2-4 THE STRUCTURE AND LIFE OF BIRDS chap. 



movement is, as we should expect, downwards. The 

 broadjumper, as he rises and descends, describes in 

 the air a curve not unlike the curves of the bird's 

 trajectory. Many of our small birds — e.g. the House- 

 sparrow — may be seen as they fly to take a regular 

 jump upward, and then with outstretched wings glide 

 onward and slightly downward. The centre of gravity 

 of course rises and falls, but not so much as a particular 

 point in the bird — the eye, for instance ; for when the 

 bird rises his wings are lowered, and this lowers the 



Fig. 6i (after Marey). 



The space between the two straight lines shows the limits of the rise and fall of the 

 eye. The cross shows the position, according to calculation, of the centre of gravity 

 at the moments when the wing is highest or lowest. 



centre of gravity ; it is at a higher point in the bird 

 when he sinks, for at this moment his wings are raised 

 aloft. Thus the centre of gravity moves along in an 

 undulating line, but its oscillations are not so great as 

 those of the eye, since every rise and fall of the bird 

 is accompanied by a fall and rise of the centre of 

 gravity. 



Long Distance Flight. 



Long wings are best suited for long distances. 

 With a single short stroke they send the bird far on 



