422 THE EMPIRE OF THE AtR. 



forearm coincides witli that of the arm; there is au almost constant 

 relation between these two parts of the wing, bnt there is a decided 

 divergence in the i-elation «»f tliese two parts (taken togethei-) with the 

 liand. 



Each flying family is prox>ortioned somewhat differently ; Miese 

 j)ecnliarities should be ascertained by detailed and minute measure- 

 ments, but in their absence some general remarks may be made. 



We may cover all genera of soaring birds by the t\vo types we jiro- 

 ceed to specify. 



(1) Among soaring birds the general posture of their wings at an 

 acute angle of about lOCP (the summit of the angle being the beak) 

 corresponds with the shortening of the arm and fore-arm, and the 

 excessive lengthening of the hand. This covers rapid soarers, such as 

 sw^allows, martins, and in lesser degree, the kites, tish-hawks, etc. 

 Let us remark, by the way. that this kind of soaring necessitates a 

 ])owerfnl tail. 



(2) In the extreme ojjposite type, that of slow soaring birds with wide 

 wingSj the angle mostly affected is about 200°, and sometimes more, 

 and here we notice a diminution in the length of the hand, ami a great 

 development of the arm and forearm. The best type of this genus is 

 the vulture. In the latter genus the tail is generally small. 



There is however a family of sailing birds with narrow wings, 

 which has an exaggerated length of arm, fore-arm, and also, generally, 

 of the han<l. This is the family of the Pel lean Ida', comprising four or 

 five genera which are constant, paradoxal sailing birds. 



The tropical phteton soars wonderfully. The P^gyptian pelican is a 

 charming instance w'hich can be closely studied, for it is easy to see 

 him in full sail over the w\ater. But the frigate-bird is the 7ie j^??/s 

 ultra of soaring creation, the elief (Pceuvre of nature in that direction; 

 in this bird the lengthening is general, the arm, the forearm, the 

 hand, all are of enormous length; therefore the creature soars fast or 

 slow just as it chooses. It is perfect of its kind but of no use as a 

 model, for man can not imitate it. Let us therefore dismiss it in this 

 essay. 



Thus we have reached the fact that the two extreme types of soaring 

 flight are the martin and the Nubian vulture. The iirst presents its 

 wings at an acute angle, and this produces instability of equipoise 

 requiring great velocity, just like the velocipede which can not remain 

 upright unless it is in motion. 



Vultures, on the contrary, can spread their wings at a re-entering 

 angle. The two wings and the tail then furnish three points of 

 support, upon which the system is balanced. The relative x)ositions of 

 these three moveable sui)ports govern the motion and the speed. 

 Thereftu'e the variable jnisitions of tliese three supports produce all the 

 evolutions of those avian acrobats w'hich have been termed soarers or 

 sailers . The variation of the angles between these ])oints of support 



