MOTION. 



429 



scent is performed with greater celerity by ele- 

 vating the wings at an angle of nearly 45 above 

 the plane of the horizon, (as in /g.223,) by which 



Fig. 223. 



a\ 



the resistance of the air, compared with the re- 

 sistance to the wing when horizontal, is dimi- 

 nished in the ratio of radius to the cube of the 

 sine of inclination, that is, as b to d c 3 ; con- 

 sequently, a bird with its wings elevated at any 

 angle to the horizontal plane will descend with 

 greater velocity than when they are in the direc- 

 tion of a b. We most frequently observe that 

 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, owing to the velocity ac- 

 quired 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 neu- 

 tralize their momentum, and thus reach the 

 ground with ease and safety. In order to pro- 

 duce lateral motion one wing oscillates more 

 rapidly than the other, thereby causing the 

 head to turn towards the side to which the 

 latter wing is attached. 



The tail of the bird performs the office of a 

 rudder in steering its course ; its plane being 

 horizontal tends chiefly, as Borelh has demon- 

 strated, to elevate and depress the head, ratiier 

 than to turn the axis of the bird laterally. Let 

 us, for instance, suppose that a bird, flying in 

 the direction of its axis gf, (fig. 224) elevates 



Fig. 224. 



its tail into the position b h parallel to o n, 

 the resistance of the air will depress b towards 

 A, and causing the bird to rotate on its centre 

 of gravity c will elevate the head from a to- 

 wards / ; on the other hand, if the tail be de- 

 pressed into the position b i, parallel to I k, by 

 the resistance of the air, the point b will be 



raised towards n, and the head depressed to- 

 wards o, consequently the direction of the bird 

 in its mesial plane is regulated by the tail.* 

 In the Grallatores the tail is short and its sur- 

 face very small, and the function of a rudder is 

 transferred to the legs, which are projected back- 

 wards inflight, to counterbalance the depressing 

 weight of their long extended neck and head 

 This fact was noticed by Aristotle.f The Swal- 

 lows, which are almost always upon the wing, 

 economise their muscular action by giving a few 

 strokes with their wings, and by keeping them 

 expanded, scud through the air with great velo- 

 city in chace of their prey ; this interval of 

 comparative repose must be of great service to 

 them during their annual migrations across the 

 sea to other countries. The velocity of some 

 birds is very great. The Eider-duck is said to 

 fly 90 miles in an hour, the Hawk 150. The 

 great Albatross wafts itself across the Pacific 

 Ocean apparently with untiring energy, owing 

 to the vast muscular power with which it is 

 endowed. 



Flight of fish and other animals. Besides 

 insects and birds, there are some other animals 

 capable of sustaining themselves during a short 

 period in the air by means of membranous ex- 

 pansions or enlarged pectoral fins. The Dac- 

 tilopterus and Exocatus, or flying fish, are en- 

 abled to raise themselves above the surface of 

 the water by the action of their enormous pec- 

 toral fins ; but Mr. Bennett, who appears to 

 have particularly observed their motions, states 

 that he has never seen these fishes sustain 

 themselves for a longer period than thirty se- 

 conds, nor ever witnessed any vibration in their 

 pectoral fins. Captain Basil Hall estimates their 

 longest flight at about two hundred yards, and 

 they have been sometimes known to rise above 

 the surface of the water as high as twenty feet. 

 The projectile force with which they emerge 

 from the water determines their elevation, and 

 the expanded pectoral fins merely sustain them 

 for a brief interval. 



The Draco Volans (fig. 225) is provided 

 with a broad disc on each side, extending from 

 the fore to the hinder extremities. It is covered 

 by the skin, supported by the first six false 

 ribs, and directed horizontally. This membra- 

 nous disc expands and closes like a fan, and is 

 elevated and depressed like the wings of birds 

 to break their fall in descending from trees, but 

 notwithstanding the extent and mobility of 

 their wings, they are said to be incapable of 

 raising themselves in the air, since their arms 

 are detached, and neither enter into the compo- 

 sition of the wings, nor assist in their elevation 

 or depression. 



The area of the wings of the Draco volans 

 (fig. 225)| estimated from the mesial section of 

 the body is nearly 5-052 square inches. Ano- 

 ther Draco volans which is preserved in spirits 



* See Rorelli, De motu Animal, c. 22, p. 235. 



t See Taylor's Aristotle's Progressive Motion of 

 Animals, c. x. p. 184. 



t In consequence of the specimen in the collec- 

 tion of Professor Grant, from which the annexed 

 outline was made, being dry, its weight in the 

 living state could not be ascertained. 



