8o THE BIOLOGY OF BIRDS 



ascend and descend a little in regular alternation. An 

 argument against the theory may be found in the fact that 

 vultures circle and circle, hour after hour, at great heights 

 where the inequality of the velocity of currents will be less 

 marked than in closer proximity to the earth. 



(7) It is possible that very slight strokes occur, such as 

 an oarsman, having attained a great speed, might give with 

 the tips of his oars. For it seems hazardous to dogmatise 

 as to what may be happening in the case of vultures at an 

 immense height. The telescope shows that there are no 

 visible strokes in the ordinary sense, but a very little might 

 go a long way. In sailing birds the tips of the primaries 

 project separately from one another, and their tilt is subject 

 to continual alteration. Again, it must be remembered 

 that the under-surface of the bird's wing is not like the 

 inside of a dish-cover ; it is a very complex curved surface, 

 and it is not rigid. The possibility of very minute strokes 

 and of alterations in the curvature of the under-surface of 

 the wing will be rejected by most ornithologists. But we 

 would point out that it is not inconsistent with the fact 

 that, at certain parts of their wheeling, vultures have been 

 seen putting on a brake as if they were going too fast. 

 There may be velocity to spare one minute, and not more 

 than enough the next. 



(8) Another possibility not to be left out of account is 

 that vortex movements of the air, which may be partly due 

 to the bird itself, may play a part in sustaining what comes 

 so near the modern " glider." 



§ 6. Velocity of Flight 



The frictional resistance which slows the movement 

 of a ship through the water has a very slight influence on 

 the bird in flight, and when the bird has got thoroughly 

 going the tendency to sink is counteracted by the horizontal 

 velocity. As Professor Roy notes : "Of the greatest 

 importance for the flight of birds is the fact that the resistance 

 offered to the motion of a flat body in a direction at right 



