156 Miscellaneous, 



According to this inclination, the animal may ascend or move hori- 

 zontally, as long as its rapidity of motion is not too much diminished 

 by the resistance of the air ; to descend requires merely a change of 

 the inclination of the wings ; to remain at the same elevation, the 

 animal must again have recourse to the beating of the air. 



Flight without locomotion is effected by many birds and insects. 

 In this mode of flight, it appears that, in ascending, the wing partially 

 destroys the ascensional effect which it produced in descending. In 

 birds, as the wing presents its convexity in ascending, and its con- 

 cavity in descending, it cannot produce the same effect in both 

 directions, even with an equal velocity ; but this difference does not 

 exist in those Neuropterous and Dipterous insects which hover in 

 one place. The explanation of this fact is to be found in the 

 different velocity with which the animals raise and depress the wings. 

 In the Frigate-birds, the wing descends at least five times as quickly 

 as it rises. The resistance of the air being in the proportion of the 

 square of the velocity of the wing, the ascending or descending velo- 

 city of the animal, caused by a movement of the wing, is in proportion 

 to this resistance multiplied by the duration of action, which is in 

 an inverse ratio to the velocity of the wing. The ascending or 

 descending velocities of a bird, caused by the movements of the wings, 

 are therefore to each other as the velocities of the wings in their 

 ascending and descending movements. 



Flight with locomotion and beating of the wings is the most 

 frequent kind, and appears to require less labour ; for the movements 

 of the wings are much less rapid. The cause of this is, that the 

 wing experiences no resistance in ascending. When a bird is about 

 to depress its wing, this is a little inclined from before backwards. 

 When the descending movement commences, the wing does not 

 descend parallel to itself in a direction from before backwards ; but 

 the movement is accompanied by a rotation of several degrees round 

 the anterior edge, so that the wing descends more in front than 

 behind, and the descending movement is transferred more and more 

 backwards, at the same time that the wing becomes more and more 

 inclined, so as to" give a movement at once ascending and accelerative 

 of the horizontal motion of the animal. Towards the close of this 

 movement, a fresh rotation takes place round the anterior margin 

 of the wing, but in the opposite direction, so as to bring the posterior 

 part on a level with the anterior, or even a little below it. This 

 also produces an ascending movement. When the wing has com- 

 pletely descended, it is both further back and lower than at the 

 commencement of the movement, but, as at this commencement, its 

 posterior part is a little lower than its anterior. It is then raised 

 in this position. 



To analyze what takes place in this process, we must take a point 

 of the anterior margin, and examine its movements, not in relation 

 to the animal, but to the mass of air in the midst of which it moves. 

 In a horizontal direction, this point is displaced to an extent equal 

 to the sum of its horizontal movement in relation to the centre of 

 gravity of the iinimal, in consequence of the movement of the wing 



