hen the train was just moving. It also decreased with the
time during which the brakes were applied; thus at 20 m. an hour the
co-efficient was at the beginning 0.182, after ten seconds 0.133,
after twenty seconds 0.099. Generally speaking, especially at moderate
speeds, the decrease in the co-efficient of friction due to time is
less than the increase due to decrease of speed, although when the
time is long the reverse may be true. When the wheels are skidded the
retardation of the train is always reduced; therefore, for the
greatest braking effect, the pressures on the brake-shoes should never
be sufficient to cause the wheels to slide on the rails. The
Burlington brake tests were undertaken to determine the practicability
of using power brakes on long and heavy freight trains. In the 1886
tests there were five competitors--three buffer-brakes, one
compressed-air brake, and one vacuum-brake. The tests comprised stops
with trains of twenty-five and fifty vehicles, at 20 and 40 m. an
hour, on the level and on gradients of 1 in 100. They demonstrated
that the buffer-brakes were inadequate for long trains, and that
considerable improvements in the continuous brakes, both
compressed-air and vacuum, would be needed to make them act quickly
enough to avoid excessive shocks in the rear vehicles. In 1887 the
trials of the year before were repeated by the same committee, and at
the same place. Trains of fifty vehicles, about 2000 ft. long and
fitted with each brake, were again provided, and there were again five
competitors, but they all entered continuous brakes--three
compressed-air brakes, one vacuum and one electric. The results of the
first day's test of the train equipped with Westinghouse brakes are
shown in Table I., the distances in which are the feet run by the
train after the brakes were set, and the times the seconds that
elapsed from the application of the brakes to full stop.

[Illustration: FIG. 3--Rapid-acting Vacuum-Brake Valve.]

TABLE I.--_Stops of a Train of Fifty Empty Cars, 1887--Automatic
Air-Brakes._

+-----------+----------+----------+---------------------+
| Speed in | Distance | Time in | Equivalent Distance |
| Miles per | in Feet. | Seconds. | at 20 m. and 40 m. |
| Hour. | | | |
+-----------+----------+----------+----------+----------+
| 19-1/2 | 186 | 9-