37^ 



NATURE 



\Feb. 1 6, J 



cannot unfortunately give an idea of the effect" produced 

 by the apparatus in motion. 



; But it may be said that this rotatory method interprets 

 the movements of the bird without indicating the forces 

 which produces them. While it would be well to know 

 that force, it is better still to measure the mechanical 



labour expended in order to sustain and transport itself 

 in the air. 



Let us see whether our photographic images reveal to 

 us anything in regard to this. 



When one knows the mass of a body, and the 

 speed with which it moves, one can calculate the force 



Fig. 8. — Bronze figures representing eleven successive positions at successive moments in the stroke of a pigeon's wing. 



which has set this body in motion, and the labour ex- 

 pended by this force. If we take a projectile of a certain 

 weight, and throw it before the photochronographic 

 apparatus, and take a series of images of this projectile 

 at intervals of i/ioo of a second, Fig. lo shows the 

 trajectory curve followed, and the space which separates 



the images from each other shows the space traversed by 

 the projectile in each of the hundredth parts of a second 

 during which its movement has lasted. From ten to ten 

 a more brilliant image has been produced by an aperture 

 in the diaphragm larger than the others : these marks are 

 useful in order to facilitate the numbering of the images, 



Fig. 9. — Zootrope, in which are placed ten figures, in relief, of a sea-gull in the successive positions of flight. 



a £xed metrical scale, photographed at the same time as 

 the object in motion, serves to measure the spaces tra- 

 versed at each moment ; then it is a problem in dynamics, 

 \i hose solution may be readily obtained by the usual 

 r ethods of calculation. 

 3i TJiB isuccessiyfi. image? of the jflying bird lepd, them- 



selves to the same dynamical analysis. The balance 

 indicates to us the weight of the bird ; we know its size ; 

 and in order that photochronography may give us to per- 

 fection the trajectory of this mass, it only requires mani- 

 fold multiplication of the images obtained (a hundred 

 may be taken in a second if need be}^ _ But.those images 



