_A._--No; friction does not increase with the velocity at all, if the
friction over a given amount of surface be considered; but it increases as
the velocity, if the comparison be made with the time during which the
friction acts. Thus the friction of each stroke of a piston is the same,
whether it makes 20 strokes in the minute, or 40: in the latter case,
however, there are twice the number of strokes made, so that, though the
friction per stroke is the same, the friction per minute is doubled. The
friction, therefore, of any machine per hour varies as the velocity, though
the friction per revolution remains, at all ordinary velocities, the same.
Of excessive velocities we have not sufficient experience to enable us to
state with confidence whether the same law continues to operate among them.

55. _Q._--Can you give any approximate statement of the force expended in
overcoming friction?

_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