104 



ANIMAL LOCOMOTION. 



necessary degree of resistance and non-resistance. Although 

 the third order of lever is particularly inefficient when the 

 fulcrum is rigid and immobile, it possesses singular advantages 

 when these conditions are reversed, i.e. when the fulcrum, as 

 happens with the air, is elastic and yielding. In this case a 

 very slight movement at the root of the pinion, or that end 

 of the lever directed towards the body, is succeeded by an 

 immense sweep of the extremity of the wing, where its elevat- 

 ing and propelling power is greatest. This arrangement in- 

 sures that the large quantity of air necessary for propulsion 

 and support shall be compressed under the most favourable 

 conditions. 



It follows from this that those insects and birds are endowed 

 with the greatest powers of flight whose wings are the longest. 

 The dragon-fly and albatrcss furnish examples. The former 

 on some occasions dashes along with amazing velocity and 

 wheels with incredible rapidity ; at other times it suddenly 

 checks its headlong career and hovers or fixes itself in the air 

 after the manner of the kestrel and humming-birds. The flight 

 of the albatross is also remarkable. This magnificent bird, I am 

 informed on reliable authority, sails about with apparent un- 

 concern for hours together, and rarely deigns to flap its enor- 

 mous pinions, which stream from its body like ribbons to the 

 extent, in some cases, of seven feet on either side. 



The manner in which the wing levers the body upwards 

 and forwards in flight is shown at fig. 52. 



In this fig. / /' represent the moveable fulcra furnished by 

 the air ; p the power residing in the wing, and b the body 

 to be flown. In order to make the problem of flight more 

 intelligible, I have prolonged the lever formed by the wing 

 beyond the body (b), and have applied to the root of the wing 

 so extended the weight w w\ x represents the universal 

 joint by which the wing is attached to the body. When the 

 wing ascends, as shown atj?, the air (= fulcrum /) resists its 

 upward passage, and forces the body (5), or its representative 

 (w\ slightly downwards. When the wing descends, as shown 

 at y, the air (= fulcrum f) resists its downward passage, 

 and forces the body (&), or its representative {w'), slightly 

 upwards. From this it follows, that when the wing rises the 



