52. _Q._--What is friction?

_A._--Friction is the resistance experienced when one body is rubbed upon
another body, and is supposed to be the result of the natural attraction
which bodies have for one another, and of the interlocking of the
impalpable asperities upon the surfaces of all bodies, however smooth.
There is, no doubt, some electrical action involved in its production, not
yet recognized, nor understood; and it is perhaps traceable to the
disturbance of the electrical equilibrium of the particles of the body
owing to the condensation or change of figure which all bodies must
experience when subjected to a strain. When motion in opposite directions
is given to smooth surfaces, the minute asperities of one surface must
mount upon those of the other, and both will be abraded and worn away, in
which act power must be expended. The friction of smooth rubbing substances
is less when the composition of those substances is different, than when it
is the same, the particles being supposed to interlock less when the
opposite prominences or asperities are not coincident.

53. _Q._--Does friction increase with the extent of rubbing surface?

_A._--No; the friction, so long as there is no violent heating or abrasion,
is simply in the proportion of the pressure keeping the surfaces together,
or nearly so. It is, therefore, an obvious advantage to have the bearing
surfaces of steam engines as large as possible, as there is no increase of
friction by extending the surface, while there is a great increase in the
durability. When the bearings of an engine are made too small, they very
soon wear out.

54. _Q._--Does friction increase in the same ratio as velocity?