MOTION. 



425 



without very sensibly increasing its weight. 

 The scapulars which have the least velocity 

 are shorter and weaker. The length and 

 strength of those feathers which contribute so 

 largely to increase the area, vary directly with 

 the velocities of that portion of the wing to 

 which they belong. The ratio of the area of the 

 wings of birds to their weight is by no means 

 constant. It is least in the Cursores, such as 

 the Ostrich, Cassiowary, and Emeu ; greater 

 in the Insessores, as the Raven, Crow, and 

 Humming-bird ; and greatestin the diurnal Rap- 

 tores, as the Eagle, Falcons, and Vultures. The 

 power of flight in birds, provided the muscular 

 forces vary in proportion, is as the areas of their 

 wings directly, and as their specific gravities in- 

 versely. Owing to the triangular figure of the 

 expanded wing, the area of its sections dimi- 

 nishes as their distance from the centre of gra- 

 vity increases,and from the same cause the areas 

 are in the inverse ratio of the velocities corres- 

 ponding to the distances of the sections from 

 the axis of motion. The centre of resistance 

 coincides very nearly with the middle of the 

 length of the wing from the shoulder-joint to 

 the tip. The wing is moved on the princi- 

 ple of the third order of levers, and the power 

 is applied sufficiently near the axis of motion 

 to produce a considerable relative velocity. 

 The power of the muscles acting on the wing 

 is increased in proportion to their mass, being 

 to those of the inferior extremity (according to 

 Borelli) in the proportion of more than three to 

 one, their absolute power in proportion to the 

 weight of the bird being as lO'OOO to 1. In 

 order to give the osseous framework the surface 

 and strength necessary for the attachment of the 

 great pectoral muscles which act on the wing, 

 the sternum is carinated, whereby its surface is 

 increased in proportion to the mass of the mus- 

 cular fibres which are inserted into it, and the 

 shoulder-joints are strengthened by the elonga- 

 tion of the coracoid processes, and the magni- 

 tude of the clavicles. The fixed condition 

 of the ribs and vertebral column tends also to 

 strengthen the thoracic sections of the bird, and 

 enables it to resist the enormous muscular force 

 applied to it during flight.* This brief outline 

 of the manifold adaptations necessary for aerial 

 progression will give some idea of the number 

 and complication of the elements which enter 

 into the composition of so small a fabric, and of 

 the enormous muscular power with which birds 

 are endowed, compared with their bulk and 

 weight. When a bird rises from the ground, 

 and at the moment it begins its flight, the first 

 impulse is communicated to its centre of gravity 

 by the sudden extension of the legs, as in leap- 

 ing. During this action the humerus is raised ; 

 the wings are unfolded and spread out hori- 

 zontally by the extension of the fore-arm, and of 



the carpal, metacarpal, and phalangeal bones. 

 The force resulting from the sudden extension of 

 the legs elevates the whole mass of the bird 

 above the plane of position, and the body thus 

 raised being deprived of the support previously 

 afforded would begin again to fall by its own 

 weight, as soon as the projectile force became 

 insufficient to sustain it, but the wings having 

 in the meantime been spread out to their fullest 

 extent, are made to descend with great velocity 

 by the contraction of the powerful pectoral 

 muscles. In consequence of the planes of the 

 wings being disposed either perpendicularly, or 

 obliquely backwards to the direction of their 

 motion, a corresponding impulse is given to the 

 centre of gravity. The resistance of the air to the 

 wings during their depression through a greater 

 or less arc of a circle becomes greater than the 

 force of gravity on the mass of the bird, together 

 with the resistance of the air on its body due to 

 its velocity, and consequently the bird rises. 

 During the ascent of the wing, the opposite 

 effect takes place. The bird has not only to 

 encounter the resistance which the air opposes 

 to the motion of the wing during its back stroke, 

 but also the resistance due to its figure when in 

 motion, as well as the force of gravity. These 

 are so many forces tending to neutralize the 

 down stroke of the wing, and to produce an 

 opposite effect, so that there will result a 

 motion, the amount of which may be pretty 

 nearly ascertained when the necessary data are 

 obtained by experiment. 



The principal data required for computing 

 the quantity of muscular action expended, the 

 velocity of the centre of the wing, and the num- 

 ber of its periodic oscillations necessary to sus- 

 tain the bird in the air, may here be briefly 

 stated. 1st. The area of the horizontal section 

 of the body of the bird. 2d. The area of the 

 two 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 velocity with which 

 the wings are lowered. 6th. The velocity with 

 which the wings are raised. 7th. The respec- 

 tive 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, llth The ratio 

 of the resistance which the air opposes to the 

 wings during their elevation and depression. 

 12th. The ratio of the resistance of the air to 

 the time of an elevation of the wings to that of 

 a depression. 



These are the principal data which are deemed 

 necessary for estimating by dynamics the amount 

 of the force employed by birds during their 

 flight.f If the area of the wings of birds always 

 preserved the same ratio to their weight, the 



* The position of the scapulo-hiimeral joint with respect to that of the great pectoral muscles throws 

 die centres of gravity and figure below the axis of articulation of the wings, so lhat the animal is 

 kept steady in flight, whilst its figure is such as to enable it to glide through the air with the least pos- 

 sible resistance. 



t Let us suppose the bird to endeavour to rise perpendicularly in the air by equal flappings of its 

 wings in a vertical direction, and let s = the area of the transvere section of the bird, A - the area 

 of the wings whilst they are depressed, A ' r= the area of the wings whilst they are elevated, u 

 the velocity with which the bird rises in the air, V = the velocity with which it depresses its 



