perate 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
friction of shafts by Mr. G. Rennie, he found that with a pressure of from
1 to 5 cwt. the friction did not exceed 1/39th of the pressure when tallow
was the unguent employed; with soft soap it became 1/34th. The fact appears
to be that the amount of the resistance denominated friction depends, in a
great measure, upon the nature of the unguent employed, and in certain
cases the viscidity of the unguent may occasion a greater retardation than
the resistance caused by the attrition. In watchwork therefore, and other
fine mechanism, it is necessary both to keep the bearing surfaces small,
and to employ a thin and limpid oil for the purpose of lubrication, for the
resistance caused by the viscidity of the unguent increases with the amount
of surface, and the amount of surface is relatively greater in the smaller
class of works.

56. _Q._--Is a very thin unguent preferable also for the larger class of
bearings?

_A._--The nature of the unguent, proper for different bearings, appears to
depend in a great measure upon the amount of the pressure to which the
bearings are subjected,--the hardest unguents being best where the pressure
is greatest. The function of lubricating substances is to prevent the
rubbing surfaces from coming into contact,