MOTION. 
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 greatest in the diurnal Rap- 
tores, as the Eagle, Falcons, and Vultures. The 
wer of flight in birds, provided the muscular 
rces 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- 
_ yity 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 
e 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 10000 to1. In 
_ order to give the osseous framework the surface 
and strength necessary for the attachment of the 
— 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 lea 
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 position of the scapulo-humeral joint with 
425 
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 are 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. 11th. 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.+ If the area of the wings of birds always 
preserved the same ratio to their weight, the 
respect to that of the great pectoral muscles throws 
the centres of gravity and figure below the axis of articulation of the wings,so that 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. 
+ 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’= the area of the wings whilst they are elevated, wu == 
the velocity with which the bird rises in the air, V = the velocity with which it depresses its 
