A.—­It varies with the nature of the rubbing bodies.  The friction of iron sliding upon iron, has generally been taken at about one tenth of the pressure, when the surfaces are oiled and then wiped again, so that no film of oil is interposed.  The friction of iron rubbing upon brass has generally been taken at about one eleventh of the pressure under the same circumstances; but in machines in actual operation, where a film of some lubricating material is interposed between the rubbing surfaces, it is not more than one third of this amount or 1/33d of the weight.  While this, however, is the average result, the friction is a good deal less in some cases.  Mr. Southern, in some experiments upon the friction of the axle of a grindstone—­an account of which may be found in the 65th volume of the Philosophical Transactions—­found the friction to amount to less than 1/40th of the weight; and Mr. Wood, in some experiments upon the friction of locomotive axles, found that by ample lubrication the friction may be made as little as 1/60th of the weight.  In some experiments upon the friction of shafts by Mr. G. Rennie, he found that with a pressure of from 1 to 5 cwt. the friction did not exceed 1/39th of the pressure when tallow was the unguent employed; with soft soap it became 1/34th.  The fact appears to be that the amount of the resistance denominated friction depends, in a great measure, upon the nature of the unguent employed, and in certain cases the viscidity of the unguent may occasion a greater retardation than the resistance caused by the attrition.  In watchwork therefore, and other fine mechanism, it is necessary both to keep the bearing surfaces small, and to employ a thin and limpid oil for the purpose of lubrication, for the resistance caused by the viscidity of the unguent increases with the amount of surface, and the amount of surface is relatively greater in the smaller class of works.

56. Q.—­Is a very thin unguent preferable also for the larger class of bearings?

A.—­The nature of the unguent, proper for different bearings, appears to depend in a great measure upon the amount of the pressure to which the bearings are subjected,—­the hardest unguents being best where the pressure is greatest.  The function of lubricating substances is to prevent the rubbing surfaces from coming into contact, whereby abrasion would be produced, and unguents are effectual in this respect in the proportion of their viscidity; but if the viscidity of the unguent be greater than what suffices to keep the surfaces asunder, an additional resistance will be occasioned; and the nature of the unguent selected should always have reference, therefore, to the size of the rubbing surfaces, or to the pressure per square inch upon them.  With oil the friction appears to be a minimum when the pressure on the surface of a bearing is about 90 lbs. per square inch.  The friction from too small a surface increases twice as rapidly as the friction from too large a surface, added to which, the bearing, when the surface is too small, wears rapidly away.