rnr. emimki: <»!■ tiii: air. 411 



would j;i(';itl\ »'ir: ;it t('iiti\«' ohscrvatioiM^r Itiid tliiilit (IciiioiistraU'S 

 tlic ('(Mistjiiit rcciinciice t»t" ineiiiilai' .unsts. not <)iil\ near tlic .nr(»uii(l, 

 l)ut t'veii n[) t() the limits ol" tlu" visible iifiiiosplicic. 



IJirds ('('I'taiiily possess the jiil't, like yood sailors, of scciiiL; t he coiuiiio- 

 .liust of wind: tli*' curliuii' cbau.iic of color on tlio water indicates to the 

 seaman the approach of the s(|ua]l. How is it that the bird perceives 

 the comiiifi- .yiist! It may be ditlicnlt to conceive how it is don<', but 

 the fact is certain, for the irreyidar puft" is often ntilized: and yet liere 

 a.yain is a basis it is not safe to bnikl on overnnich, for the lieavy soar- 

 ing- birds seem to disdain to use these puffs of wnnd ; they a(;cept them, 

 they store up, asit were, theaccruinii- momentum. l>uttliev iiev<'i' trouble 

 themselves to profit fully by them. 



In order to <;-ain a sound idea of what is ,iioing on in sailin.y- tiij^ht, to 

 understand and to account for it, we must separate two conditions of 

 wiiul \\hich are usually confounded — the regular current of wind and 

 the irregular gust. 



It would seem at first consideration that when in a regular current 

 of air the bird sweeps a circle, he must lose, against the current, just 

 so nuich momentum as he has received in going with the wind, plus the 

 frictional losses, etc. But we have observed that this is not the case, 

 and as we say because the bird breasts the wind with his smooth 

 cleaving- shape, a shape more perfect as he excels as a sailer; which 

 cleaving shape differs nuieh from the rear, which latter is arranged in 

 (piite another form to catch the wind as a sail, ^ow to the difference 

 in the coefficient of result upon these different shapes we must add the 

 effect of the varying- angle of the incidence of different spreads of 

 wing as velocities change, the relative short course against the wind, 

 and finally that mysterious first cause which we call life, which exhib- 

 its marveh)us wonders(»f equilibrium of rest and of motion and governs 

 the active i)art of existence. 



Vet, as I said before, very large soaring birds do not seem to trouble 

 themselves nun-h to utilize all these little accessories; the experts in 

 the art, having adjusted their surfaces at an average angle, judged 

 sufficient from their exjierience, do not readily modify their attitude; 

 they know there is small i)rofit to them in small manonivers, such as 

 the furling and unfurling the w'ing, to modify the extent of sustaining- 

 surface in diifenuit ])ortions of the circling sweep; one might say that 

 they adjust their areoplane up to a fixed notch, which they know to be 

 practi(;ally good, ami trust to the wind gust for material uprise. There 

 are probably minute changes in adjusting the eiiuilibrium which the 

 telescope does not disclose, such, for instance, as movement of the head, 

 which is a precious balancing i)endulum a<lmirably located; there are 

 even unconscious movements of the whole body; but as to intentional 

 changes in the size and set of the sails (that is, the attitude of the 

 bird's tiight), they may remain for whole hours at the point fixed, with 

 reasonably steady wind, just like the sails of ships. V\'e nuist there- 



