PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



259 



I emphasize these effects because tbey are frequently produced in tlie 

 flight of birds. The okl treatises on falconry describe the interesting- 

 evolutions of the birds employed in hunting. Without going- back 

 further, we lind in Iluber (octavo, published at (leneva in 1784) a descrip- 

 tion of the curvilinear movements of the falcon, to which they gave the 

 name of j>rt6'6'«//e&, and which consisted in an oblique descent of the bird, 

 followed by a rise in its course. "The bird," says Huber, "when about 

 to strike the earth, carried away by its own rapidity, would be dashed 

 to i)ieces if it did not call into action a certain faculty which it possesses, 

 stronger than its descending motion, to rise even high enough to make 

 a second swoop. This motion is suhicient, not only to arrest its descent, 

 but even to carry it without eftort as high as the elevation from which 

 it came." 



Fiff. 15. 



~o~ 



The posterior corners of the paper have been 

 bent downward. After passing through a para- 

 bolic curve the object takes a very rapid desceud- 

 insj course. 



There is certainly exaggera- 

 tion in the statement that the 

 bird remounts as high as the ele- 

 vation from which it descended 

 without further eftort. The re- 

 sistance of the air must over- 

 come part of the force acquired 

 during the descent, and which 

 is transformed into ascending- 

 force. We see, however, that 

 the phenomena above described 

 is confirmed by observation, and 

 that it has been considered in 

 some sort as a passive act in 

 which the bird expends no mus- 

 cular power. The act of hover- 

 ing in st)me cases i)resents a 

 great analogy with the phenom- 

 ena just described. When some 

 birds, pigeons for instance, have 

 used their wings during a certain distance, the wings are seen to be 

 perfectly quiet during a few seconds gliding through the air, either 

 horizontally or rising or falling. The descending motion has the long- 

 est duration; in fact it is only an extremely prolonged descent in which 

 motion is maintained by the force of gravity, which diminishes it in the 

 horizontal or ascending plane. In these latter forms the wing, more or 

 less obliquely directed, takes hold on the air like the toy kite, with this 

 difference, that motion is imparted to this by pulling the string when 

 the air is calm, while the bird utilizes momentum previously acquired by 

 an oblique descent or previous strokes of the wings. 



I have already said that observers have admitted that certain birds, 

 which they call sailors, can sustain and direct themselves in the air by 

 means of the wind alone. This theory appears paradoxical. It is in- 

 comprehensible that a bird, motionless in the wind, shoidd not yield to 

 the resistance of the air through which it glides. If the passades or 

 swoops which the falcon executes can sometimes carry it against the 

 wind, this can only be a transient effect, compensated for by being car- 

 ried away by the wind more rapidly in another moment. However, 

 this theory has been sustained with great talent by some observers, 

 especially the Count d'Esterno, the author of a remarkable memoir 

 on the flight of birds. "Every one," he says, "can see some birds 

 practicing this method of flight; to deny it is to deny vself-evident 

 facts." I myself have noticed this mode of flying, but it has seemed 



