274 ANIMAL MECHANISM. 



always to sustain the body of the bird, sometimes by acceler- 

 ating the rapidity of its flight, sometimes by slackening it.* 

 Fig. Ill shows this change of plane. 



As to the re-actions to which the body of the bird is sub- 

 jected, experiment has clearly demonstrated them ; it has 

 furnished us with the means of estimating their absolute force. 

 \\V have seen that these re-actions differ according to the 

 species of bird which is observed. They are powerful and 

 sudden in birds which have a small surface of wing; longer 

 and more gentle in birds formed for hovering; the re-action 

 of the period of the re-ascent of the wing disappears almost 

 entirely in the latter kind. 



If we could compare terrestrial locomotion with the flight 

 of birds, and assimilate alternate with simultaneous move- 

 ments, we might find certain analogies between the walk 

 of man and the flight of the bird. In both, the body is 

 urged forward by an intermittent impulse ; man, like the 

 bird, raises himself by borrowing the necessary work from 

 the ilifiiuiiiic energy which he has acquired by his muscular 

 efforts. 



As to the estimation of the work expended in flight, we 

 must, before we can undertake it, have a perfect knowledge 

 of the resistance which the air presents to surfaces of every 

 form, inclined at different angles, and possessing varied velo- 

 cities. We only know as yet the movements of the wings ; 



* "We onglit to beg the reader to remark that the inclinations repre- 

 sented iii fig. Ill are referred to a line which probably is not horizontal 

 during flight. In fact, this line does not correspond with the axis of the 

 body of the bird, for it was suspended in the apparatus by a corset placed 

 behind its wings, and thus had its centre of gravity in trout of the point 

 of suspension, which caused its beak to hang .slightly down. In five 

 11 glit, mi the contrary, the axis of the bird is horizontal or rather turned 

 somewhat upward. l.YMoi-ed to this proper position, a fresh direction 

 would be given to each of the positions of the wing (tig. Ill), which 

 would alter them all by the same number of degrees. Then, probably, 

 we should sec that the wing always presents its lower Miil'acc to the air, 

 as the only one which can find in it a point of resistance. This supposi- 

 tion rci|iiiivs for its verification some fresh experiments, which we hope to 

 be soon aUe to make. 



