THREE MODES OF FLIGHT 79 



Having got the bird well up into the air by 

 means of the uplifting power of the wings, it 

 now behoves us to investigate the secret of 

 forward momentum. 



Progression through the air, in my belief, is 

 acquired in three ways, viz., by undulating 

 flight on outstretched motionless pinions ; 

 " coasting " flight, and flight resulting from 

 wing action only ; the latter being the ordin- 

 ary flight of the generality of birds. 



Undulating flight on motionless pinions is 

 confined to " gliders," such as gulls and others. 

 Several species of the gull tribe, without 

 stirring their wings, are capable of following, 

 and even overtaking, a ship against a head 

 wind, and I have little doubt that a gull or 

 other " soaring " bird, on attaining a certain 

 altitude, has the power (not that it uses it) to 

 travel in this fashion indefinitely. Undulating 

 flight might be compared with the " switch- 

 back " system, only more so, because in the 

 bird's case, the impetus gained by the down- 

 ward slope, aided by the wind, suffices to 

 bring the " glider " back to the same level, or 

 even above it. This, however, is " gliding " ; 

 flpng directly through the air is another 

 matter. 



When ducks, geese, plover, etc., are seen 

 overhead, travelling to some distant spot, the 



