It is well known that one of the most common expedients for reducing the friction between two rubbing surfaces is to interpose some lubricant, which seems to form a coating to the projecting points, making the whole surface more continuous, and thus lessening the resistance. At, very low pressures and velocities the viscosity of the lubricant occasionally causes the resistance to be increased instead of lessened, but in general the effect of an unguent is to decrease the friction in quite a large ratio. Careful experiments have been made with regard to the friction between two surfaces when they were perfectly dry and clean, and when different lubricants were used. In this manner it has been found that good oil, such as olive-oil, is one of the best lubricants; that lard is better than tallow, and that the use of water, instead of lessening the friction, generally increases it. Experiments upon the manner of applying the lubricant show that there is a great advantage in a continual application, so as to keep a film constantly interposed between the rubbing surfaces, over the case in which the surfaces are merely kept slightly greasy. Below are given mean values for the coefficient of friction, in cases arising from the sliding of one plane surface upon another, the surfaces being supposed to be true, and, in common language, smooth:

In the case of journals, the coefficient of friction is generally much less than for plane surfaces. Mean values of this coetlicient, both for wood and metals, vary from 0.15, when the journal is only slightly unctuous, to 0.05, when there is a continual sup ply of the lubricant. In regard to journal-friction, the amount is independent of the diameter of the journal, but the work required to overcome friction will of course be greater with a large than with a small journal, because the distance passed through by the periphery of the journal in a given time will be greater in the former case. B.