4^6 THE POPULAR SCIENCE MONTHLY. 



"ber of observers, and have been concisely demonstrated by Strauss- 

 Durckeim, Liais, and Marey. 



But, in considering the difficulty of the construction of our me- 

 chanical bird, we were obliged, notwithstanding our desire to make a 

 machine which should be siraj^Ie and easy to understand, to try to 

 perfect those actions we have somewhat summarily described. It is 

 evident that the different parts of the wing, from its base to its extrem- 

 ity, act on the air under very different conditions. The interior part 

 of the wing, having small velocity, produces little propelling effect 

 at any moment of its beat ; but it is far from being useless, and one 

 may imagine how, by presenting its lower face downward and slightly 

 facing the front, it acts during the rapid translation of the bird, like a 

 kite, as well while the wing is being elevated as during its down- 

 ward motion, and thus sustaining in a continuous manner a portion of 

 the weight of the bird. The middle portion of the wing has a junc- 

 tion intermediate between that of the interior and that of the outer 

 portion, or end, of the wing ; so that the wing, during its action, is 

 twisted on itself in a continuous manner from its base to its extrem- 

 ity. The plane of the wing at its base varies but little during flight ; 

 the plane of the median part of the wing is very much displaced on 

 one and the other side of its mean position ; finally, the outer part of 

 the wing, and especially its tip, experiences considerable change of 

 plane. This w^arping of the wing is modified at each instant during 

 its elevation and depression, in the manner just indicated ; at the 

 extreme points of its beat the wing is nearly plane. The action of 

 the winof is thus seen to be intermediate between that of an inclined 

 plane and that of a screw with a very long and continually variable 

 pitch. 



Notwithstanding the differences found to exist in the hypotheses 

 of various authors when compared with one another and with the one 

 just given, still one or the other of these writers confirms the greater 

 portion of the ideas just advanced. Thus the torsion of the wing had 

 already been pointed out by Dutrochet, and especially by Pettigrew, 

 who long maintained this opinion ; only he has taken, according to 

 our view, the change of form occurring during the elevation of the 

 wing for that of the form occurring during its depression, and vice 

 versa. These authoi's clearly saw how the articulations of the bones^ 

 the ligaments of the wing, the imbrication and elasticity of the quills, 

 bring about the above result. M. d'Esterno had explained the con- 

 tinuous effect, like that of a kite, of the interior portion of the wing 

 during its depression and elevation ; and M. Mai'ey had very appropri- 

 ately designated that portion of the wing as " passive," at the same 

 time, liowever, maintaining that the most important action of the 

 wing during flight is due to a general change of its plane produced by 

 the rotation of the humerus on itself. 



According to our view there is a sharp distinction to be made be- 



