PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 247 



toward a useful result, embarrassing the ueigbboriug parts, witliout com- 

 pensation of any kind. The membrane should not exist except when 

 vitality itself exists in a corresponding degree. Finally, the extent which 

 the alar membrane should have, to best utilize the disposable force, can 

 be determined experimentally. M. de Lucy has compared, in the case 

 of a certain number of animals, the surfaces of the wings to the total 

 weight of the body. He finds an extent of 30 square millimeters in a 

 gnat weighing 3 milligrams: 1,003 square millimeters in a butterfly 

 weighing twenty centigrams; 750 square centimeters in a pigeon weigh- 

 ing 290 grams ; 4,500 square centimeters in a stork weighing li,205 grams; 

 8,543 square centimeters in an Australian crane, weighing 9,500 grams. 

 But to facilitate the comparison it is necessary to reduce these figures 

 to a common measui"e ; and, in spite of the barbarous xthrases to vrhich 

 they lead us, we obtain : 



Srxnarc meters. 



The kilogram of the gnat represents 10. 



The kilogram of the butterfly represents 8. 



The kilogram of the pigeon represents 2, 580 



The kilogram of the stork represents 1. 988 



The kilogram of the Australian crane represents 0. 899 



The extent of the wings, therefore, is not proportionate to the size 

 of the animal. A wing being given, a maximum rapidity of stroke cor- 

 responds to it. To augment the rapidity of the stroke, in hope of indef- 

 initely accelerating the rate of flight, would be illusory ; it is possible 

 to accelerate it up to a certain point, but beyond this maximum limit 

 additions become useless. Increasing progressively the action of the 

 air-pump, the strokes of the wings are more rapid, and at first the rapidity 

 of flight will be augmented. Continue the increase, and the rate of flight 

 diminishes. The amplitude of the motion also experiences a consider- 

 able reduction, so that at the limit the wings appear motionless, or 

 animated only by a slight quivering. Passing this extreme limit, the 

 api)aratus retrogrades. A given wing then corresponds to a fixed rate of 

 progressive strokes; for, by the effect of inertia, the frequency of the 

 strokes is increased oul^- at the expense of their extent, and, when the 

 extent diminished, the propelling force diminishes with it. I leave to 

 yourselves the task of explaining these facts, which are the simple con- 

 sequences of the principles I have previously explained. 1 also leave to 

 you the comparison of the mode of progression of insects with theotlier 

 modes which are seen in other animals or in various mechanical contriv- 

 ances. You will discover almost everywhere the mechanism of the 

 revolution of forces on the principle of the inclined plane. You will find 

 it in the niotiou of the tail of a fish, the principal organ of its locomo- 

 tion ; in the sculling motion of a waterman's oar, and even in the screw 

 of a steam ]3ropeller. 



lY. Flight of bieds. 



By the simple inspection of a bird's wing it is easily seen tliat its 

 mechanism for flight is not the same as that of an insect. Let the 

 manner in which the feathers of birds are laid, one over another, be 

 observed, and it will be evident that the air resists the motion of the 

 wing only from below, so that in an inverse direction it finds an easy 

 passage between the long beards of the feathers, which, in this motion, 

 are no longer pressed together. This well-known arrangement, the 

 effect of which Precblt* has clearly pointed out, has led to tlie belief 



* Uutcrsuchungeu liber deu Flug der Vogel. 8vo. YiouDa, 184G. 



