PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 279 



In a remarkable study of tbe flight of birds, M. Liais has been led, 

 through observation and deduction, to adoi)t this theory, to which the 

 experiments about to be described, I trust, will add new j)roofs in its 

 favor. 



Before leaving the subject, it is necessary to mention the existence of 

 certain other motions in the flight of small birds. I refer to the folding 

 and unfolding of the wings. But the existence of these motions does 

 not seem to be constant, and the eye cannot perceive the least trace of 

 them during the flight of the large birds upon which I have experi- 

 mented. I slmll, therefore, omit the study of these motions, and of their 

 l^ossible effects, and restrict my conclusions on the mechanism of flight 

 to a certain number of determinate species of birds. 



The study of the motions of tlie wings of birds during flight necessa- 

 rily inebides the effect produced by each of these movements. We are 

 tempted to deduce these efflnjts from the nature of the motions which 

 generate them, but it is safer to obtain the solution of this complicated 

 problem from experiment. Two distinct effects are produced during 

 tiiglit: first, the bird is upheld against the force of gravity ; second, it 

 is propelled horizontally. Is the bird in the air sustained at a constant 

 elevation, or is it rather subject to oscillations in the vertical plane? 

 Does it not exhibit, by the intermittent eflect of the strokes of its wings, a 

 series of ascents and descents, the frequency and extent of which can- 

 not be observed by the eye "? Is not the bird also subjected to a varia- 

 ble velocity in its horizontal course! Does it not receive a jerking 

 motion from the action of its wings f These questions can be solved by 

 experiment, in the following manner : Since we possess the means by 

 which distant motions iiroduced. by pressure exerted upon a drum filled 

 with air are made to record themselves, we must seek to connect the 

 movements which we would study with a pressm-e of this kind. The 

 oscillations which the bird executes in the vertical plane should be made 

 to produce alternately strong or feeble pressure on the membrane of the 

 drum, according as the bird rises or falls. The same should be done in 

 seeking the variations of its horizontal velocity. Supi^ose that a flying 

 bird carries upon its back a light metallic drum, like the one already 

 described; that the membrane of this drum be turned upw^ard, and that 

 this instrument be put in communication with the registering appara- 

 tus by means of a long tube. If the membrane of the drum freely par- 

 takes of the motions of the bird it will not produce any displacement of 

 the air in the apparatus, and the registering lever will remain motion- 

 less. But if we prevent the membrane from partaking of all the motions 

 of the bird, if we can give it a tendency to remain at rest while the 

 drum is moved, motion will be produced in the air with which the drum 

 is filled, and the signals will be registered by the lever. Now, we can 

 produce this tendency to remain at rest upon the membrane by loading 

 it with an inert body, such as a disk of lead. 



Fig. 29 shows the drum with an inert mass upon its membrane. 

 This mass is formed of disks of lead, of which a certain number can be 

 added or taken off", until the apparatus responds satisfactorily to the 

 motions of vertical oscillation imparted to it. In this arrangement the 

 movements in the horizontal plane are without influence upon the appa- 

 ratus. If the drum is suddenly raised, the inert body, not participat- 

 ing in this elevation, dei^resses the membrane exactly as if the mass 

 itself had been depressed, and the drum had remained motionless. Con- 

 versely, when the drum descends, the inertia of the mass resists the 

 motion, as if it or the membrane had been raised and the drum had 

 remained motionless. We may remark that the movement of the lever 



