224 THE STRUCTURE AND LIFE OF BIRDS cuHap, 
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—eg. 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 
Fic. 61 (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 
