168 SPIRAL FLIGHT OF THE LARK. 



always on the surface of that logarithmic column (or funnel, rather), 

 which is the only figure that, on a narrow base, and spreading as it 

 ascends, satisfies the eye with its stability and self-balancing in the 

 thin and invisible fluid. Nor can it seem otherwise, for it is true 

 to nature. In the case of smoke or vapour, it diffuses itself in the 

 exact proportion as the density or power of support in the air 

 diminishes, and the Lark widens the evolutions of its spiral in the 

 very same proportions. Of course it does so only when perfectly 

 free from disturbance or alarm, because either of these is a new 

 element in the cause, and as such it must modify the effect. When 

 equally undisturbed, the descent is by a reversal of the same spiral, 

 and when that is the case, the song is continued during the whole 

 time that the bird is in the air. 



The accordance of the song with the mode of the ascent and 

 descent is also worthy of notice. When the volutions of the spiral 

 are narrow, and the bird changing its altitude rapidly, in proportion 

 to the whole quantity of flight, the song is partially suppressed, and 

 it swells as the spiral widens, and sinks as it contracts, so that, 

 though the notes may be the same, it is only when the Lark sings 

 poised at the same height, that it sings in an uniform key. It gives 

 a swelling song as it ascends, and a sinking one as it comes down, 

 and even if it take but one wheel in the air, as that wheel always 

 includes either an ascent or a descent, it varies the pitch of the song. 



This author further observes that 



The song of the Lark, being a most acceptable and delightful 

 subject for common observation, is a very curious one for the physi- 

 ologist. Everyone in the least conversant with the structure of 

 birds, must be aware that, with them, the organs of intonation and 

 modulation are inward, deriving little assistance from the tongue, 

 and none or next to none from the mandibles of the bill. The 

 windpipe is the musical organ, and it is often very curiously formed. 

 Birds require that organ less for breathing than other animals 

 having a windpipe and lungs, because of the air cells and breathing 

 tubes with which all parts of their bodies, even the bones, are fur- 

 nished. But these different breathing organs must act with most 

 freedom when the bird is making the greatest efforts in motion, that 

 is, when ascending or descending, and in proportion as these cease 

 to act, the trachea is the more required for the purposes of breathing. 

 The Skylark thus converts the atmosphere into a musical instru- 

 ment of many stops, and so produces an exceedingly wild and 

 varied song a song which is, perhaps, not equal in power or com- 

 pass in the single stave to that of many of the Warblers, but one 

 which is more varied in the whole succession. All birds that sing 

 ascending or descending have similar power, but the Skylark has 

 it in a degree superior to any other. 



