MOTION. 
furnish a large surface for the attachment of the 
muscles which move them. The fore-arm, con- 
sisting almost solely of the radius, does not 
possess the power of pronation and supina- 
tion, which would tend to lessen the resist- 
ance of the air to the wing in flight; the 
hand rotates on the radius by abduction and 
adduction, as in birds, so that, when folded, 
the little finger lies along the outside of the ra- 
dius: the fingers, which are of great length, 
contribute to the expansion of the wing in 
flight ; the thumb, which is not enclosed by the 
interdigital membrane, terminates by a strong 
hook for prehension, and for suspending the 
animal when in repose. The wing, taking its 
commencement from the neck, extends to the 
arm, feet, and tail. The interfemoral membrane, 
when developed, has its margin supported by 
an osseous extension from the calcanium; this 
membrane serves to elevate and depress the 
axis of the animal, its functions in this respect 
being analogous to that of the tail of birds. 
The elastic though delicate membrane of 
which the wings are composed, gives its stroke 
upon the air a great mechanical effect. 
The proportion of the area of the wings to 
the weight of the body is greater in Bats than 
_ inmany species of birds, and nearly approaches 
that in the Lepidopterous Insects, consequently 
their power of flight is very considerable. 
Bats are capable of increasing the area of 
their wings during their descent, and of con- 
tracting them during their ascent by the alter- 
nate flexion, extension, abduction, and adduc- 
tion of their elbows, fingers, and hands; and 
they can also vary their velocity, and conse- 
quently the resistance of the air during the 
elevation and depression of their wings in the 
Same manner as birds. The ratios of the times 
and of the resistances during these move- 
ments of the wings, as likewise the number 
of their oscillations in a given time, may be 
“computed very nearly by the formula appli- 
cable to the flight of birds, but owing to the 
extensive area of their wings compared with 
their weight, their oscillatory movements are 
comparatively slow. Their power of flight pre- 
ponderates greatly over the force of gravity and 
the mass of their bodies, so that they are ca- 
eit of flying with great ease, even when 
en with one or two young ones. Their 
centres of gravity and magnitude lie beneath 
the axes of the articulation of the wings with 
the trunk, an arrangement which keeps them 
Steady during flight. In repose they suspend 
themselves by their hind feet to some elevated 
object, from which on being alarmed they can 
fly off instantly. Their inferior extremities pos- 
sess neither the length necessary to raise the 
body sufficiently to expand their wings, nor the 
power to project it vertically, like birds on 
taking flight; but by dropping suddenly from 
the ment of suspension, they are enabled to ex- 
pand their wings instantaneously and without 
obstruction in the air. Their velocity is so great 
that they can overtake and capture their insect 
food on the wing. 
The amount of force requisite for aerial pro- 
gression is so enormous, owing to the rarity of 
431 
thé atmosphere, that it would be impossible for 
a man to sustain himself in the air by means of 
his muscular strength alone, in any manner 
he is capable of applying it. It is calculated 
that a man can raise 13 25 lbs. avoird. to a 
height of 3.25 feet per second, and can conti- 
nue this exertion for eight hours in the day, he 
will therefore exert a force capable of raising 
381600 Ibs. in the day to a height of 3.25 feet, 
or 47700 lbs. to a height of 26 feet, which, ac- 
cording to Chabrier, is the height to which a 
bird would raise itself in one second by the 
force it is obliged to exert in order to sustain 
itself in the air. Now, if we suppose the con- 
ditions necessary for flight in man to be the 
same as for birds, and that a man whose weight 
is 150 Ibs. could concentrate the muscular 
power of a day’s labour into as short a period 
as the ac¢omplishment of this object required, 
we might find the time ¢, during which he 
could support himself in the air, from the fol- 
lowing equation :— 
150 ¢ = 47700, 
hence ¢ = 318”, or about five minutes. 
It is, however, impossible that a man could 
concentrate the force of eight hours’ labour into 
the short interval in which he would have to 
expend it when supporting himself in the air. 
The opinions of Borrelli and Chabrier agree with 
these views, and we are not so sanguine as to 
suppose with Bishop Wilkins, Sir G. Cayley, 
and others, that with the assistance of some 
mechanical contrivance men will some day be 
enabled to fly by the force of their muscular 
system. Such hypotheses, like the ancient 
stories of Dedalus and Icarus, &c. serve only 
to deceive the ignorant, amuse the credulous, 
and misdirect the human mind to attempt the 
accomplishment of impossible objects. 
Sect. III. Swimming.—In swimming, as 
in flying, the fulcrum which affords the requi- 
site resistance to the action of the locomotive 
organs of animals is the fluid medium in which 
they move, and as this medium yields to the force 
impressed on it by the organs, it is evident that 
these modes of locomotion are regulated by 
different principles from those applicable to 
animals whose progression is performed upon 
solids. 
The reaction of the water in swimming is 
equal to theaction impressed on it by the impulse 
of the locomotive organs ; and if motion ensues, 
it results from a surplus force in the body in 
motion, equal to the difference between the 
force of the locomotive organs, and the resist- 
ance of the medium. The motion is accelerated 
as long as the force of the locomotive organs is 
greater than the resistance of the medium react- 
ing against the surface of theanimal. When the 
mean forces urging the animal forwards and the 
resisting force are in equilibrio, the motion be- 
comes uniform. When these forces are at the 
maximum, the velocity is also at a maximum. 
If the weight of the animal be equal to that of 
the water it displaces, there will be no ten- 
dency to rise or sink, as the vertical force of the 
water upwards will be equal to the force of 
gravity upon the animal vertically downwards, 
and forces need only be employed to urge the 
