194 ON SOARING FLIGHT. 



amount of effort would cause the strands to rise. It seems, therefore, 

 that very slight disturbances, such as the circling and flapping of the 

 bird might produce, are sufficient to produce ascending currents under 

 the conditions which observation shows to be favorable to flight. 



THE CONDITIONS KENDERING SOARING PLIGHT POSSIBLE. 



We are now in a i)osition to turn to the soaring birds and inquire if 

 in the manner of their soaring or otherwise they furnish any evidence as 

 to the correctness of the hypothesis here set forth. And first we shall 

 consider the conditions of the atmosphere which, as observation shows, 

 render soaring flight possible. 



Of recent years writers upon this subject have almost universally 

 agreed that winds are necessary to flight. But notwithstanding this 

 Avidespread belief among scientific observers I venture to say that while 

 winds may attain a velocity sufficient to furnish through their internal 

 movements all the energy necessary for sustained flight without flap- 

 ping, the birds may yet soar perfectly when the winds are in them- 

 selves too feeble to support them, and that the condition of the atmos- 

 phere which gives rise to local winds — that is, an unstable condition of 

 the lower strata — is also the condition which renders soaring flight pos- 

 sible in the absence of strong winds. A light wind on this view is 

 to be regarded not as a cause at all but as an effect, and we may, on this 

 view, at once dismiss the question as to the power which such a wind 

 may furnish, and look instead to the condition of the atmosphere which 

 gives rise to it for the explanation of the flight of the birds, whose abil- 

 ity to soar seems to be so entirely independent of the strength of the 

 winds that they may occasionally be found soaring in what appears to 

 be an absolute calm. I have many times seen the turkey buzzard 

 soaring in sheltered localities when there was no indication of any 

 wind whatever. Early one morning, while following the railroad up the 

 narrow valley of the French Broad Eiver, near the Warm Springs, 

 N. C, I saw a turkey buzzard fly from the western slope of a high 

 ridge and begin soaring in circles above a narrow sunlit meadow upon 

 the banks of the river, alternately beating the air violently with its 

 wings and sailing. This he continued to do for the space of perhaps 

 two minutes, after which he entirely ceased flapping, and rose steadily 

 along a spiral j)ath to a height of 300 feet, before sailing away along a 

 direct descending course. On this occasion there was not the slightest 

 breath of wind to be detected. Not a blade of grass moved, not a 

 tassel of corn, not a leaf upon the trees. I came to the spot where the 

 bird had been soaring, hemmed in by high ridges and mountains, and 

 could nowhere detect any sign of motion in the air. On another occasion 

 I witnessed one of these birds soar, without flapping, 30 feet above the 

 tops of some cedars in a valley and 75 feet from where I stood on a 

 railroad embankment, while the smoke from some neighboring chimneys 

 rose almost vertically and no motion whatever was to be detected in 



