MOTION. 



431 



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 

 interdigilal 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 

 in many 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- 

 pable of flying with great ease, even when 

 laden 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 point 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 



the 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 Ibs. 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 Ibs. 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 accomplishment of this object required, 

 we might find the time t, during which he 

 could support himself in the air, from the fol- 

 lowing equation : 



150 t = 47700, 

 hence t = 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 O. 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 Deedalus 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 

 orga'ns 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 



