206 



ANIMAL MECHANISM. 



very nearly horizontal ; in the hymenoptera, the wing moves 

 in a plane of nearly 45° , but the lepidoptera flap their wings 

 almost vertically, after the manner of birds. 



In order to render the influence of the plane of oscilla- 

 tion more evident, and to show that the force derived from the 

 resistance of the air has the double efi'ect of raising the insect 

 and directing its course, we must arrange the flight-instrument 

 in a peculiar manner. It will be necessary, in the first place, 

 to be able to change the plane of oscillation of the wings, 

 which is effected by placing the drum on a pivot at the ex- 

 tremity of the horizontal tube, at the end of which it turns. 

 To show the ascensional force which is developed in this new 

 arrangement, the instrument must no longer be confined to a 

 simple movement of rotation in the horizontal plane, but it 

 must be able to oscillate in a vertical plane like the scale beam 

 of a balance. 



Fig. 88 shows the new arrangement which we have given 

 to the instrument in order to obtain this double result. 



In this modification of the apparatus, the air-pump which 

 constitutes the moving force is retained ; as is also the turn- 

 ing column which moves in the mercurial gasometer. But 

 above the disc which terminates this column at the upper end, 

 is fixed a new joint, which allows the horizontally-balanced 

 tube at the end of which the artificial insect is placed, to 

 oscillate in the vertical plane like a scale-beam. In order to 

 establish a communication between the revolving column and 

 the tube carrying the insect, we make use of a little india- 

 rubber tube, sufficiently flexible not to interfere with the 

 oscillatory movements of the apparatus. 



Other accessory modifications may be seen in fig. 88 ; one 

 consists in employing a glass tube to convey the air from the 

 pump which moves the insect ; the other in a change of the 

 mechanism by which motion is imparted to the wings. The 

 most important alteration is the introduction of a joint which 

 allows us to give every possible inclination to the plane in 

 which the wings oscillate. 



The apparatus being arranged so that the counterpoise, 

 having been brought nearer to the point of suspension, does 

 not exactly balance the weight of the insect, the latter is 



