August 21, 1890] 



NATURE 



397 



absorption of the first manganese fluting at 557'6, and the 

 same argument might be employed to abolish carbon 

 from many cometary spectra. 



My recent work has entirely justified Dr. Copeland's 

 observations, and to him certainly belongs the credit of 

 having established the existence of the carbon bands 

 bright in a newr star. J. Norman Lockyer. 



ON THE SOARING OF BIRDS. 



'T^HE interesting problem of the soaring of birds, 



-•■ though repeatedly discussed, especially in Nature, 



has not yet found a satisfactory solution. This is the 



explanation I propose. 



Suppose that a bird soaring horizontally with a certain 

 velocity enters a current of air cutting his own course 

 rectangularly. The bird will be seized and partly borne 

 by the wind. Instead of passing by calm the distance 

 a to b, he will advance from a to ^ in the same space of 

 time (see Fig. i ; the arrow ef indicating the direction of 



/^ 



the wind, and the cross-lines the length-axis of the wing- 

 area). The way a\o c evidently being longer than a to b, 

 the bird, on arriving at c, has a greater absolute velocity 

 than if he had pursued, in a calm, his course a to b. It is 

 equally evident that, if the initial velocity of the bird and 

 the velocity of the wind are properly adapted, the velocity 

 of the bird at the point c can, in spite of the resistance of 

 the air to his advancing, be greater than at a. If arriving 

 at c the bird can turn against the wind^ without consider- 

 able loss of velocity, it is clear that he is able to continue 

 his new course for a short space, before his velocity sinks 

 to the initial velocity which he possessed at the point a. 

 During this part of his course, the relative velocity of the 

 bird (with relation to the air) is more than twice the 

 absolute velocity of the wind, supposing the initial 

 velocity of the bird equal or superior to that of the 

 wind. Let d be the point where the absolute velocity 

 of the bird has sunk to the initial velocity. If the bird 

 turns at d, so that his course crosses the direction of the 

 wind at right angles, he is again ready to begin the same 

 course as when starting from a. Thus, on the way a\.o c 

 the absolute velocity increases, on t to ^ it diminishes as 

 much. 



Let us now suppose the direction of the wing-plane 

 unchanged ; the course of the bird will no longer lie in 

 the horizontal plane, but, from reasons now easily under- 

 stood, a X.O c will gradually drop down to the earth, 

 according as the relative velocity diminishes ; on the 

 other hand, c to d will rise according to the increment of 

 the relative velocity. Which will be the greater, the 

 sinking or the rising, depends on several circumstances, 

 but principally on the force of the wind, the adaptation 

 of the wing-plane, the size and form of the bird and the 

 corresponding proportions between the bearing of the 

 wings and the resistance of the air. This resistance is, 

 of course, in proportion to the weight, less to the 

 advancing of large birds than to the advancing of small 



' It has long been acknowledged that some birds possess the power of 

 changing their direction without any sensible loss of velocity. 



birds. This is the reason why large and heavy birds are 

 ttie best soarers. 



It results from this that a bird suitably built for the 

 purpose can not only maintain the same level without 

 working his wings, by a uniform and moderate wind, but 

 also gain in height by adroit movements. 



It may perhaps be objected that, according to this 

 scheme, the course of the bird will not be spiral, but 

 run in figures of eights gradually moving in the direction 

 of the wind" or in continuous windings on the one or on 

 the other side and partly with the wind (Fig. i). Indeed 

 it is likely that the movements of the birds will often 

 prove that they profit by this principle in manoeuvres the 

 purpose of which has not yet been understood. 



The spiral soaring is still to be explained. I think we 

 must suppose that commodiousness is the principal motive 

 thereof. Let us fancy that a bird, having acquired the 

 necessary initial velocity, soars in a calm without working 

 his wings, not in a rectilinear course, but by suitable 

 inclinations and turnings of the wings in circular courses. 

 We know that, in order to perform this manoeuvre, the 

 bird drops the interior wing a little and raises the exterior 

 wing just as much, so that the wing-plane, during this 

 motion, forms a conic ring, the top of the cone pointing 

 downwards. If the velocity did not diminish, the bird 

 would be able to continue this course indefinitely, or he 

 would rise or sink in a screw-formed course, according as 

 the velocity should increase or diminish. By greater 

 inclination of the wing-plane to the axis of the cone, the 

 circles would become narrower ; by diminishing inclina- 

 tion, they would become wider : both these motions are 

 easily produced by minimal changes of the form of the 

 wing-plane or of the place of the centre of gravity. Let 

 us further suppose that the stratum in which the bird 

 soars is continually moving in a certain direction. From 

 the moment the course of the bird is perpendicular to the 

 direction of the wind (point a in Fig. 2) till the moment 



it grows parallel with it (/5),the bird obtains from the wind 

 an addition to his absolute velocity (not considering the 

 loss occasioned by the resistance of the air) and also an 

 increment of velocity from the moment his course deviates 

 from the direction of the wind {b) till the moment it 

 grows perpendicular to it {c). From this moment again 

 the absolute velocity gradually diminishes, till, at last, at 

 the point/, it reaches its minimum. From this point (/), 



NO. 1086, VOL. 42] 



