330 ON THE VARIOUS MODES OF FLIGHT 



Bess of body is conccrnccl, ho not inaptly compared to a compass set i;pon 

 gimbals, where the universality of motion in one direction insures comparative 

 fixedness in another. 



Many instances might lie quoted of the marvellous powers of flight residing 

 in insects as a class. The male of the silkworm moth (Attacus Paphia) is stated 

 to travel more than 100 miles a day;* and an anonymous writer in ^Nicholson's 

 Journal calculates that the common house fly (Musca domesticus) in ordinary 

 flight makes GOO strokes per second, and advances 25 feet; but that the rate of 

 speed, if the insect be alarmed, may be increased six or seven fold, so that under 

 certain circumstances it can outstrip the fleetest race-horse. Leeuwenhoek relates 

 a most exciting chase which he once beheld in a menagerie about 100 feet long, 

 between a swallow and a dragon liy (mordella.) The insect flew with such 

 incredible speed and wheeled with such address that the swallow, notwithstand- 

 ing its utmost efforts, completely failed to overtake it.f 



Whig of bird. — There are few things in nature more admirably constructed 

 and where design can be more readily traced than in the wing of the bird. Its 

 great strength and extreme lightness, the manner in Avhich it closes up or folds 

 during flexion, and opens out or expands during extension, as well as the method 

 according to which the feathers are strung together, and slate each other in 

 divers directions to produce at one time a solid resisting surface, and at another 

 an interrupted and comparatively non-resisting one, present a degree of fitness 

 to which the mind must necessarily revert with pleasure. The wing of the bird, 

 like that of the insect, is concavo-convex, and more or less twisted upon itself 

 when extended, so that the upper or thick margin of the pinion presents a dif- 

 ferent degree of curvature to that of the nether or thin margin, the curves of 

 the two margins in some instances even intersecting each other. This twisting 

 is in a great measure owing to the manner in which the bones of the wing are 

 twisted upon themselves, and the spiral nature of their articular surfaces, the 

 long axes of the joints always intersecting each other at right angles. As a 

 result of this disposition of the articvdar surfaces the wing may be shot out or 

 extended, and retracted or flexed in nearl}' the same plane, the bones composing 

 the wing rotating on their axes during either movement. This secondary action, 

 or the revolving of the component bones upon their own axes, is of the greatest 

 importance in the movements of the wings, as it communicates to the hand and 

 forearm, and consequently to the primary and secondary feathers which they 

 bear, the precise angles necessary for flight. It in fact insures that the wing, 

 and the curtain or fringe of the wing which the primary and secondary feathers 

 form, shall be screwed into and down upon the wind in extension, and unscrewed 

 or withdrawn from the wind during flexion. The wing of the bird may, there- 

 fore, be compared to a huge gimlet or auger, the axis of the gimlet representing 

 the bones of the wing, the flanges or spiral thread of the gimlet the primary 

 and secondary feathers. As the degree of rotation made by the bones of the 

 forearm and hand during extension amounts as nearly as may be to a quarter of 

 a turn of a spiral, it follows that in flexion the wing presents a knife-like edge 

 to the wind ; whereas in extension the curtain of the wing is rotated in a down- 

 ward direction until its anterior or concave surface makes an angle of 45° with 

 the horizon. From this description it will be evident that by the mere rotation 

 of the bones of the forearm and hand the maximum and minimum of resistance 

 is secured nmch in the same way that this object is attained by the alternate 

 dipping and feathering of an oar. 



In the majority of quick-flying birds — at all events in such as do not glide or 

 skim — considerable advantage is gained by the primary and secondary feathers 

 being thrown out of position during flexion, this arrangement preventing retard- 



*" Liun. Trans, vii, 40. 



t The liobhy falcon which abounds iu Bulgaria is equal to this task, the dragon-fly form- 

 ing a principal constituent of its food. 



