28 Natural and Artificial Flight. [January, 
five years ago, and although the Society has not, as a body, 
undertaken experimental investigation, individually there 
has been much done to prevent wasteful work, to point out 
essential principles, and the causes of failure hitherto. But 
the thanks of the Society are mainly due to MM. Marey and 
De Lucy, who, by their able investigations of the laws of 
natural flight, have done much to elucidate the subject. 
Aérostation may be considered under two heads. 1. Bal- 
looning, in which ascent is gained by means of a gas 
specifically lighter than air. 2. True flight, in which the 
acts of rising and suspension are due to expended force. 
There are two obvious reasons why balloons have not been 
successfully navigated. It is difficult to apply a directive 
force at the point of suspension of the balloon, while any 
force applied to the car merely serves to tilt the balloon. 
Again, a body to be propelled against a current of air, even 
that created by its own motion, must have a weight in pro- 
portion to its surface. This law will become apparent in 
endeavouring to throw a block of wood and a cube of paper 
to the same distance. 
It was for long generally supposed that birds were sus- 
pended or balanced by a certain volume of rarefied air 
confined in the lungs, bones, and feathers. But this ex- 
planation will not bear the least reflection, for (the density 
of the air being 781 times less than that of water) a bird 
weighing a kilogramme, together with its wings, cubes about 
4 decimetres, and can therefore only displace 4 decimetres 
of air, the weight of which is 5 grammes 20 centigrammes. 
Hence it follows that a raven weighing 1 kilogramme, to 
be supported in the air, should have a volume of at least a 
cubic metre. 
In considering the phenomena of flight it is necessary, ~ 
then, to observe the weight, or gravitating force, the surface 
or resistance of the air, and the force of projection. The 
point immediately presenting itself, under this consideration, 
is the determination of the relation of the extent of wing- 
surface to the weight of the bird. This question gave rise 
to much controversy amongst naturalists, until M. de Lucy 
undertook its settlement by a series of decisive measure- 
ments. He sought to establish a common unit between 
birds and insects in this matter, and, although here not 
perfectly successful, the experiments clearly proved that 
birds of large size and great weight are sustained by a much 
smaller proportional wing-surface than those of smaller 
size. If the wing is considered as the means of elevation, 
that is, as an instrument with which to strike the air, and 
