ON SOARING FLI(;HT. 197 



earth before attempting to rise, and yet witli not more than thirty 

 seconds of time remaining he will select a spot and perhaps without a 

 single beat of the wings once more begin rising. Furthermore, his 

 object when near the earth is apparently to search for food, not to find 

 a spot from which he may risej and if at any time he detects a carcass, 

 or fancies that one may be concealed beneath him, he will at that spot 

 turn upon a curve and begin an investigation, circling round and round 

 and meanwhile maintaining his elevation, so that the same maneuver 

 which enables him to investigate the locality also enables him to remain 

 in the air. When we consider that he is under the necessity of finding 

 a new locality from which to rise on an average of perhaps once in 

 every ten minutes during several successive hours, and that when he is 

 near the earth a suitable spot must be found quickly or he must resort 

 to flapping, and further, that he rarely finds it necessary to flap, it 

 seems clear that he must in some way be able to rise almost when and 

 where he will. How this is accomplished is easily explained upon the 

 hypothesis of rising currents artificially produced when the air is in a 

 state of unstable equilibrium, and as already stated it seems impossible 

 to find a satisfactory explanation on any other hypothesis hitherto 

 advanced. 



TENDENCY TO DRIFT INTO ASCENDING CURRENTS. 



Early in the morning the fine sands of the Modjave Desert rise in 

 slender columns to a height of 1,000 feet, being fed doubtless by the 

 stratum of heated air near the surface, and we have here almost posi- 

 tive proof of unstable equilibrium long continued, the warm air flowing 

 underneath the cold air above in order to reach an ascending column, 

 for if the air near the surface were not warmer than neutral equilibrium 

 demanded it would not rise at all, and as it appears to rise only in 

 widely separated localities, as indicated by the columns of sand, at 

 other points the rate of decrease in temperature must be greater than 

 that which gives rise to neutral equilibrium; and yet the air does not 

 rise at those points. It is probable that similar currents of warm air 

 are constantly being formed in the warmer sections of the country, and 

 whenever an ascending current exists the surface winds in the vicinity 

 will be found blowing toward it. This aids to a fuller comprehension of 

 a difficulty which may seem to attach to the previous explanation — the 

 difficulty, that is, of explaining how the bird finds the ascending cur- 

 rents. Let us suppose that on a warm day, when light, local, irregular 

 winds are succeeded by intervals of calm, on an extended plain, a 

 number of vultures or a pair of hawks be frightened into the air. They 

 will soon be found soaring near the spot from which they took wing 

 and often flapj)ing vigorously. If there be a wind they will drift with 

 it as they soar, and as all local winds in general blow toward a rising 

 column of air the birds in soaring will drift toward the rising column, 

 and if it be not too far distant will drift into it, and the difficulty of 



