Feb. 1 6, 1888] 



NATURE 



in 



will be partially confused, because the bird, in the 

 hundredth part of a second, only traverses a space equal 

 to the length of its body : the image of the second will 

 therefore partly cover that of the first, the third that of 

 the second, and so on. In this confusion one can 

 scarcely distinguish the moment in which the wing lowers 

 itself, or that in which it is raised. But this is of no im- 

 portance : we fix on the head of the bird a small but very 

 brilliant metallic point, and the image of this point, 

 clearly seen in the series of figures, reveals the trajectory 



of the bird, together with its speed, and the accelerations 

 and slackening of speed produced by the movements of 

 the wings. One may then face the dynamic problem 

 of flight. It is granted first that the bird does not oscil- 

 late sensibly in the vertical sense, whence one must con- 

 clude that the resistance of the air under its wings is 

 precisely equal to its weight. On the other hand, it is to 

 be observed that the motion of the animal presents 

 alternations of speed and slowness, showing that the 

 propelling force and the resistance of the air predominate 



Fig. 10.— Trajectory of a white ball thrown in front of a black screen. The interval between two successive images is measured on the metrical scale. 



The time taken to travel over this interval is i/ioo of a second. 



by turns. From the value of these accelerations there 

 must be deducted the value of the horizontal component 

 of the bird's motion, and that of the resistance of the air. 



The calculations based on these experiments have 

 given the following results for the forces which act during 

 the flight of the sea-gull : — 



Vertical component o'623 kilogramme 



Horizontal component o' 



Total ... 



These forces develop themselves during the act of 

 lowering the wings ; the ascent is passive, and is due to 

 the pressure of the air upon the lower surface of the wings, 

 which act then for the support of the bird, as in a paper 

 kite. 



As the resistance of the air under the wings acts?' at a 

 point a considerable distance from the articulation of 

 the shoulder, and as the pectoral muscles, by which the 

 wings are lowered, act very near the articulation — that is 

 to say, on the arm of a very unfavourable lever — it results 



Fig. II. — Curves and nodes produced by a vibrating stalk, one end of which is fixed. (Fac-simile of instantaneous photographs taken by the author.) 



that the effort of the muscles is much greater than the 

 resistance of the air which they surmount. For the 

 pectorals of the sea-gull, the effort developed would be 

 19 kilogrammes. 



It is frequently asked whether the muscles of birds 

 have not a specific strength greater than those of other 

 animals — that is to say, whether two bundles of the same 

 thickness of muscles belonging, one to a bird, the other 

 to a mamuial, would have different powers. In the sea-gull 



which served for my experiments, one transverse section 

 of the pectoral muscles arranged perpendicularly to the 

 direction of their fibres had about 1 1 centimetres square 

 of surface, or about r6oo kilogramme per square centi- 

 metre. Other birds had formerly given me nearly similar 

 returns for their specific strength ; thus, the buzzard de- 

 veloped 1200 grammes per square centimetre, the pigeon 

 1400 grammes. 

 Aeronauts hope that they will one day invent a machine 



