204 ON SOARING FLIGHT. 



answer we Lave only to point to the flight of the vultures, which, unlike 

 the sea birds, fly high and may maintain their course uninterruptedly 

 into a high wind at great altitudes. Nor need we, in fact, look away 

 from the sea birds for an evidence to show that wind breaks, although 

 often utilized by the bird, offer no satisfactory solution of the problem. 

 Quite recently I spent some time in watching the flight of two gulls on 

 the Potomac Eiver. A stiff breeze was blowing, but the waves which 

 it produced were scarcely 12 inches high — entirely too small to be effect- 

 ive as wind breaks — while the broad expanse of water, the level nature 

 of the surrounding country, and the direction from which the wind 

 came precluded any assumption of a general rising current of the air. 

 Considered iu its entire mass, there is every reason to believe that the 

 wind was horizontal. In this horizontal wind, by rising to a height of 

 about 40 feet the gulls were able to soar with the utmost ease, sailing 

 steadily into the wind or across it without loss of elevation and without 

 flapping. This and similar occurrences go to show that a strong hori- 

 zontal wind, though not, so far as we have any evidence, a homogeneous 

 one, suffices for sustained soaring flight. Only the small and rapid 

 changes in the direction of the actual wind, whose course, in general, is 

 horizontal, are here supposed to become effective in sustaining the bird. 

 However, in his work on The Internal Work of the Wind, Mr. Langley, 

 while including under this title the energy arising from all changes in 

 the condition of the wind, whether in velocity or direction, applies the 

 theory more particularly to a consideration of the available power 

 resulting from changes in the velocity. In addition, then, to the energy 

 obtained from the winds iu the manner just described, there must be 

 considered that which arises from the latter source, and all these varia- 

 tions taken together constitute, in their effect upon resisting bodies, 

 the internal work of the wind. 



We have finally to consider flight in winds of moderate velocity. 

 Looking again to the birds for the facts from which to reason, we find 

 in such winds they still soar, but in a manner which is neither that 

 adopted in feeble winds nor in strong ones, but intermediate iu char- 

 acter. In moderate winds the bird avails himself of the power found 

 in ascending currents and also in the internal movements of the wind. 

 In light winds it rises only by soaring in circles; in heavy winds this 

 method may be, and usually is, wholly discarded, the flight becoming 

 sustained and direct; while in moderate winds it is still under the 

 necessity of rising in circles, but less frequently than when the winds 

 are light, and while sustained flight in direct courses is no longer pos- 

 sible as in the case of high winds, it yet obtains from the winds a cer- 

 tain amount of uplift which renders its rate of descent less rapid than 

 it would be in a calm or in light winds. Observation abundantly shows 

 that in direct flight the rate of descent is indirectly proportional to the 

 velocity of the wind. Thus if a bird in a calm descends at tlie rate of 

 3 feet per second, the rate of descent will be reduced to 2 feet per second 



