It may be noted that the velocity of 60 ft. per sec., assumed at the
foot of the grade, is probably higher than should be expected in
practice; it insures, on the other hand, that quite enough has been
allowed for momentum, and that the results are conservative.
Arguments like the foregoing are always more or less treacherous; being
based on statistics, they are naturally subject to material
modifications in the presence of a larger array of data, therefore,
material assistance in reaching practical conclusions can be given by
the presentation of additional data.
DISCUSSION
C. D. PURDON, M. AM. SOC. C. E. (by letter).--Some years ago the writer,
in making studies for grade revision, found that the tractive power of a
locomotive up grade becomes less as the length of the grade increases,
and in some unknown proportion. This was a practical confirmation of the
saying of locomotive engineers, that the engine "got tired" on long
grades. On a well-known Western railroad, with which the writer is
familiar, experiments were made for the purpose of rating its
locomotives. The locomotives were first divided into classes according
to their tractive power, this being calculated by the usual rule, with
factors of size of cylinders, boiler pressure, and diameter of drivers,
also by taking one-fourth of the weight on the drivers, and using the
lesser of the two results as the tractive power.
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