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
bearing, when the surface is too small, wears rapidly away.

57. _Q._--Has not M. Morin, in France, made some very complete experiments
to determine the friction of surfaces of different kinds sliding upon one
another?

_A._--He has; but the result does not differ materially from what is stated
above, though, upon the whole, M. Morin, found the resistance due to
friction to be somewhat greater than it has been found to be by various
other engineers. When the surfaces were merely wiped with a greasy cloth,
but had no film of lubricating material interposed, the friction of brass
upon cast iron he found to be .107, or about 1/10th of the load, which was
also the friction of cast iron upon oak. But when a film of lubricating
material was interposed, he found that the friction was the same whether
the surfaces were wood on metal, wood on wood, metal on wood, or metal on
metal; and the amount of the friction in such case depended chiefly on the
nature of the unguent. With a mixture of hog's lard and olive oil
interposed between the surfaces, the friction was usually from 1/12th to
1/14th of the load, but in some cases it was only 1/20th of the load.

58. _Q._--May water be made to serve for purposes of lubrication?

_A._--Yes, water will answer very well if the surface be very large
relatively with the pressure; and in screw vessels where the propeller
shaft passes through a long pipe at the stern, the stuffing box is
purposely made a little leaky. The small leakage of water into the vessel
which is thus occasioned, keeps the screw shaft in this situation always
wet, and this is all the lubrication which this bearing requires or
obtains.

59. _Q._--What is the utmost pressure which may be employed without heating
when oil is the lubricating material?

_A._--