From the day of Solomon onward the way of a bird in the air has been a subject of general interest, and the attention given to the problem of aerial navigation of late years has caused the flight of birds to be carefully studied in the hope that it might throw some light on the subject. There have been many conceptions, not to say misconceptions, regarding the flight of birds; it has been assumed that their muscles exerted a power quite beyond that of other animals, that the air sacs of some birds and the hollow bones of others gave them a degree of lightness quite unattainable by the use of ordinary materials, while some have even gone so far as to suggest the presence of some mysterious power, something like Stockton’s negative gravity, whereby birds could set at naught the law of gravitation and rise at will like a balloon. The strength of a bird’s muscles, of some birds’ muscles at least, is not to be underrated; a hawk will plant its talons in a bird of nearly its own size and weight and bear the victim bodily away, and an osprey will carry a fish for a long distance. But a tiger has been known to fell a bullock with a single blow of the paw, to carry a man as a cat would carry a rat and to drag an Indian buffalo heavier than himself. On the other hand, some of the petrels, birds which can pass a day or so on the wing with ease, cannot rise from the water after a hearty meal, and the humming-bird, unsurpassed in aerial evolutions, may be trapped in a spider’s web. This shows no great power, and long ago Marey found that the pulling force of a hawk’s great breast muscle, applied through the humerus, amounted to 1,298 grams per square centimeter, something like seven pounds to the square inch; not a very heavy pull. So it seems fair to assume that while the power exerted by a bird is great, it is very far from marvelous, probably far less in proportion to size than the engine of Maxim’s great aeroplane, or the naphtha motor of Professor Langley, which weighs less than ten pounds per horse power. We may get a fair idea of what this means by remembering that a bald eagle weighs from nine to fifteen pounds and that he exerts but a small fraction of a horse power. Turning to the question of the part played by the air sacs it may be said that their value is not proved; some of the fastest birds get along without them, while birds of the most labored flight are sometimes well provided. In birds like the gannet and brown pelican the air sacs and cellular tissue about the body undoubtedly serve as buffers to break the 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.