510 POWER OF FLIGHT POSSESSED BY BIRDS. 



during the down-stroke ; otherwise the effect of the former 

 would neutralise that of the latter. This is partly accom- 

 plished by the great velocity of the down- stroke compared 

 with the up-stroke, which causes the resistance of the air to be 

 much greater against the former than against the latter. 1 But 

 it is by the alteration in the surface of the wing, as it acts upon 

 the air, that the chief difference is made in Birds ; the arrange- 

 ment of their great feathers being such, that they strike the air 

 with their flat sides, but present only their edges in rising. 

 What is called " feathering the oar " in rowing, is a similar 

 operation, performed with the same intention, and deriving 

 its name from this resemblance. 



672. The degree in which the wings act in raising the body 

 or in propelling it through the air, varies considerably in 

 different species, according to the way in which they are set. 

 Thus in Birds of Prey, which require a rapid horizontal 

 motion, the surface of the wings is very oblique, so that they 

 strike backwards as well as downwards, and thus impel the 

 body forwards whilst sustaining it in the air. Such birds find 

 a difficulty in rising perpendicularly ; and can in fact only do 

 so by flying against the wind, which then acts upon the 

 inclined surface of the wings just as it does upon that of a 

 kite. On the other hand, the Lark, Quail, and such other 

 birds as rise to great heights in a direction nearly vertical, 

 have the wings so disposed as to strike almost directly dowTi- 

 wards. It has been estimated that a Swallow, when simply 

 sustaining itself in the air, is obliged to use as much force to 

 prevent its fall, as would raise its own weight to a height of 

 about twenty-six feet in a second. Hence, we may form some 

 idea of the enormous expenditure of force which must take 

 place, when the body is not only supported, but raised and 

 propelled through the air. The Eider-duck is said to fly 

 90 miles in an hour, and the Hawk 150. The Swallow 

 and Swift pass nearly the whole of the long summer days 

 upon the wing, in search of food for themselves and their 



1 This resistance varies as the square of the velocity of the stroke. 

 Hence, if the down-stroke be made three times as fast as the up-stroke, 

 the resistance it experiences will be nine times as great. But as this 

 only operates during one-third of the time, it is in effect equal to three 

 times that which operates against the up-stroke, and which would tend 

 to lower the Bird in the air. 



