TllH EMIMKi: Ol" llli: Allv'. 40^V 



tlieii' be iioAvind licAvill .ylide totlic .urouiMl, to a distance ]>r()i)orti(»iial 

 always to liis surface, and above all, to his mass; theietbie an avrian 

 vulture will ^lide farther thau a tawny vulture, and this latter farther 

 than an Egyptian vulture, yet they are all constructed very nearly in 

 the same ])ro]>ortions. 



\N'heu there is no wind, sailing- birds must come down: Higlil is not 

 jmssible unless they choose to flap. This sore necessity rarely brings 

 early birds, for the morning is usually calm, particularly in the tropics. 

 But let there be a current of air, a circumstance almost always 

 present at a certain height in the atm<Ksphere; at once the scene 

 changes, the sailer sweeps in circles, he rises upon the air to great 

 altitudes, and thence he glides downward where h<' Avishes to go, even 

 against the wind. 



Let us try to explain this circling act. The bird glides in his sweej* 

 in the direction with the wind, losing as little height as [)ossible, the 

 wind imparting velocity almost eijual to its own by imi)act in the rear. 

 This push is effective: there is a good hold against upturned feathers, 

 whereas when the bird turns again against the wind all feathers are 

 smoothed down snug against each other and preseiiting surfaces of 

 least resistance. This difference in action is akin to the revolving (uips 

 which serve as anemometers through the different resistances of the 

 C(mvex and concave surfaces.* As the bird further sweeps around lu' 

 faces the wind, Avith fr«\shly accjuired velocity, and utilizes this in gam- 

 ing fresh elevation. 



In all soberness is this fraction of an exi)lanation ])resented; for this 

 circling action is little understood and is evidently of great use to the 

 bird. When he faces the win<l in the sweep h«' describes, the bird ad- 

 justs his wings and tail so as to rise slightly, his own ac(piii'<Ml speed in- 

 creasing the normal wind i»ressnre, he rises more than he has diopi)ed 

 to develop his own speed. 



To sum it uj), there is a balance of benetit; tin* result, a lift ])ro- 

 duced by the force of the wind, which does not act with equal etfect 

 whether the bird's front or rear is presented thereto. 



The soaring bird repeats this circling sweep, and gams elevati<m at 

 every la]>. The greater is the mass of bird and the more nearly con- 

 centric are the sweeps, especially when the breeze is light. Vet even 

 among those birds best able to produce those decompositions of force 

 approaching theory, the sweeps are only exactly concentric in the sole 

 case when there is no wind; Avhile he is awaiting the vivifying cur- 

 rent the bird simulates the (tircling i-ise, he still sweeps around to sus- 

 tain himself, but be gains no elevation. This action almost always de- 

 ceives the ()l)server, and h^ads him to l)elie\-e tliat tlie bird is rising 

 unless they are both placed on the same level. 



It is not well however to ascribe undue importance to the varying 

 effect of the wind on the feathers, front and rear, and to rely on this 



[M. Monillnrd imw coiisiders tliis cxpLmatioii ononcons. 



