FORM OF THE BIPwD. 



221 



certain birds which they called sailing birds " could sustain 

 and direct themselves in the air solely by the action of the 

 wind. This theory has all the appearance of a paradox ; we 

 cannot understand how the bird, when in the wind, and using 

 no exertion, should not be affected by its force. 



If the passadeSj or the changes which it effects in the plane 

 of its wings, can sometimes carry it in a direction con- 

 trary to that of the wind, these can be only transient effects, 

 compensated afterwards by a greater force driving them 

 before the wind. 



Nevertheless, this theory of sailing flight has been advo- 

 cated with great talent by certain observers, and especially by 

 Count d'Esterno, the author of a remarkable memoir on the 

 flight of birds. 



Every one,'' says this author, must have seen certain 

 birds practise this kind of sailing flight ; to deny it, is to 

 contradict evidence." 



We know so little yet of the resistance of the air, especially 

 with reference to the resolution of this force when it acts 

 against inclined planes under different angles, that it is im- 

 possible to decide on this question as to sailing flight. It 

 would be rash absolutely to condemn the opinion of observers, 

 by depending on a theory or on notions as vague as those 

 which we possess on this subject. 



Ratio of the surface of the wings to the weight of the hody. — 

 One of the most interesting points in the conformation of birds 

 consists in the determination of the ratio borne by the surface 

 of the wings to the weight of the bird. Is there a constant 

 relation between these two quantities ? This question has 

 been the cause of many controversies. 



It has already been shown that, if we compare birds of 

 different species and of equal weight, we may find that some 

 have their wings two, three, or four times more extended 

 than the others. The birds with large wing surfaces are 

 those which usually give themselves up to a kind of hoveriDg 

 flight, and have been called sailing birds ; while those whose 

 wing is short or narrow are more usually accustomed to a 

 flight which resembles rowing. If we compare together 

 two rowing'' or two ^'sailing" birds; if, to be more 



