222 



ANIMAL MECHANISM. 



exact, we choose them from the same family, in order to have 

 no difference between them except that of size, we shall find 

 a tolerably constant ratio between the weight of these birds 

 and the surface of their wings. But the determination of this 

 ratio must be based upon certain considerations which have 

 been long disregarded by naturalists. 



Mons. de Lucy has endeavoured to compare the surface of 

 the wings with the weight of the body in all flying animals. 

 Then, in order to establish a common unit between creatures 

 of such different species and size, he referred all these esti- 

 mates to an ideal type, the weight of which was always one 

 kilogramme. Thus, having ascertained that the gnat, which 

 weighs three milligrammes, possesses wings of thirty square 

 millimetres of surface, he concluded that in the gnat type 

 each kilogramme of the animal was supported by an alar sur- 

 face of ten square millimetres. 



Having drawn up a comparative table of measurements 

 taken in animals of a great number of different species 

 and sizes, Mons. de Lucy has arrived at the following re- 

 sults : 



From these measurements we obtain the following im- 

 portant consideration, that animals of large size and great 

 weight sustain themselves in the air with a much less pro- 

 portionate surface of wing than those of smaller size. 



Such a result plainly shows that the part played by the 

 wing in flight is not merely passive, for a sail or a parachute 

 ought always to have a surface in proportion to the weight 

 which it has to support ; but, on the contrary, when con- 

 sidered in its proper point of vie\v, as an organ which strikes 

 the air, the wing of the bird ought, as we shall see, to pre- 



