through heavy roads or up grades tell on them,--the former has
trouble in keeping up steam, the latter rapidly consumes its store
of electricity. The gasoline machine does not tire. Within its
limitations it can keep going indefinitely, and it is immaterial
whether it is up or down grade--save in the time made; it will go
all day through deep mud, or up steep hills, quite as smoothly,
though by no means so fast, as on the level; but let it come to
one hole, spot, or hill that is just beyond the limit of its
power, and it is stuck; it has no reserve force to draw upon. The
steam machine can stop a moment, accumulate two or three hundred
pounds of steam, open the throttle and, for a few moments, exert
twice its normal energy to get out of the difficulty.

It is not a series of hills that deters the gasoline operator, but
the one hill, the one grade, the one bad place, which is just
beyond the power he has available. The road the farmer calls good
may have that one bad place or hill in it, and must therefore be
avoided. The road that is pronounced bad may be, every foot of it,
well within the power of the machine, and is therefore the road to
take.

In actual road work the term "horse-power" is very misleading.

When steam-engines in early days began to take the place of
horses, they were rated as so many horse-power according to the
number of horses they displaced. It then became important to find
out what was the power of the horse. Observing the strong dray
horses used by the London breweries, Watt found that a horse could
go two and one-half miles per hour and at the same time raise a
weight of one hundred and fifty pounds suspended by a rope over a