FRICTION OF LVSRICATED SURFACES. 3'3j

With the more perfect lubrication attainable by the oil-batlu
the friction varies neariy as the square root of the speed, at^
velocities customarily met with in engineering. At very low
speeds, as shown by the experiments already quoted, the coef-
ficient decreases with increasing speed — presumably in conse-
quence of the greater freedom of supply so secured. At speeds J
exceeding loo to 150 feet (30 to 46 m.) per minute, the resist-fl
ance increases slowly with ordinary lubrication, and more rap-|
idly with more perfect oil-supply. The experiments of the 4
Institution of Mechanical Engineers give, for oil-bath lubric^ I
tion and flooded journals, approximately: I

Sperm-oil /oe \ ~p' I

Lard-oil /<x —p- ; ■

♦^ 1

Olive-oil ftx. —5-; I

W I

Mineral-oil / k —^, I

The apparent law thus varies with the character of the 1
lubricant, with variation of pressure, although usually giving!
values of friction varying as the square root of the velocity. I

The work of the Author, exhibited in Figs. 48, 49, illus-
trates the peculiar variation of friction with velocity of rubbing,
through a wide range of speeds, pressures, and temperatures.
These curves, which were constructed for the Author by the
late Mr. W, G. Cartwright, indicate the existence of a definite
law of variation of the coefficient, for each definite set of con-
ditions, taken as unvarying in other respects. At low speeds
the coefficient decreases, in ail cases, with great rapidity; passes
a minimum, usually at between 100 and 200 feet (30 and 61 m.)
per minute, and then gradually increases again up to the high- J
est speeds attained.
