Page:Popular Science Monthly Volume 44.djvu/765

Rh all that time, I never saw this feat. The reason was, that I lived in a perfectly flat country. I saw it for the first time when, at the age of fifteen, in going to college, I moved to a rolling country. It is best seen in a bare rolling country, like much of the western portion of the United States. The most perfect poising I have ever seen done was by the red-tailed hawk (Buteo montanus), on the bare, rolling lava plains of eastern Oregon. The conditions absolutely necessary are a rolling country and a steady breeze. The bird places himself above the brow of a hill with face to the wind. As long as the wind remains steady the bird retains his position, with outstretched wing, motionless.

The explanation is as follows: As already said, the bird places himself facing the wind just above the brow of a hill. The wind is deflected upward by the slope of the hill. The bird places his aëroplane (wings and tail) in a plane inclining slightly downward, but not so much inclined as the slope of the hill, so that the wind still strikes the under side of the aëroplane. In this position the force of gravity would carry him downward and forward, while the wind would carry him upward and backward. The bird skillfully adjusts the position of the aëroplane so that these two opposite forces shall exactly balance one another. As long as the wind remains steady his position is unchanged. If the wind changes in direction or in velocity, he wiggles himself a little, perhaps flaps once or twice, until he finds a new position of equilibrium, and again remains steady. This explanation is, I believe, complete.

. It is well known that many large and long winged birds, such as vultures, hawks, pelicans, etc., will sweep about in wide circles with motionless, outstretched wings, not only maintaining their level, but rising in ascending spiral until they disappear from view. I have often watched their easy, graceful motion for hours, and am quite sure that it is accomplished without any expenditure of energy at all commensurate with the work of elevation. How is it done? There is no problem of bird-flight upon which so much has been written, and so little of any value. Let us see first what are the necessary conditions.

1. Every careful observer must have noted that the bird slopes downward along one half of the circle, so as to acquire high velocity, and then rises along the other half to a higher level than that from which he descended. How can he rise higher?

2. Every clear thinker must see that this feat is impossible, and every careful observer must have noted that it is never done—in still air. For if air is still, even if there were no friction and no tendency to fall toward the ground, the most that the velocity acquired by the down slope could do would be to carry the bird back to the same level again. Therefore, in still air the bird must