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Page:Popular Science Monthly Volume 57.djvu/484

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474
POPULAR SCIENCE MONTHLY.

shock of a headlong plunge into the water from a height of a hundred to a hundred and fifty feet. Or, again, they equalize the internal and external pressure when a soaring bird drops suddenly from a great height, or still more often aid in oxygenating the blood.

The hollow bones of birds are frequently cited as beautiful instances of providential mechanics in building the strongest and largest possible limb with the least expenditure of material, and this is largely true. And yet birds like ducks, which cleave the air with the speed of an express train, have the long bones filled with marrow or saturated with fat, while the lumbering hornbill that fairly hurtles over the tree tops has one of the most completely pneumatic skeletons imaginable, permeated with air to the very toe tips; and the ungainly pelican is nearly as well off. Still it is but fair to say that the frigate bird and turkey buzzards, creatures which are most at ease when on the wing, have extremely light and hollow bones, but comparing one bird with another the paramount importance of a pneumatic skeleton to a bird is not as evident as that of a pneumatic tire to a bicycle.

While it may not be easy to disprove Herr G├Ątke's assertion that birds sustain themselves in the air by the exercise of some power beyond our own, it is pretty safe to assume that they do not, and it would seem that the burden of proof should lie with those who take the affirmative side of the question.

If we have nothing to learn from birds in the way of building an engine that shall exert great power for its size and weight we may still have something to gain in the matter of speed, although here the popular idea is apt to be exaggerated. We often read that ducks fly at the rate of a mile a minute, or that the swallow has a speed of two hundred miles an hour, but it is very difficult to lay hands upon any facts that will sustain these assertions.

So, too, homing pigeons are frequently stated to have travelled for long distances at the rate of sixty miles an hour, but some of the published records show that one hundred and twenty miles in two hours and a quarter is unusually fast traveling, and this is at the rate of only nine-tenths of a mile per minute, a speed not unusual for express trains. However, it may be said that actual observations show that ducks do travel from forty to fifty miles an hour, and any sportsman will readily believe that under some conditions they attain a velocity of a mile and a quarter a minute, although a confession of faith is not a demonstration of an assured fact.

So far the lesson taught by the bird is that a machine of low power may attain a very considerable speed and it remains to be seen if there is anything to be learned concerning methods of flight. Broadly speaking, there are two, possibly three, distinct modes of flight, by repeated strokes of the wings and by soaring or sailing, although we find