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

_A._--That will depend upon the velocity. When the pressure exceeds 800
lbs. per square inch, however, upon the section of the bearing in a
direction parallel with the axis, then the oil will be forced out and the
bearing will necessarily heat.

60. _Q._--But, with, a given velocity, can you tell the limit of pressure
which will be safe in practice; or with a given pressure, can you tell the
limit of velocity?

_A._--Yes; that may be done by the following empirical rule, which has been
derived from observations made upon bearings of different sizes and moving
with different velocities. Divide the number 70,000 by the velocity of the
surface of the bearing in feet per minute. The quotient will be the number
of pounds per square inch of section in the line of the axis that may be
put upon the bearing. Or, if we divide 70,000 by the number of pounds per
square inch of section, then the quotient will be the velocity in feet per
minute at which the circumference of the bearing may work.

61. _Q._--The number of square inches upon which the pressure is reckoned,
is not the circumference of the bearing multiplied by its length, but the
diameter of the bearing multiplied by its length?

_A._--Precisely so, it will be the diameter multiplied by the length of the
bearing.

62. _Q._--What is the amount of friction in the case of surfaces sliding
upon one another in sandy or muddy water--such surfaces, for example, as