42 
xis to regard it with anything approaching indifference. The seem- 
ing impossibility of a heavy body supporting itself in mid-air, and 
with graceful and rapid motion gliding along, changing its direction 
at will, apparently violating all the known forces of nature, is 
sufficiently astonishing to attract the attention, and engage the re- 
searches of scientific men — and yet, till of late, this subject has been 
neglected or the theories formed to account for so remarkable a 
phenomenon, have been altogether erroneous. 
In attempting to ascertain the means by which a bird is enabled 
to rise into the air, it has been the custom to regard its weight as 
the chief impediment, and tire great stumbling-block to the arrival 
at the truth seems to have been the very natural idea that buoyancy 
was the first essential to flight, this however is now clearly shown 
not to be the case, so far from being an essential, too great a degree 
of buoyancy is an actual impediment, and weight is found in reality 
to be the very main-spring of flight. When Hunter discovered the 
presence of air-cells in the bones, and dispersed over various parts 
of the bird’s body, it was believed that by means of inflating these 
cavities with heated air, it was possible to increase its bulk, and at 
the same time decrease its weight • unfortunately however, additional 
bulk without a corresponding increase of weight (as will be shown 
presently) would only enlarge the surface presented to atmospheric 
resistance, rendering the too buoyant body of the bird a helpless 
object, the sport of every changing current of air with which it came 
in contact. If, as Sir Charles Bell'^' says, “ it is necessary that birds 
as they are buoyed in the air, be specifically lighter,” and this is to 
be accomplished by inflation with hot air, let us see what the result 
would be. Captain Hutton, in a most interesting jjaper “ on the 
Birds inhabiting the Southern Ocean,” printed in the “ Ibis,” for 
1865, (to which I shall have occasion to refer more than once) has 
given an admirable account of the flight of the albatross, and very 
cleai’ly pointed out the mechanical principles by which it is effected. 
He there shows that in order to bring the specific gravity of the 
albatross to that of the atmosphere, the air-cells in its body should 
contain 1,820 cubic feet of air, heated to 108 degrees — equal to a 
sphere of more than 15 feet in diameter ; “or, in other words, they 
must be 1,200 times the size of the body itself of the bird,” which 
he adds would give it, when flying, an Aldermanic appearance he 
* Bridgewater Treatise on the Hand, i>. 72. 
