FRICTION. All surfaces have certain de grees of roughness arising from the innume rable small asperities with which they are covered ; and from these, when two surfaces move upon one another, there arises a force acting in a direction opposite to that in which the surfaces move. This is friction, which is therefore a retarding force capable of destroy ing but incapable of generating motion, and capable of acting powerfully as a mechanical force, of which the tendency is to bestow sta bility.
The following are a few of the laws which guiern friction, useful in all considerations respecting moving machinery:—], Friction is increased by time; thus it requires the appli cation of a greater force to move a weight along a horizontal plane from its position of rest than to keep it afterwards moving on the same plane. 2, Between substances of the same nature the friction is proportional to the pressure ; thus, if a block of oak be double the weight of another, and both, having equal surfaces of contact, are placed on one plane of uniform nature, the force necessary to move the first will be double of that requisite for the second. 3, The amount of friction is in. dependent for one and the same body of the extent of the surface of contact. 4, When the fibres of two substances are parallel, fric tion has a tendency to increase ; thus, when a rectangular block of oak is placed on an oak table so that the fibres in both lie parallel, the friction is gre'ater than in the case where the fibres of the block lie transversely to those of the table. 5, The friction is independent of
the velocity; at least when the velocity is neither very small nor very great.
Mr. Rennie made many valuable experi ments on friction a few years ago, with various substances drawn on a horizontal plane. He found that metals moving upon each other produced friction in the following order :— Brass on wrought iron.
Steel upon steel.
Brass upon cast iron.
Brass upon steel.
Hard brass upon cast iron.
Wrought iron upon wrought iron.
Cast iron upon oast iron.
Cast iron upon steel.
Cast iron upon wrought iron.
Brass upon brass.
Tin upon tin.
Amongst the conclusions which Mr. Rennie draws, the following are perhaps the most important. With fibrous substances, such as cloth, Sm., friction is increased by surface and time, and diminished by pressure and velocity. With harder substances, such as woods, me tals and stones, the amount of friction is sim ply as the pressure, without regard to surface, time, or velocity. Friction is greatest with soft, and least with hard substances. The di minution of friction by unguents depends on the nature of the unguents, without reference to the substances moving over them.
M. Coulomb and M. Morin have made valu able experiments on the friction produced by one kind of wood moving over another.