are small, the adhesion appears to be very great compared with the friction, but if the pressures are great it becomes but a very small portion of the friction, and can generally be neglected. Unguents, generally, increase the adhesion, since they produce a greater number of points of contact." The following illustrations are valuable as suggestions to aid in understanding the case under consideration: 1st. The careful pushing of one surface plate over another, and the lifting of the lower one by the upper, is, certainly, a proof of atmospheric aid in the case. 2d. When a carpenter uses an iron plane which is both level and smooth, and finds that his labors are largely increased by what he calls suction on the plane, that is good evidence of easy exclusion of air, and consequent atmospheric pressure. 3d. In the case of the boy's leather sucker on the stone, air is excluded. To compare exactly the action of a belt with the boy's sucker, it would be necessary that the belt should be so saturated with oil, and be made so very pliable, that it would in no sense act as a suitable belt. 4th. When the house-fly puts his feet upon a pane of glass, and the enclosing claws find no surface into which they can interlock, then from the continued enclosing of the claws there exudes from two lobes, under the claws on each foot, an adhesive substance, in globules, which adheres to the glass, shutting out air at the points of contact; then the fly has atmospheric aid in addition to adhesion (a reverse movement of the claws throws the lobes free). 5th. By putting a liquid into a vessel, air is excluded; putting a liquid upon a flat surface excludes air. Putting any substance upon another excludes air at the points of contact. Intimate contact between two surfaces acts to exclude air. “Two substances cannot occupy the same space at the same time." It may be said that the best working of leather belts, with respect to tractive force, has been carried out in many cases, but an explicit, clear statement has apparently never been made as to what should be sought for as the best conditions for obtaining the greatest tractive force of a belt. The experience of all users of belting has taught them that they get the best results-particularly when calling for the full power of belts-from the use of belts with smooth surfaces and even textures, working on smooth pulley faces; hence, the idea of "friction," as an aid in belt traction, must be largely excluded. It is very generally known and appre ciated that there is a material benefit from the presence on the surface of belts and pulley faces of a suitable "unguent” in very limited quantity, as explained in my paper; which I claim fills the pores of each so that when the two surfaces come together there is a practical union, and a shutting out of air, with consequent atmospheric aid. The practical effect-value-of a belief in atınospheric aid is to induce the running of belts very or comparatively slack, thus avoiding unnecessary stress on bearings, and maintaining the integ rity of belts. My observation of the every-day use of belting, particularly during the last ten years, is in full confirmation of this, and, per contra, I have seen that a total disregard of this belief has resulted in the destruction (throwing out) of belts in a few weeks or a few months, when they might have served well on towards the full life of the best-made belts, which, as I have stated, is from thirty to forty years. Atmospheric aid is obtained in the easiest manner as follows (read in connection with paragraph commencing "The most intimate contact" etc.): 1st. By the use of the best-made belting, as explained. 2d. By the use of belts made without rivets-rivets, generally speaking, are only a make-shift to stand as aids to poor, incomplete work-poor cement-at the laps; or to invite the abuse of belts in use-as, for instance, in the over-tightening of belts when slipping occurs, owing to the presence of rivets, on pulleys of very small diameter. 3d. By the use of very low crowns on pulleys, as explained. 4th. By keeping belts clean and free from oil. 5th. By the use of a suitable "unguent," as explained. 6th. By running the hair-grain-smooth side to pulley face, which will eventually (not at first) best exclude air. To meet Professor Denton's request, I give an account of an experiment, so called. I have had in use a 4" single belt driving off from a 10" pulley into a 10" pulley, at an angle of 80° from the horizontal, with the pull on the top side. It was, for "experimen purposes, purposely put on very loose; it measures 3" more, in length, than the distance around the pulleys, which have flat faces. One use to which this belt is put is to elevate 500 lbs. 50' high in one minute. Speed of driving pulley 150 R. P. M. Distance from centre to centre of shafts, 5'. The belt does this work in a perfectly satisfactory manner; it has not been shortened or re laced since putting on, two years since. The faces of both belt and pulley are kept in such clean condition as to ensure intimate contact; and, as nothing of an adhesive, sticky nature is used, the belt leaves the pulley faces with perfect freedom. In a recent trial the pulley faces were thoroughly cleaned, and then carefully washed with naphtha; the belt was thoroughly scraped, and rubbed with clean cotton waste; nothing further was done to either. The condition of pulley faces was that the pores were filled; the condition of belt was that it was free from oil, but had its pores filled with a suitable" unguent," sufficient to exclude air on contact with pulley faces. While the slackness of this belt is excessive, in view of its being a nearly vertical belt, yet even in a succession of damp days it gives no real trouble. Adhesion, as explained, and atmospheric pressure, cannot be dissociated one from the other; the two work harmoniously together. Thus, I maintain that the chief thing to be sought for in belt traction is aid from atmospheric pressure, as indicated. NOTE-As these remarks are intended to have a wholly practical bearing, I would say that some exigencies in the use of belts involve their rapid destruction; hence a "cheap belt" may result in giving as great (or greater) economy as an expensive one; also there are some conditions (dampness-steam-oil) in the use of belts which involve the necessity for their being riveted. A suitable crown is usually necessary on pulleys, particularly where the full power of belts is used. CCCLII. AN ERROR IN THE ENCYCLOPEDIA BRITANNICA. BY J. BURKITT WEBB, HOBOKEN, N. J. (Member of the Society.) In the article on Hydromechanics by Professors A. G. Greenhill and W. C. Unwin, section 150 (page 514), on the "Reaction of a Jet Issuing from a Vessel," a statement is made which would, if true, lead to most remarkable results, enabling us, indeed, to deter Fig. 153. mine the absolute direction and velocity of the earth's motion in space by a simple mechanical method. The first part of Section 150 runs as follows: "Suppose a vessel filled with water, having an orifice of area , from which water issues horizontally with a velocity v2gh: The volume discharged per second, neglecting contraction, ww. -The momen G tum generated per second in a horizontal direction = wv2; and this is equal to the force producing the change of momentum. Hence the horizontal force or reaction R, acting on the side of the vessel opposite to the orifice, and equal and opposite to the force producing the momentum, is— G this is the weight of a column of water the section of which is the area of the orifice, and the height is twice the head. If the vessel moves in a direction opposite to that of the jet with the velocity u, the absolute velocity of the water leaving the vessel is v и. The momentum generated per second is an (v — u) = R.” What is said with reference to the stationary vessel is mainly correct, but the last paragraph contains remarkable and contradictory assumptions, for while the velocity, u, of the whole apparatus is not supposed to alter the issuing velocity, v, it is supposed to change the reaction, R, of the issuing water. Or, to put the contradiction in another way, it is held that while in a stationary vessel R depends on alone, in a moving one it depends on vu. Now a vessel is stationary with respect to all objects moving at the same speed in the same direction as it moves, and it is moving with respect to all others, so that stationary and moving are terms referring to the relation between the vessel and exterior objects, while the reaction is an interior relation between the vessel and the issuing water, and therefore cannot be affected by u. The absurdity of the thing may be made clearer by considering that, were the formula given for R in the last paragraph correct, we could, by experimenting upon the reaction of a jet which could be directed toward all points in space, find directions of maximum and minimum reaction, the former of which must be the direction of the absolute motion of the vessel in space, for in that case we should have and with the jet turned opposite to the motion in space, G By measuring v also we should be able to arrive at the value of u. Having experimented upon the jet in a sufficient number of places, we could deduce therefrom the absolute motion of the earth in space, which is an absurdity in itself; at least we can form no definite conception of such a motion, nor of any fixed point in space to reckon such a motion from. APPENDIX.* A description of the apparatus required and the method of using it may make clearer the exact nature of the experiment proposed. In Fig. 202 a is a vertical pipe supporting the apparatus and * Added since the meeting. |