PHENOMENA OF FLIGHT IN THE AMIMAL KINGDOM. 



265 



The frequency of the strokes varies accordiDg- as the bird is starting, 

 is in full motion, or at the end of its flight. Some birds, as we know, 

 have periods when the wing is motionless, and when they move by 

 means of the momentum acquired. 



It is interesting to observe the relative duration of the periods of 

 ascent and descent of the wings. Contrary to the opinion expressed by 

 some observers, the descending- period is generally longer than that of 

 elevation. The inequality of the two periods is especially evident in 

 birds which have large wings and make few strokes. Thus, whde the 

 periods are almost equal in the duck, which has very narrow wings, they 

 are unequal in the pigeon, and much more so in the harrier. 



The following figures exhibit the results obtained from several species 

 of birds : 



It is more difticult than might be supposed to determine the precise 

 instant of the change of direction in the line traced by the telegraph. 

 The attraction of the magnet and the relaxation have an appreciable 

 duration, if the blackened cylinder turns with snfticient velocity to 

 measure the rapid motions which we seek to analyze. The inflections 

 of the line traced by the telegraph then become curves, of which it is 

 somewhat diflicult to determine the precise origin. There is therefore 

 a limit to the precision of the measurements which can be made by the 

 electric method. I think that we cannot approximate by this method 

 nearer than J^^ of a second to the duration of a motion." 



Another kind of signal allows the estimation of tlie frequency of the 

 stroke at the same time that it furnishes indications of the successive 

 action of the principal motive muscles of the wing. 



Mi/og}'(q)Jtic method. — In 1807 I indicated a myographic method winch 

 might be api)lied without mutilating the animal upon which the experi- 

 ment was performed. It consists in enq)loying the swelling of a muscle 

 to afford evidence of its changes in length — that is to say, by its con- 

 traction or relaxath)n. Muscles, not being sensibly compressible, cannot 

 change their length Avithout at the same time changing their transverse 

 diameter. A rapid or short, feeble or energetic contraction of a muscle, 

 hence, is accompanied by an increase in diameter, aflording the same 

 features of rate or intensity. At each descent of the bird's wing the 

 great pectoral muscle thus exhibits an increase of size, which can bg 

 indicated by the registering apparatus. 



I have made use of flexible air tubes of India-rubber in transmitting 

 these effects, a method which has enabled me at times to register at 

 some distance the beating of the heart, the puJse, and the motions of 

 resi>iration. 



The bird flies in an inclosure fifteen meters square and eight meters 

 high. The registering apparatus being placed in the center of this 

 enclosure, twelve meters of rubber tubing are enough to establish a con- 

 stant communication between it and the bird. A sort of corset is applied 

 to a pigeon, (see Fig. 10.) Under this corset, between it and the pectoral 



