velocity, he considered to be referable to the first three causes, besides the velocity of the planes in contact.

With regard to the physical cause of friction, he coincides with the opinions of Amontons and others, that it arises from the entangling of the asperities, which can only be disengaged by bending or breaking. These experiments led to some important results, viz.

1st. That the friction of wood on wood, without unguents, was in proportion to the pressure, which attained its maximum in a few minutes after repose.

2nd. That the effects of velocities were similar; but the intensities were much less to keep the body in motion, than to detach it from a state of rest, oftentimes in the ratio of 22: 95.

3d. That in the case of the metals, the results were likewise similar; but the intensity was the same, whether to disturb or maintain the motion of the body.

4th. That with heterogeneous surfaces, such as those of woods and metals, gliding over each other, the intensity did not attain its limit sometimes for days.

In general, however, with woods and metals, without unguents, velocities were found to have very little influence in augmenting friction, except under peculiar circumstances.

The treatise of Coulumb is illustrated by a great variety of interesting experiments, and forms the most valuable work we possess on the subject.

In the year 1784, Dr. Vince endeavoured, by some very ingenious experiments, to determine the law of retardation, together with the quantity and the effect of surface on friction. The results were, that the friction of hard bodies in motion, was a uniformly retarding force, but not so with cloth and woollen, which were found in all cases to produce an increase of retardation with an increase of velocity.

That when the surface varied from 1.61: 1 to 10.06: 1 the smallest surface gave the least friction; and, finally, that friction was greatly influenced by cohesion.

Dr. Vince's conclusions regarding the laws of retardation,

were partly confirmed by the late ingenious Mr. Southern, of the Soho, who, in a letter to Dr. Vince, in 1801, communicated the results of several experiments on the surfaces of the spindles of grindstones, moving with great velocities; when it was found that with the rubbing surfaces moving at the rate of four feet per second over a length of surface of one thousand feet, the resistance arising from the friction of three thousand seven hundred pounds of matter only amounted to one-fortieth of the weight.

In the year 1786, and subsequently, the late Mr. Rennie made several experiments on the friction of heavy machinery. The results varied under different circumstances; but it appeared that an augmentation of resistance took place in proportion to the quantity of machinery put into action. In one instance, in the ratio of one to five, when it absorbed from one-fifth to one-tenth of the power expended.

This anomaly, as compared with the ratio of surfaces in the present experiments, can only be accounted for from the irregularity of the movements, and the difficulty of producing simultaneous actions, in complicated machinery; the more especially as the results were affected by contingencies which could not be properly estimated; some of the elements on which the deduction is founded not being stated. The resistance was, likewise, increased by reversing the direction of motion. The velocities being very moderate, and hardly exceeding one hundred and twenty feet a minute, appeared to have had no influence; but the experiments related principally to the resistances produced by different kinds of machinery. The experiments of M. Boistard* on the gliding of stones, with a view to develop the equilibrium of arches, led him to conclude that the relation of the friction to the pressure was constant; that asperity of surface did not alter its value, which generally amounted to four-fifths of the pressure.

*Recueil d' Experiences et d' Observations, &c. sur le Pont de Nemours.

From similar experiments M. Rondelet concluded*, 1st. That the rougher the surface of stones, the greater the power required to move them.

2nd. That the greater the insistent weight, the greater the resistance; but as the inequalities are apt to be broken, the maximum force required to overcome the friction, ought to be equal to produce that effect, whatever be the weight of the stone.

3d. That this force ought rather to be in the ratio of the hardness of the stone, than of its weight.

4th. The amount of friction varied from one-half to onethird of the insistent weight.

5th. The angle of equilibrium of similar stones was about thirty degrees. And,

6th. Finally, extent of surface did not alter its value.

The experiments of Morisot on the grinding and polishing of stones, and of Maniel and Pasley on the pressure and equilibrium of earths, present some interesting results; but it is only recently that our knowledge of the subject has been materially enlarged.

The agitation of the canal and rail-road question in the years 1824 and 1825, and the invention, or rather revival, of a mode of applying steam in lieu of animals to carriages on rail-roads, led to the most extravagant conclusions: and although the doctrines of Coulomb and Vince, relative to the equality of resistances under different velocities, have been still farther confirmed by the experiments of many able persons in this country, such as Chapman, Grimshaw, Wood, Tredgold, Palmer, Roberts, and others, and much valuable information elicited; our progress in the science has been but slow and unsatisfactory. Sensible of these defects, and being unable to profit by the valuable treatises subsequently published, it occurred to me that a series of experiments founded on the omissions of former writers, would be extremely desirable..

L'Art de Batir, tome III. 1808.

The present series of experiments relates to the friction of attrition. This branch of science comprehends the resistance occasioned by solid bodies, such as ice, cloth, paper, leather, wood, stones, metals, &c., gliding over each other simply, or by the intervention of semi-fluids or unguents, such as oil, tallow, &c.

The object has likewise been to determine the powers to resist abrasion under the circumstances of surface, pressure, and velocity. Examples have been sought,

1st. From ice, by the resistance of its surface to sledges, skates, &c.

2nd. From cloth, by its remarkable properties of resistance in opposition to the law observed by solids.

3rd. From leather, by its great utility in the pistons of pumps, &c.

4th. From wood, in its application to pile-driving, carpentry, launching of ships, &c.

5th.

From stones, as relating to the equilibrium of arches and buildings. And,

6th. From metals, from their universal application to machinery; but more particularly to wheel carriages and rail and other roads, on which a great many experiments have been made.

Experiments on a great scale, however, frequently involve so many contradictions, from the difficulty of obtaining the necessary elements, that I have deemed it preferable to offer the present series, as comprehending in a greater degree most of the cases in question, and affording a more systematic view of the nature of the investigation.

(To be continued.)

168

XL.-On Improved Melting-Pots, for Iron, Steel, Brass, &c. By Mr. CHARLES SYDNEY SMITH *.

IT appeared in evidence before the Committee of Chemistry, that Mr. Smith's pots were made while he was in the service of a manufacturer of metallic axle-trees†, who therefore had occasion for a considerable quantity of metal castings. The fusions were performed in earthen pots, each of which was required to stand an entire day's work without cracking, or becoming leaky, by the formation of small holes, called pin-holes.

The failure of a pot is a serious inconvenience, both on account of the loss of time, fuel and metal, as well as of the interruption which it creates. Great variations are observable in the duration of pots from different makers, and even in those by the same maker; arising not so much from difference in the materials employed, as from a difference of skill or care in mixing of the ingredients, and in the other parts of the manipulation. Whenever a bubble of air is left in the clay after being tempered, a pinhole in the pot made of such clay will be the common result, for the pressure of the melted metal will probably force a way through this weak part.

In order to submit the pots made by Mr. Smith to a very severe trial, one was kept constantly at work for two days and the intervening night; during which time it received twenty-three charges of seventy pounds each, of cast-iron. Another pot was worked for three successive days, the fire being raked at night, in order to prevent it from cooling ; under this management it received eighteen charges of castiron, of the same weight as the former.

Neither of the pots had cracked or leaked in the least, but were now become unserviceable, from the lip having

* From vol. XLVII. of the Transactions of the Society for the Encouragement of Arts, Manufactures, and Commerce, Adelphi. The Society presented the sum of Twenty Pounds to Mr. Smith.

+ Mr. William Mason, patent axle-tree maker, Margaret Street, Cavendish Square, whose superio r melting-pots, have been frequently mentioned in our work.-EDITOR.