the
tractive force is expended in overcoming the friction, which is made up
partly of the friction of attrition in the axles, and partly of the rolling
friction, or the obstruction to the rolling of the wheels upon the rail.
The rolling friction is very small when the surfaces are smooth, and in the
case of railway carriages does not exceed 1/1000th. of the load; whereas
the draught on common roads of good construction, which is chiefly made up
of the rolling friction, is as much as 1/36th of the load.

494._Q._--In reference to friction you have already stated that the
friction of iron sliding upon brass, which has been oiled and then wiped
dry, so that no film of oil is interposed, is about 1/11th of the pressure,
but that in machines in actual operation, where there is a film of oil
between the rubbing surfaces, the friction is only about one third of this
amount, or 1/33d of the weight. How then can the tractive resistance of
locomotives at low speeds, which you say is entirely made up of friction,
be so little as 1/500th. of the weight?

_A._--I did not state that the resistance to traction was 1/500th of the
weight upon an average--to which condition the answer given to a previous
question must be understood to apply--but I stated that the average
traction was about 1/300th of the load, which nearly agrees with my former
statement. If the total friction be 1/300th of the load, and the rolling
friction be 1/1000th of the load, then the friction of attrition must be
1/429th of the load; and if the diameter of the wheels be 36 in., and the
diameter of the axles be 3 in., which are common proportions, the friction
of attrition must be increased in the proportion of 36 to 3, or 12 times,
to represent the friction of the rubbing surface when moving with the
velocity of the carriage, 12/429ths are about 1/35th of the load, which
does not differ much from the proportion of 1/33d as previously determined.

495. _Q._--What is the amount of adhesion of the wheels upon the rails?

_A._--The adhesion of the wheels upon the rails is about 1/5th of the
weight when the rails are clean, or either perfectly wet or perfectly dry;
but when the rails are half wet or greasy, the adhesion is not more than
1/10th or 1/12th of the weight or pressure upon the wheels. The weight of a
locomotive of modern construction varies from 20 to 25 tons.

496. _Q._--And what is its cost and average performance?

_A._--The cost of a common narrow