264 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



each point of the wiug will travel twice as far as the similar part of the 

 smaller bird. It we call the space traversed g, the resistance r, which 

 the wing: of the small bird enconnters, we shall have rg for the work 

 done by the wing, and 4 r 2 g or 8 rg for the work done by the biixl. 

 We see, then, that this work increases in the same proportion as the 

 weight of the animals we are comparing. 



Another conclnsion resnlts from the preceding considerations. If we 

 admit that the wing jiossesses the same velocity in both birds, the dura- 

 tion of the stroke will increase with the space traversed by the wing; 

 that is, it will be i)rop()rtioned to the linear dimensions of the bird. 

 Observation confirms this view by showing tbat large birds make fewer 

 strokes than small ones do. We have not yet been able to determine 

 exactly the number of strokes of the wings of birds to ascertain if their 

 frequency presents an exact inverse ratio to the size of the animal, but 

 it is easy to see that it is in this manner that the frequency of the wing- 

 strokes of birds varies. 



The graphic method, which is easily emi^loyed in deterinining the 

 frequency of the wing-strokes of insects, cannot be similarly employed 

 with birds. It is necessary to adopt some method of transmitting 

 signals from the flying bird to the registering appaiatus. For this pur- 

 pose, I have first used the electric telegraphy which furnishes the means 

 of solving the following questions: 1. What is the frequency of the 

 strokes of the wings of a bird"? 2. What are the relative durations of 

 the periods of elevation and depression of the wings ? The experiment 

 consists in placing at the extremity of the wing an apparatus which 

 breaks or closes an electric circuit at each of the alternate motions, 

 while at the further i)art of the circuit is placed an electro-magnetic 

 apparatus, which makes a trace upon a turning cylinder. Fig. IG 

 shows this method of studying the flight of a pigeon, together with 

 another method of transmitting signals. In this figure the two wires 

 are separated from each other. 



The writing style traces a crenulated line of which the changes of 

 direction correspond to a change in the direction of the motion of the 

 wing. 



In Order that the flight may be as free as possible, a fine, flexible cord, 

 containing two wn'es, establishes the communication between the bird 

 and the writing telegraph. The two ends of the two wires are attached 

 to a very small, light apparatus which, from the resistance of the air, 

 executes a kind of valvular motion. When the wing is elevated the 

 valve opens, the circuit is broken, and the line traced b^^ the telegraph 

 rises. When the wing descends the valve closes ; the circuit is also 

 closed, and the line is depressed. 



Applied to different kinds of birds, this api^aratus registers the fre- 

 quency of the strokes of the wing in each. The number of species 

 which I have as yet been able to study is very small ; I have, however^ 

 obtained the following results: 



Nmiiber of vibrations of the icing licr second. 



Sparrow 13 



Wild duck 



Pigeon 8 



Hen-hawk, Buteo vulgaris, ahaw^k called in England and France the 



" buzzard " or " husarW 5| 



Screech-owl 5 



Harrier, Circus rufus, marsh harrier of England, huse of France 3 



