116 ANATOMY OF VERTEBRATES. 



the sine of inclination, that is, as « 5 to ^ c ; consequently, a 

 bird with its mngs elevated at any angle to the horizontal plane 



37 



will descend mth greater velocity than when they are in the 

 direction of a b. Pigeons elevate their wings in this manner 

 until they arrive within a foot or two of the ground, when, to 

 prevent the shock they would otherwise receive, omng to the 

 velocity acquired during their descent, they suddenly turn their 

 axis perpendicular, which had previously been parallel, to the 

 direction of their motion, and by a few rapid strokes of the wing 

 neutralise their momentum, and thus reach the ground with ease 

 and safety.'^ 



The manner of flight varies in different birds : some dart for- 

 ward by jerks, closing their wings every three or four strokes ; 

 the Woodpeckers and Wagtails show this kind of undulatory 

 motion : most birds have an even continuous flight : the Kite and 

 Albatross sometimes buoy themselves in the air without any per- 

 ceptible motion of the wings. The best flyers often economise 

 their forces by availing themselves of the impetus of a few rapid 

 strokes to scud along with the wings expanded, until the interval 

 of rest requires to be broken by a fresh eflbrt, — a phase of flight 

 beautifully defined by an old observer of nature ; — 



Mox acre lapsa quieto 

 Radit iter liquidum, celeres neque commovet alas. — Virgil. 



> CCiv. p. 428. The principal data requisite for estimating by dynamics the 

 amount of the force employed by birds during flight are briefly stated by Mr. Bishop 

 to be : — ' 1st, the area of the horizontal section of the body of the bird : 2nd, the area 

 of the wings whilst they are lowered : 3rd, the area of the wings whilst they are 

 raised: 4th, the velocity with which the bird is driven through the air: 5th, the velo- 

 city with which the wings are lowered : 6th, the velocity with which the wings are 

 raised: 7th, the respective durations of the elevation and depression of the wings: 

 8th, the weight of the whole bird: 9th, the weight of an equal volume of air: 10th, 

 the resistance of the air due to the figure and velocity of the bird: 11th, the ratio of 

 the resistance which the air opposes to the wings during their elevation and depres- 

 sion : 12th, the ratio of the resistance of the air to the time of an elevation of the 

 wings and to that of a depression.' lb. p. 425. 



