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 pulley; this is equivalent to thirty-three thousand pounds raised one foot in one minute, which is said to be one horse-power.
No horse, of course, could raise thirty-three thousand pounds a foot or any portion of a foot in a minute or an hour, but the horse can travel at the rate of two and one-half miles an hour raising a weight of one hundred and fifty pounds, and the horse can do more; while it cannot move so heavy a weight as thirty-three thousand pounds, it can in an emergency and by sudden strain move much more than one hundred and fifty pounds; with good foothold it can pull more than its own weight along a road, out of a hole, or up a hill.

It could not lift or pull so great a weight very far; in fact, no farther than the equivalent of approximately thirty-three thousand pounds raised one foot in one minute; but for the few seconds necessary a very great amount of energy is at the command of the driver of the horse.

Hence eight horses, or even four, or two can do things on the road that an eight horse-power gasoline machine cannot do; for the gasoline machine cannot concentrate all its power into the exertion of a few moments.

If it is capable of lifting a given load up a given grade at a certain speed on its lowest gear, it cannot lift twice the load up the same grade, or the same load up a steeper grade in double the time, for its resources are exhausted when the limit of the power developed through the lowest gear is reached.

The grade may be only a mud hole, out of which the rear wheels have to rise only two feet to be free, but it is as fatal to progress as a hill a mile long.
Of course it is always possible to race the engine, throw in the clutch, and gain some power from the momentum of the fly-wheel, and many a bad place may be surmounted step by step in this way; but this process has its limitations also, and the fact remains that with a gasoline machine it is possible to carry a given load only so fast, but if the machine moves it all, it will continue to move on until the load is increased, or the road changes for the worse.
When the farmer hears of an eight horse-power machine he thinks of the wonderful things eight good horses can do on the road, and is surprised when the machine fails to go up hills that teams travel every day; he does not understand it, and wonders where the power comes in.