MOVEMENTS OF THE WINGS OF BIRDS. 247 



allows the bird to fly under conditions almost normal, and 

 at the same time will permit the artificial instruments to make 

 attempts at flight, without any fear of letting them fall, if 

 the movements which they produce should be insufficient to 

 sustain them in the air. We will now describe this suspen- 

 sory apparatus. 



There is a sort of frame-work of six or seven metres in 

 diameter, in which the bird moves continuously, being thus 

 able to furnish us with an observation of a circular flight of 

 long duration. We give the instrument a large radius, that 

 its curve, being less abrupt, should modify less the nature of 

 the movement which the bird may make. Harnessed to some 

 extent to the extremity of a long arm which turns on a central 

 pivot, the bird ought to be as free as possible to go through 

 its movements of vertical oscillation. We shall presently see 

 that a bird passes through a double oscillatory movement in 

 a vertical plane for each revolution of its wings. 



Arrangement of the frame. — The conditions to be fulfilled are 

 the following : in the first place, a great mobility of the 

 instrument, that the bird may have the least possible resist- 

 ance to overcome in its flight ; then, a perfect rigidity of the 

 arm of the machine, to prevent any vibrations peculiar to 

 itself, which might render unnatural the movements executed 

 by the bird. 



Fig. 103 shows the general arrangement of the apparatus. 

 A steel pivot, resting on a solidly-cast socket of great weight, 

 is placed on the platform of a photographic table. This table 

 is raised by means of rack- work, so that the operator, after 

 having arranged his apparatus so as to suit the experiment, 

 may place the platform sufficiently high for the instrument to 

 turn freely above his head. 



The frame- work, properly so called, is a bow formed of a 

 long piece of fir- wood slightly curved. The string of this 

 bow is an iron wire, which is fixed by the middle to a cage 

 of wood traversed by the central pivot. Care is taken to 

 balance the two ends of the apparatus, by gradually adding 

 weights to the arm not carrying the bird which is the subject 

 of the experiment. 



If we did not take this precaution, the apparatus, as it 



