MAY 91 



flight of a skilfully thrown card and needs 

 little effort. But the humming-bird flies 

 by main muscle, and he works that muscle 

 at a rate of which we can scarcely conceive. 

 Nothing hums, to human ears at least, that 

 does not shiver at a rate of at least sixteen 

 times each second. We can tell the rate by 

 catching the pitch. Our grouse, rapidly 

 as his wings seem to the eye to move, flutters 

 but little oftener than just enough for the 

 beats to blend into a very low hum, and this 

 is obscured by the whir of the separate 

 feathers, which is quite a different and 

 much higher note. More rapidly by far 

 goes the wing of the hummer, and this for 

 him is not in the short, infrequent flight of 

 the grouse. In long-sustained, constant 

 exercise, the humming-bird probably keeps 

 continually a wing motion of perhaps twen- 

 ty-five or thirty beats to the second. With 

 this quick stroke he can advance, stop in 

 mid air, veer from side to side, and retreat 

 with precision. Indeed, so constantly is he 

 on the wing that his feet are but puny 

 affairs in contrast with the firm legs of the 



