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.