116 



BRITISH BIRDS. 



horizon, and the wind due to his own velocity acts at 

 right angles to the plane, offering much support and very 

 little resistance. Great velocity is in all cases essential. 

 When a Rook flaps leisurely along, yet the pace at which 

 the extremities of his wings move is very great. 



The next point to grasp is that the downward movement 

 of the wings is able not only to lift the bird but to propel 

 him. In rapid flight the wing is first raised high aloft. 



Fig. 1. — Lesser Black-biieked Gulls off Mogador. 



and then its movement is at once downward and forward, 

 the front margin being all the while lower than the hinder. 

 The force of the air impinging against the wing acts at 

 right angles to it, and, the wings having the incline I 

 have described, the bird is not only lifted but urged forward. 

 In fact the parallelogram of forces is energetically at 

 work. The force acting at right angles to the wing is 

 resolved into two — one lifting, the other j)ropelling. On 

 this resolution of one force into two all flight depends. 



