PROGRESSION IN OR THROUGH THE AIR. 



169 



distance, or throws its body into the air by a sudden leap, 

 the wings being simultaneously elevated. When the body is 

 fairly off the ground, the wings are made to descend with 

 great vigour, and by their action to continue the upward 

 impulse secured by the preliminary run or leap. The body 

 then falls in a curve downwards and forwards ; the wings, 

 partly by the fall of the body, partly by the reaction of the 

 air on their under surface, and partly by the shortening of 

 the elevator muscles and elastic ligaments, being placed above 

 and to some extent behind the bird — in other words, elevated. 

 The second down stroke is now given, and the wings again 

 elevated as explained, and so on in endless succession ; the 

 body falling when the wings are being elevated, and vice 

 versa (fig. 81, p. 157). When a long- winged oceanic bird 

 rises from the sea, it uses the tips of its wings as levers for 

 forcing the bod}" up ; the points of the pinions suffering no 

 injury from being brought violently in contact with the 

 water. A bird cannot be said to be flying until the trunk is 

 swinging forward in space and taking part in the movement. 

 The hawk, when fixed in the air over its quarry, is simply 

 supporting itself. To fly, in the proper acceptation of the 

 term, implies to support and propel. This constitutes the 

 difference between a bird and a balloon. The bird can 

 elevate and carry itself forward, the balloon can simply elevate 

 itself, and must rise and fall in a straight line in the absence 

 of currents. When the gannet throws itself from a cliff, the 

 inertia of the trunk at once comes into play, and relieves the 

 bird from those herculean exertions required to raise it from 

 the water when it is once fairly settled thereon. A swallow 

 dropping from the eaves of a house, or a bat from a tower, 

 afford illustrations of the same principle. Many insects 

 launch themselves into space prior to flight. Some, however, 

 do not. Thus the blow-fly can rise from a level surface when 

 its legs are removed. This is accounted for by the greater 

 amplitude and more horizontal play of the insect's wing as 

 compared with that of the bat and bird, and likewise by the 

 remarkable reciprocating power which the insect wing pos- 

 sesses when the body of the insect is not moving forwards 

 (figs. 67, 68, 69, and 70 p. 141). When a beetle attempts 



