428 
dissimilar orders, the areas of the wings do not 
as the weights of the birds. 
e ratio of the times of the descent and as- 
cent of the wing will cause a corresponding 
difference in the ratio of the resistance of the 
air, which is not as the velocity simply, but as 
the square of the velocity. The velocity of the 
wing varies according to the celerity with which 
the bird moves, and it moves through a greater 
or less arc according as the bird merely sus- 
pends itself in the air or is in rapid motion, on 
the rational supposition that birds employ their 
locomotive organs in such a manner as to econo- 
mize as much as possible the expenditure of 
their muscular power. We find by the an- 
nexed analysis that for this purpose the ratio of 
the time of the descent of the wings to that of 
their ascent is nearly as one to two, and that the 
ratio of the resistance of the =p to that of the 
down stroke lies between one-half, one-fourth, 
and one-fifth. In the Swallow, for example, in 
order that the bird may merely sustain itself in 
the air, the centre of the wings, according to 
Chabrier, must descend with a velocity of about 
seven metres, or 22°9662944 feet per second, 
which, we find by the annexed analysis, gives 
15°3 for the number of vibrations, and for the 
minimum amount of action expended in the 
same time, a force which would raise its own 
weight to the height of 26°246 feet. 
The ratio of the time of the ascent and de- 
scent of the wing becomes much greater when 
the bird moves against the wind, suppose about 
forty-eight feet per second, or in rapid flight; 
and the velocity of the descent of the wings, 
and quantity of action expended, will augment 
in proportion. The great quantity of action ex- 
pended in flight tends to confirm the views of Bo- 
rellirespecting the vast power with which the pec- 
toral muscles of birds are endowed. In small 
birds the oscillations are performed with such 
great rapidity, that they cannot be numbered 
by the eye; but in the finches and humming- 
birds, the oscillations of which produce a musi- 
cal note, the pitch will enable us to determine 
with accuracy the number of oscillations in a 
given time. In large birds, the wings move 
through arcs of greater circles than in small 
ones, and the times of their periodic oscilla- 
tions decrease in the same ratio, and may thus 
be more easily numbered: the areas of their 
wings, and the resistance which they encounter, 
where the formula gives 7.38, and observation from 2 to 3 flappings per second. It is 
remark, that by supposing V to be equal to the cube root, instead of the square root of ? +P 3 
P-q # 
the number of fappings in each of the last two cases by the formula, agrees very closely 
by the mean of several observations. 2% 
The quantity of force expended would be greater if the density of the air were less, but 
number determin 
would only increase in the ratio of 1.4 to 1 if the air were but half as dense. We may, ther 
that the height to which a bird can raise itself is limited not so much by want of su 
conclude 
MOTION. 
bear some proportion to the greater weight o 
the body : bat adieeek theory ascribes to th 
wings a large number of oscillations, it by 
no means follows that they perform t 
exact number assigned at least for any leng! 
ened period. On the contrary, we observe 
that many birds, such as the Woodpecker,* 
and most Insessores, give a few strokes of the 
wings by which the body acquires a projec~ 
tile velocity sufficient to elevate it through a 
considerable space, and that when the im- 
pulse thus given is nearly expended, they re- 
peat this action, and again suspend it. If they 
ate moving horizontally, their is 
ek izes ina similar manner; the axis of the 
ird is inclined upwards at each impulse 
apa but the mean motion is horizontal. 
e curve described during each projection is — 
a parabola. After a few strokes, during th 
ascent, the wings are folded until the bird has 
passed the vertex of the curve, and has de 
scended to some distance on the opposite sid 
when they suddenly expand their wings again, 
and by a few strokes describe a new curve. — 
this mode of progression the velocity is ‘ 
variable, hiehae ual to that which a body 
would acquire by falling through one-fourth of 
the parameter of each point in the curve 
Many large birds, such as the Rooks, Pige 
&c. when descending from great heights 
iets their wings, and incline the axis of then 
ies obliquely downwards, as in fig. 222. 
“ 
Ls 
Fig. 222. 
\\ 
In this case the air opposes sufficient resistane 
in a vertical direction upwards to keep in equil 
brio the force of gravity acting upon the bad 
vertically downwards, so that the motion of th 
bird becomes uniform, without requiring an 
movement of the wings.+ Another mode of de 
‘ened 
ae 
support in the resistance of the air as by the difficulty of respiring in too rare an atmosphere. 
* The Rook appears to make from ten to fifteen, and the Pigeon from ten to twenty-three e! , 
strokes of the wing in five seconds. 
t+ The soft downy feathers which line the wings of the nocturnal rapacious birds, as the Owl, p 
the wings to perform its evolutions during flight in search of their prey without noise. On Y 
in the diurnal species of this order, which chase and capture thei 
secrecy would suffice, the feathers are strong, and their passage threugh the air is accompanied 
a rushing noise, 
a 
eir prey in open day, and where 
aj 
