Importance of the Air- Spaces in Flying Animals. 153 



function in tlie act of soaring we must first seek to determine 

 how soaring birds perform this remarkable movement. It is 

 certain that soaring and circling depend on overcoming the 

 force of gravity by the aid and employment of the force 

 inherent in the wind. Now the way in which this is turned 

 to account appears to a certain extent doubtful. In the event 

 of the speed of the wind increasing sufficiently quickly with 

 increasing altitude the bird is able, by using the increment of 

 the force of the wind, to mount in circles without a stroke of 

 its wings (Lord Rayleigh). The wind is also said to be very 

 irregular, to blow at any given point with quickly changing 

 strength and, to a slight extent, with quickly changing 

 direction. By utilizing the differences in the force and 

 direction of the wind the bird can likewise keep itself poised 

 and also ascend without flapping its wings (Langley) . Lastly, 

 the bird is able by circling, precisely like a rapidly rotating 

 top, to acquire a certain inherent stability, which, operating 

 like the string of a kite, renders it capable, when the current 

 of air is continuous and perfectly equable in force, of soaring 

 and ascending like a kite (Lendenfeld). 



Be this as it may, soaring at any rate depends upon an admir- 

 able utilization of the force of the wind, which is only attained 

 by the bird always setting its expanse of sail, the whole 

 underside of its body (MiillenhoflF) , precisely in the proper 

 angle with the horizontal plane and with the direction of the 

 wind. This process must be very difficult — -indeed, it is 

 hardly possible to imagine how a free-soaring bird, which 

 presents a large surface of sail to tlie wind, in the midst of 

 violent currents of air and in a constantly changing position, 

 without any fixed point of attachment, can maintain its 

 equilibrium, and can regulate efficiently, with ease and pre- 

 cision, the inclination of the surface of sail which it forms. 



Since the bird soars free, the position of the surface of sail 

 can really be attained only by changes in the position of the 

 centre of gravity with reference to the expanse of sail, as 

 well as by alterations of the parts that feel the resistance of 

 the air. But changes of this kind in the position of the 

 centre of gravity and alterations of the resisting surfaces can 

 be brought about by alterations in the degree of fullness of 

 the various air-sacs, since by their inflation parts of the body 

 are enlarged and forced away from the centre. It is true 

 that these changes are small, but even small changes of this 

 sort will be enough to produce a considerable and sufficient 

 result. 



Although this action of the air-sacs is most distinctly 

 marked in soaring, it will nevertheless also come into opera- 



