The ingenious Mr. Emerson, in his Principles of Mechanics, has given the following rules deduced from experiments; but they require some variation under different circumstances, which must be left to the judgment of the artist.
1. Wood and all metals, when greased, have nearly the same friction; and the smoother they are, the less friction they have; yet metals may be so far polished as to increase friction by the cohesion of their parts.
Wood slides easier upon the ground in wet weather than in dry, and easier than iron in dry weather; but iron slides easier than wood, in wet weather. Lead makes a great deal of resistance. Iron or steel running in brass, makes the least friction of any. In wood acting against wood, grease makes the motion twice as easy, or rather ²⁄₃ds easier. Wheel-naves, greased or tarred, go 4 times easier than when wet.
Metals oiled make the friction less than when polished, and twice as little as when unpolished.
In general, the softer or rougher the bodies, the less or greater their friction.
2. As to particular cases: a cubic piece of soft wood of 8 pounds weight, moving upon a smooth plane of soft wood, at the rate of 3 feet per second; its friction is about ¹⁄₃d of the weight of it; but if it be rough, the friction is little less than one half the weight.
Upon the same supposition, other soft wood upon soft wood very smooth, the friction is bout ¹⁄₄th of the weight.
Soft wood upon hard, or hard wood upon soft, ¹⁄₅th or 1-half of the weight. Hard wood upon hard wood, ¹⁄₇th or ¹⁄₈th of the weight.
Polished steel moving upon steel or pewter, ¹⁄₄th of the weight; moving on copper or lead, ¹⁄₅th of the weight; on brass, ¹⁄₅th of the weight. Metals of the same sort have more friction than different sorts.
The friction, cæteris paribus, increases with the weight almost in the same proportion. The friction is also greater with a greater velocity, but not in proportion to it, except in very few cases. A greater surface also causes somewhat more friction, with the same weight and velocity; yet friction may sometimes be increased by having too little surface to move on; as upon clay, &c. where the body sinks.