NEW LIGHTS ON THE PROBLEM OF FLYING. 747 



ing 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 (Buieo 

 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 

 aeroplane (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 aeroplane. In this position the 

 force of gravity would carry him downward and forward, while 

 the wind would carry him upward and backward. The bird skill- 

 fully adjusts the position of the aeroplane so that these two oppo- 

 site 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, com- 

 plete. 



SOARING. 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 main- 

 taining their level, but rising in ascending spiral until they disap- 

 pear 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 ele- 

 vation. 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 ve- 

 locity, 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 



