was requisite between the area of the surface exposed to the friction, and the pressure upon it, to bring it within the general law. For practical purposes, he submitted that the law should be received with limitations.

Mr. Gravatt said, that even allowing, for the sake of argument, that the second proposition, assumed by the author, was incorrect, still, as the paper was only a theoretical examination of the comparative friction of those parts of two kinds of engines, which were most subjected to strain, supposing them both to be of similar power and dimensions, equally well-proportioned and constructed, and the same sort of lubrication of the bearings employed, he would contend that the circumstances being equal,

equal results would be obtained, and that the conclusions arrived at by the author should be received as correct.

Mr. Pole observed, that the objections brought forward were important, as they referred principally to the fundamental laws of friction.

He would first give some explanation respecting the communication itself.

The investigation was commenced at the request of his late friend, Mr Samuel Seaward; it was originally intended to have especial reference to the Gorgon engine, but had subsequently been extended to others.

The paper, necessarily containing much mathematical reasoning, could only be read in abstract, and might, therefore, have been partially misunderstood, both as to its objects and results.

The object was not to enter into a discussion of the whole question of the respective merits or defects of beam and directaction engines, but simply to ascertain the value of the one objection named.

The whole friction of an engine at work, with its load upon it, might be divided into two distinct parts. 1st. The friction due to the engine itself, or such as would be produced by the working of the engine, if unloaded. 2nd. The additional friction caused by the strain consequent upon the load; for it must be evident, that when the engine had its work upon it, the friction upon the bearings, through which the strain passed, must be increased, and additional friction produced beyond that which would exist when the engine was working without a load. The latter of these alone required to be calculated, and to this, mathematical analysis was more peculiarly adapted. The friction of the engine unloaded, might be ascertained by the indicator, as described by Mr. Field; but as he had remarked, there was no practical method of finding what was the additional friction when the load was applied; indeed, it would be as difficult to find the latter by experiment as the former by theory.

He then explained the manner in which the amount of friction upon each bearing had been calculated, and engines of different constructions compared with each other. He had adopted precisely the plan suggested by Mr. Murray, namely, by taking engines of the same length of stroke and diameter of cylinder, supposing them to be equally well proportioned and constructed, and in equally good condition. But instead of assuming, as Mr.

Murray had done, that there was somewhat more or less friction on any particular bearing, his object had been to ascertain what was its actual value. If it were impossible to measure the pressures, and spaces moved through, an approximation might be received; but since these quantities were ascertainable, it was more satisfactory to obtain results deduced from them.

The conclusions drawn from the paper accorded, however, with Mr. Murray's, viz., that "neither construction could be said to possess advantages over the other, in such a degree as to be appreciable in practice, so as to render the point of importance in a choice between them." The difference between Mr. Murray's process and that in the paper, was, that what the former only assumed, the latter endeavoured to prove.

Mr. John Seaward's pamphlet on the Gorgon engine had been referred to. The conclusions he there drew, were more favourable to the direct-action engine, but were derived, like Mr. Murray's, merely from approximate consideration, rather than from strict investigation. Mr. Seaward confessed, that the friction. caused by the strain was difficult to be calculated, and had therefore contented himself with assuming, that those gudgeons, through which the strain passed, had three times as much friction as was due to the others. He also assumed, that the friction was proportional to the area of the rubbing surface, a principle which no experiments had ever shown. On these grounds, it was contended that Mr. Seaward's results were open to objection.

Mr. Pole then proceeded to notice the objections urged against the fundamental laws of friction which he had stated, and to give authorities for them.

The first of these had not been questioned since the days of Vince, by whom it was proved; it might therefore be considered as established. With regard to the second and third, it must be noticed that they depend, in some measure, upon each other, for it could be proved, that if the third was true, the second must be true also.

The principal experiments, which had been made upon the friction of solids, were those by Amontons, in 1699; Coulomb, in 1779; Vince, in 1784; Wood, in 1818; Rennie, in 1828; and Morin, in 1831, 32, and 33.

Amontons was the first who devoted any considerable attention to the subject, and he found that friction was not augmented by an increase of surface, but only by an increase of pressure.

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Coulomb's researches were more elaborate, the experiments were on a large scale, and were submitted to a great variety of trials; they fully proved that the friction was proportional to the pressure, and that the extent of surface did not affect it.

These results were further confirmed by the experiments of De la Hire, Ximenes, Boistard, Rondelet, and others.

Mr. George Rennie's experiments were very valuable, as having been conducted on a large scale, and with much care; they were also of a comparatively recent date. The results were conclusive on the point in question, for he found, that when the surfaces were to each other as 6:22: 1, the friction remained the same; and one of the general conclusions he deduced was, “ that the amount of friction was as the pressure directly, without regard to surface, time, or velocity."

The last and most extensive series of experiments were those by M. Morin; they were conducted at Metz, by order of the French Government, and extended over a period of three years, (1831, 1832, and 1833,) no expense or trouble having been spared to render them conclusive and satisfactory.

The results were given by Professor Moseley, in his new work on the Mechanical Principles of Engineering. They proved that "the friction of any two surfaces was directly proportioned to the force with which they were pressed perpendicularly together," and that "the amount of friction was, in every case, wholly independent of the extent of the surfaces of contact."

The before-mentioned experiments all agreed, that the friction was proportional to the pressure, and was independent of the extent of surface. In opposition, however, to these, stood the experiments of Professor Vince, of Cambridge, which led him to the conclusion, that the friction increased in a less ratio than the pressure, and that it was not altogether independent of the area of surface. These experiments were probably conducted with care and accuracy; but it was also probable that equal precision had been used in those which proved the contrary; and if this was allowed, the majority of coinciding experiments might, as in all other cases, be safely received in preference to one dissentient. But if the particulars of Professor Vince's experiments were examined, many circumstances appeared which would render them less worthy of regard than others. It was not shown that he experimented upon metals, but that he used pieces of wood, either bare or covered with paper; and the experiments were on a small

scale, the moving bodies being at the utmost a few ounces weight: while Coulomb, Rennie, and Morin, had extended their trials to all kinds of materials, and had used considerable weights. Professor Vince himself, although satisfied with the method of conducting his experiments, did not seem equally so with their results, as regarded the influence of surface and pressure, for he had remarked, "that no general rule could be established to determine it, even for the same body."

Quotations were then given from Gregory, Brewster, and others, corroborating this view of the inconclusive and unsatisfactory nature of Vince's experiments.

The law of the influencce of pressure and surface upon friction, was occasionally modified by accidental circumstances, two of which might be noticed, as they had been expressly treated of by Rennie and Morin.

1o. It was only applicable within the limit of pressure which would not injure and abrade the surfaces; for when heating and undue attrition commenced, it was natural that the law would not hold good. Well-constructed machinery, however, was never supposed to pass this limit, and therefore this cause of irregularity might be rejected in calculation.

2o. Another modification was produced by the application of unguents; this was treated of by Mr. Wood, whose experiments showed, that when unguents were introduced, there was a certain area of bearing surface, proportioned to the weight, which was peculiarly favourable as regarded the loss by friction, but that when this area was preserved, the friction was in strict ratio to the pressure.

It could not, however, have been Mr. Wood's intention, from these results, to impugn the applicability of the established general laws to the purposes of calculation, but only to show the existence of modifying cirumstances under certain conditions; for the formula he had given, assumed the friction to be as the weight, and had no element in it expressing the area.

Mr. Rennie and M. Morin had also examined the influence of unguents, and had found that that their introduction did not materially alter the general laws of friction, but only affected the value of the co-efficient or multiplier to be used in ascertaining its numerical amount.

Having thus brought before the meeting the results of the principal experiments on friction, Mr. Pole concluded by ad