MOVEMENTS OP THE WINGS OF BIRDS. 227 



the bird the method which has enabled us to analyse the other 

 modes of locomotion. 



Frequency of the strokes of the wing. — The graphic method 

 which enabled us so easily to determine the frequency of the 

 strokes of the insect's wing cannot be employed under the same 

 conditions when we experiment on the bird. It will be neces- 

 sary to transmit signals between the bird as it flies and the 

 registering apparatus. We have here to deal with a problem 

 similar to that which we solved with respect to terrestrial 

 locomotion, when we registered the number and the relative 

 duration of the pressures of the feet upon the ground. We 

 must now estimate the duration of the impacts of the wing 

 upon the air, and the time which it occupies in its rising 

 motion. 



Electrical method, — We made use at first of the electric 

 telegraph. The experiments consist in placing on the ex- 

 tremity of the wing a kind of apparatus which breaks or closes 

 an electric circuit at each of the alternate movements which 

 it is induced to make. In this circuit is placed an electro- 

 magnetic arrangement which writes upon a revolving cylinder. 

 Figure 94 shows this mode of telegraphy applied to the study 

 of a pigeon's flight, simultaneously with the transmission of 

 signals of another kind, to be hereafter described. In this 

 figure the two conducting wires are separated from each other. 



The writing point will trace a wavy line, the elevations and 

 depressions of which will correspond with each change in the 

 direction of the movement of the wing. In order that the 

 bird may fly as freely as possible, a thin flexible cable, con- 

 taining two conducting wires, establishes a communication 

 between the bird and the telegraphic tracing point. The two 

 ends of the wires are fastened to a very small light instrument 

 which acts like a valve under the influence of the resistance of 

 the air. When the wing rises, the valve opens, the current 

 is broken, and the line of the telegraphic tracing rises. When 

 the wing descends, the valve closes, the current closes at the 

 same time, and the tracing made by the telegraph is lowered. 



This instrument, when applied to difierent kinds of birds, 

 enables us to ascertain the frequency peculiar to the move- 

 ments of each. The number of species which we have been 



