246 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



An air-pump, moved by a rotary apparatus, alternately comppesses 

 and relaxes the air in a tube which traverses the central pivot of tbe 

 ajiparatus, wliere a sort of mercurial gasometer lieruietically seals it 

 ■while permitting the free rotation of the arms. The horizontal branch 

 is hollow, and conducts the air into the apparatus, which is' closed by a 

 liollow metallic drum, of which the two circular faces are closed by two 

 sheets of rubber. By the i)lay of the air-pump these two sheets are 

 inflated or contracted both together. They comnuinicate the rapid mo- 

 tions of elevation or depression to the wings by two angular levers. 

 The wings, presenting, like those of an insect, conditions of unequal 

 flexibility, decompose the resistance of the air, and impart to the appara- 

 tus a rapid rotary motion around the central pivot. 



Imagine two artificial wings, as nearly alike as possible, both inserted 

 on one of these little drums, which I liave frequently described. They 

 receive through this drum absolutely synchronous motions of eleva- 

 tion and depression. This apparatus is fixed at the extremity of an arm 

 balanced by a counterpoise, and turning u^^on a pivot. This arm is 

 hollow, furnishing a canal by which the effect of inflation can be trans- 

 mitted to the movable drum of the wings. ^Ye may consider the drum 

 as representing the body of the insect, and nothing prevents us from 

 really giving it the shape of this animal. The rigid nervures, furnished 

 with flexible membranes disposed to the right and left, will be the two 

 "wings, and the animal, instead of being free, will be fixed at the extrem- 

 ity of a movable rod ; there is, therefore, only a single motion possible, 

 which is that of turning around the pivot, carrying the attached rod with 

 it. In efl'ect, if I put the air-pump in motion, the artificial insect moves, 

 flaps its wings, and really flies. At each stroke there is a change of 

 plane of the alar membrane; at each stroke the point of the wing de- 

 scribes a figure of eight; and in a general way this theoretical animal, 

 this artificial insect, rej^roduces all the particulars which the observation 

 of real insects has revealed to us. 



This apparatus aftbrds many other advantages besides those of veri- 

 fying theoretic ideas. It enables us to make new experiments, to which 

 living beings will not lend themselves. AVe can change one of the con- 

 ditions, for example the form of the wings, their extent or the rapidity 

 of the stroke, or any other of the circumstances, while all the others 

 remain constant; we may thus discover the influence which each of them 

 singly may have on the mechanism of flight. It is by such experiments 

 that we can assure ourselves of the following fact. In the course tra- 

 versetl by the wing there is only one region useful in the propulsion 

 of the insect; that is the median region. In the two extreme portions 

 the wing has not experienced that change of plane which renders its 

 action effective. Thus we see if we diminish the extent of the motions 

 of the wing, the tractile power produced b}' the apparatus diminishes 

 considerably, and finally ceases altogether. If the membrane of the 

 wing is too broad, another phenomenon results. The hinder edge of the 

 wing remains almost immovable in space, especially during motions of 

 small amplitude ; the nerxnire only is animated with rapid motion. The 

 air, therefore, is struck by planes inclined inversely to those v\'hich act 

 upon it in normal flight, so that the apparatus retrogrades and turns 

 around its pivot in a direction contrary to its usual motion. 



Experimental flight also shows the adaptation of certain forms of 

 ■wings to obtain the most rapid translation of motive force. These are 

 precisely the forms which we find in nature. The nervure of insects 

 does not carry the wing membrane back to its point of insertion. Those 

 parts near the articulation have little vitality ; they contribute very little 



