Page:On the economy of machinery and manufactures - Babbage - 1846.djvu/68

34 (39.) An effect not very different from this might take place if very great velocity were communicated to boats. Let us suppose a flat-bottomed boat, whose bow forms an inclined plane with the bottom, at rest in still-water. If we imagine some very great force suddenly to propel this boat, the inclination of the plane at the fore-part would cause it to rise in the water; and if the force were excessive, it might even rise out of the water, and advance, by a series of leaps, like a piece of slate or an oyster shell, thrown as a "duck and drake."

If the force were not sufficient to pull the boat out of the water, but were just enough to bring its bottom to the surface, it would be carried along with a kind of gliding motion with great rapidity; for at every point of its course it would require a certain time before it could sink to its usual draft of water; but before that time had elapsed, it would have advanced to another point, and consequently have been raised by the reaction of the water on the inclined plane at its fore-part.

(40.) The same fact, that bodies moving with great velocity have not time to exert the full effect of their weight, seems to explain a circumstance which appears to be very unaccountable. It sometimes happens that when foot-passengers are knocked down by carriages, the wheels pass over them with scarcely any injury, though, if the weight of the carriage had rested on their body, even for a few seconds, it would have crushed them to death. If the view above taken is correct, the injury in such circumstances will chiefly happen to that part of the body which is struck by the advancing wheel.