45 
tliought will show how erroneous is the conclusion. Without 
weight the object would be helpless, it might floai, but it could 
never /y; there would be no resisting force to form a fulcrum to 
Its movements; it would in fact be part of the atmosphere, and 
subject to it, wafted hither and thither without the means of 
guiding itself or directing its motions. » No bird,” says the Duke 
of Argyll, “ is ever for an instant of time lighter than the air in 
which it flies ; but being on the contrary always greatly heavier, it 
keeps possession of a force capable of supplying momentum, and 
therefore capable of overcoming any lesser force, such as the ordi- 
nary resistance of the atmosphere and even of heavy gales of 
wind. * Ihe bird being elevated in the air po.ssesses, in virtue of 
its weight, a force, always exerting itself in a downward direction, 
thereby producing motion, which if it has the power to control, will 
prove the mainspring of its flight. 
2nd. In order to counteract this downward force, the next 
property is called into request— viz.. Surface. The expanded wing 
of the bird is presented to the column of air perpendicular to 
itself, and a new law of nature comes into operation — that of 
atmospheric resishmce. This is not suflicient to counteract the 
force of gravity without some mechanical action on the part of the 
biril, but it would in a great measure break the force of the fall, 
causing It to descend in a series of zig-zags, as a sheet of paper 
falls from a balloon, or rooks descend from a great height to their 
roosting trees. Although in a vacuum the force of gravity is abso- 
lute, and all bodies of an equal mass fell through a given space in 
precisely the same period of time, and with a velocity which 
increases with the square of the time, yet, through the resistance 
of the air, aU bodies passing through it are light in proportion to 
their surface, and “ every body, wliatever its absolute weight or 
bulk IS, may become, in the air, as light as a feather, by a simple 
alteration of its form;”+ a bii-d, therefore, which with closed 
Avings would come to tlie ground with great force and increasing 
rapidity, by simply expanding its wings, so as to increase the sur- 
face offered to the resistance of the atmospliere, in a great measure 
overcomes tlie action of the well known force of gravitation. 'SVe 
should expect to find the surface increase in duo proportion to the 
weiglit of tlie animal to be supported in the air, but strange to say, 
* “ Reign of Law,” i«vge 130. f Dc Lucy, paragraph iii. 
