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, whereby abrasion would be
produced, and unguents are effectual in this respect in the proportion of
their viscidity; but if the viscidity of the unguent be greater than what
suffices to keep the surfaces asunder, an additional resistance will be
occasioned; and the nature of the unguent selected should always have
reference, therefore, to the size of the rubbing surfaces, or to the
pressure per square inch upon them. With oil the friction appears to be a
minimum when the pressure on the surface of a bearing is about 90 lbs. per
square inch. The friction from too small a surface increases twice as
rapidly as the friction from too large a surface, added to which, the