410 THE EMPIRE OF THE AIR. 



;is ;i sole explanation; tlie problem contains many mure elements. 

 Tlie variations in the amount of wing- surfaces unfolded to tlie wind, 

 in the different portions of the circles described, the variations in 

 speed, and the shifting- in position of the center of gravity of the bird 

 are all of them factors to be taken into account. 



The bird's elevation is gained by the skilfnll utilization of all these 

 elements, and by the happy use of a number of casual circumstances, 

 beginning- with those of ascending currents, which have been so much 

 discussed of late, but which are not to be reckoned upon as a steady, 

 sole cause, and ending with the utilization of the coming- puff" of wind, 

 which the bird takes advantage of by breasting it at the best angle of 

 incidence, just at the right moment; tinally, and especially, by the 

 difference in length of course while sweeping with the wind or against 

 it, the latter being shorter, and the difference being nioic marked, as 

 the rise becomes steeper. 



The advantage obtained in rising- on circling- sweeps is easily ob- 

 served and understood, yet it must be confessed that there is a weak 

 point in the analysis, an insufficient exijlauation. This pertains to the 

 stage when the bird is going with the wind. Is the acquired momen- 

 tum, the velocity necessary to support the bird, sufficient to account 

 for the subsequent phenomena? I scarcely think so, and I feel that 

 the explanation is not absolutely correct, for observation shows that 

 there is often complete arrest of motion. In any case, whether my 

 analysis be good or inadequate, the circling sweep is much practiced 

 by birds, and observation indicates that it is the manoeuver which af- 

 fords easiest ascent, for it is the process always employed by the sail- 

 ing bird when there is a minimum of wind. 



While still lacking a clear, convincing- explanation, we may hold to 

 that above given ijrovisionally. Ifelying on the instinct of the birds, 

 we may without risk accept the usefulness of the sweeping circle. 



Instead, a manteuver which supports analysis, and which is easily 

 understood, is that of direct ascension against the wind, either by 

 drifting- back, which is an easy feat, or vertically, which is more diffi- 

 cult, or even while advancing- against the wind, the most difficult 

 of all. 



When we note the conect angle of incidence presented by the bird, 

 the adjustment of his surfaces, and his skillful utilization of the vary- 

 ing velocities of the wind, advancing forward in the calm, and ascend- 

 ing on the increased velocity, we understand his mananivers and find 

 his solution of the problem easy to follow. 



But we remark that this particular process of rising in the air re- 

 quires that the wind shall i)Ossess such speed as to sustain at all times 

 an areoplane with no velocity of its own, while, if in circling sweep, 

 this same areoplane would have an initial velocity, thus enabling it to 

 utilize breezes too feeble to serve in direct ascensions. 



We must never think of the wind as a regular current of air — we 



