44 
instanced by Sir Charles Bell, and which he tells us the difference 
of a full meal is sufficient to out-weight, and prevent from rising 
until it has disgorged, probably owes its helpless condition to quite 
another cause, namely the stupor attendant upon repletion, more or 
less observable in all birds of prey, and not the mere weight of its 
meal. 
Of late years the possibdity of man being enabled to navigate the 
air by means of suitable machinery, has been seriously discussed by 
many men of undoubted learning and ability, particularly in France, 
and, as might have been expected, their attention has been given in 
the first instance, to the study of the laws which govern the flight 
of animals, and the mechanical principles involved. The late Duke 
of Argyle was engaged for many years in studying the same subject, 
and his son has given us the result of his investigations in a 
masterly chapter on flight, in “ The Keign of Law.” Thanks to the 
patient and careful investigations of modern scientific men, “ The 
way of an eagle in the air ” is no longer a mystery — its wonderful 
powers can be dissected, as it were, and the various contending 
forces called into action, to produce the phenomenon of flight, 
examined separately — strange as it may appear at first sight, we shall 
find it to be weight, and not buoyancy, which forms the motive power. 
M. de Lucy, a French Doctor of Medicine, has written an elabo - 
rate paper, “ On the Flight of Birds, of Bats, and of Insects, in 
reference to the subject of Aerial Locomotion,” (a translation of 
which has been made by Dr. Fox, of Scarborough,) in which he 
has shown that three great properties are absolutely essential to all 
winged animals : First, weight — -or the force of gravity. Second, 
surface — or the area presented to atmosjjheric resistance; and 
third, force . — or the power of projection inherent in the animal, 
and exercised by it. On each of these it wdl be necessary to 
dwell a short time, in order thoroughly to understand them. 
1st. Weight, or gravity. “The idea of a weight, even very 
light, suspended and moving itself in the air without any ajjparent 
support — without the least supporting material — seems so little in 
accordance with the daily experience of men, that we do not 
willingly admit the fact of the bird possessing weight.” * No 
wonder that so simple a solution of the difficulty, as what may bo 
called the hot-air thegry, was universally accepted ; but a littlo 
* De Lucy, “ Flight of Birds,” &c., paragraph iv. 
