268 FACULTIES OF BIRDS. 
CHAPTER XXI. 
FLIGHT OF BIRDS. 
Tue flying of a bird in the air is performed on 
similar principles to that of a fish swimming in the 
water, with this difference, that the bird is more 
heavy, in comparison with the air, than the fish in 
comparison with the water. At first view, it might 
be thought impossible for so huge an animal as the 
Ostend whale (Balenoptera boops ?), weighing four 
hundred and eighty thousand pounds, to swim in 
the sea, considering that its body, se far as the 
bones and muscles were concerned, must have been 
considerably heavier than water. Yet, by a singu- 
lar contrivance, it is at once buoyed up in the sea, 
and rendered so much lighter than water, that it 
floats on the surface when dead. ‘This consists in 
an enormous layer of an oily substance called blub- 
ber, immediately under the skin. We are well jus- 
tified in using the epithet ‘‘ enormous,” from the 
fact that in the Ostend whale the blubber measured 
four thousand gallons, and weighed a twelfth of the 
whole body. With a large proportion, therefore, of 
a light body like this to buoy it up, so far from find- 
ing it difficult to swim, it would require a great ef- 
fort in order to dive deep into the water. 
In fishes (for the whale, being a warm-blooded 
animal, and breathing the air, is not considered a 
fish), the buoyancy indispensable for swimming is 
effected by a very different contrivance. A blad- 
der, varying in form in different species, is filled 
with air (azote in fresh water, and carbonic acid gas 
in marine, fishes), over which the animal appears to 
possess a voluntary power, either to empty it by 
