44 The Fli(jlil and Anatomy of Birds. 



wings, measuring in superficial extent in some cases several of such 

 kites, with the additional difficulties, which mathematical knowledge 

 would prove to be proportionally hicreased at every step in his 

 progress." 



The flight of birds differs exceedingly ; some fly with jerks, 

 closing their wings every three or four strokes, which gives them 

 an undulating motion, as the woodpeckers, the wagtails, and in fact 

 most small birds: others fly smoothly and evenly, as pigeons and 

 partridges ; others again seem to buoy themselves up in the air 

 without perceptible motion of the wings, as the kite and kestrel 

 hawk. Most birds fly with their legs drawn up and their necks 

 stretched out, but there are some whose length and weight 

 of neck makes it necessary for them to contract it in flight, and in 

 order to keep up their balance, to stretch out their legs behind also. 

 Such are the heron and the bittern. Others fly with the neck ex- 

 tended, but still are obliged to throw out their legs behind, as the 

 ducks and geese, and most of the water fowl. 



There are various ways of discovering the rapidity of a bird's 

 flight; the most practicable method of course is by noticing the 

 number of seconds in which a bird flies over a certain space, the 

 length of which can be ascertained, and then by a common pro- 

 portion sum it can easily be discovered at the rate of how many 

 miles an hour it flies. In America there is a curious way of gues- 

 sing at speed of flight; birds have been shot with coffee berries 

 in their stomachs so fresh that they cannot have been eaten more 

 than three or four hours, and as the nearest coffee fields were known 

 to be some hundreds of miles off, it was calculated that they must 

 have flown at the rate of sixty or seventy miles per hour. An eagle 

 was once watched while crossing a valley in the Pyrenees, and the 

 number of seconds noted which elapsed between its leaving one 

 lofty peak and settling on the brow of a mountain opposite, and 

 the distance between these two heights being known it was calcu- 

 lated that the bird flew at the rate of one hundred and forty miles 

 per hour. But extraordinary as this rate may appear, Audubon 

 calculates the flight of the large white fishing eagle of America to 

 be even very much faster than the one just mentioned; he says, 

 " On perceiving their prey they glide downwards from an immense 

 height with such rapidity as to cause a mighty I'ushing sound, not 

 unlike that produced by a violent gust of wind passing through the 

 branches of trees ; and the fall of this bird, enormous as it is, can 

 scarcely be followed by the eye." But we need not look abroad 

 only for instances of great rapidity of flight; among our own 

 commonest birds there are two of extraordinary rapidity, viz. the 

 swallow and the swift, the flight of the swallow has been com- 

 puted at ninety, that of the swift at the extraordinary rale of one 



