PHENOMENA OF FLIGHT IX THE ANIMAL KINGDOM. 257 



nature of tlie movements of a bird's wing is the question now presented, 

 and this will be the object of our next experiments. 



Form of the bird. — All those who have studied the flight of birds have 

 called attention, with great i)ropriety, to the form of these animals, 

 which renders them eminently suited to tiight. They have seen in it 

 the conditions necessary for perfect stability in the midst of the air. 

 They have well understood the office of those great surfaces formed by 

 the wings, which sometimes serve as a parachute to enable the aiiinud 

 to descend very slowly, while at others these surfaces glide over the 

 air, following the plane of their inclination, and permitting the bird to 

 descend oblicpu'ly, or even to rise, or, as we shall see, to advance with 

 motionless wings. But many observers have gone so far as to assert 

 that some species of birds have an entirely passive iiight, and that, sub- 

 mitting their wings to the action of the wind, they obtain a force sufti- 

 cient to propel themselves in any direction, even against the wind itself. 

 It seems to me important to discuss, in a few words, this essential point 

 in the theory of tlight. 



The stability of the bird has been fully explained; there is nothing to 

 add to the remarks which have been uuuleon this subject. The attach- 

 ment of the wings is situated exactly at the most elevatetl part of the 

 thorax of the bird, and consequently when the extended wings take 

 hold on the air the weight of the body is to be found below this surface ot 

 suspension. It is known also that in the* body itself the lighter organs, 

 the lungs and air sacs, are above, while the denser mass of the intestines 

 is vsituated below. Lastly, the thoracic muscles, so heavy and thick, 

 occn[)y the lowest point of the system, so that the densest portion of the 

 body is placed as much below the point of suspension as ])ossible. The 

 bird which descends with outstretched wings, therefore, always has its 

 ventral region lowest ; it does not need to exert itself in order to preserve 

 its equilibrium, but takes this attitude passively-, as does the parachute 

 when let fall throngh the atmosphere, or as the shuttlecock Avliich falls 

 back upon the battledore. But this vertical descent of which I have 

 spoken is exceptional; the descending bird is almost always actuated 

 by a motion previously obtained, so that it slides obliquely upon the air 

 as does a light body of large su])erticial extent when placed in the above 

 mentioned situation. Mr. J. Pline has carefully studied the different 

 kinds of sliding which may occur in this manner, and has even repro- 

 duced them by means of a very simple and easily-constructed theoreti- 

 cal apparatus. Take a square sheet of paper and fold it midway, so as 

 to form a rather obtuse angle, as in Fig. 13; then, with a little wax, 

 fix in this angle a small metal rod, furnished with two balls of the same 

 weight, and the result will be an apparatus which will possess stability 

 in the air. 



If the center of gravity is exactly in the center of the apparatus, it 

 will be seen to fall vertically when dropped into the air, with the edge 

 of the angle downward. If the center of gravity is displaced by taking 

 away one of the two balls, instead of falling vertically, it will pursue an 

 oblique course and slide over the air with an accelerated motion. The 

 trajectory of the apparatus in this case will be described in a vertical 

 plane if the two sides of the apparatus are perfectly symmetrical ; if 

 this is not the case it will be deflected toward the side of least resist- 

 ance. This effect, easily comprehended, is identical with that produced 

 in the course of a ship by the resistance of the rudder. It can also be 

 produced in a vertical plane, so that the trajectory of the apparatus 

 may present a curve with a superior or inferior concavity, according to 

 the conditions of its projection. 

 17SG9 



