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.

Locomotives of different classes, and hauling known loads, were run over
a freight division, the cars being weighed for the purpose; thus the
maximum load which could be handled over a division, or different parts
of a division, was ascertained, and this proportion of tonnage to
tractive power was used in rating all classes.