MOTION. 
scent is performed with greater celerity by ele- 
vating the wings at an angle of nearly 45° above 
the plane of the horizon, (asin fig.223,) by which 
Fig. 223. 
=>psd 
~- 
Fy eee: ee 
\ 
oy 
~~ 
_ 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 a b to d c3; 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 ab. 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- 
pipe 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 
pound 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 
‘Tudder in steering its course; its plane being 
horizontal tends chiefly, as Borelli has demon- 
_ Strated, to elevate and depress the head, rather 
_ than to turn the axis of the bird laterally. Let 
__ us, for instance, suppose that a bird, flying in 
the direction of its axis g f, (fig. 224) elevates 
Fig. 224. 
its tail into the position b A parallel to on, 
the resistance of the air will depress b towards 
k, 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 2, parallel to / k, by 
| the resistance of, the air, the point b will be 
Tien’. 
429 
raised towards m, and the head depressed to- 
wards 0, 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 in flight, to counterbalance the depressing 
weight of their long extended neck and head 
This fact was noticed by Aristotle.t 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 Exocetus, 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 i 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 hinderextremities. 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 Borelli, De motu Animal. c. 22, p. 235. 
+ 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. 
