44 THE FLIGHT OF BIRDS 



I will first give some account of a long and com- 

 plete stroke. Preparatory to the down-stroke, the 

 wing is raised till it points vertically upward, its front 

 margin being turned in the direction of the bird's 

 flight. There may then be a moment's pause, the 

 wing, as it were, resting before it strikes its blow. In 

 the case of gulls the next move seems to be a slight 

 bend at the wrist- joint. After this begins the serious 

 work. The wing descends with lightning speed, so 

 fast indeed that this early phase of the down- stroke 

 does not always appear in the series when photo- 

 graphy has succeeded in depicting all the other 

 phases. The great rapidity at this stage seems to 

 indicate that it is not till the wing is approaching the 

 horizontal that it begins to feel the resistance of the 

 air and do its work of lifting and propelling. When 

 this work is going on, the upward bending of the 

 primary- feathers leaves us in no doubt about the 

 fact. As the wing descends it points more and more 

 forward. The way in which this is brought about 

 is highly interesting. The big muscle which lowers 

 the wing attaches to the front part of the lower face 

 of the humerus (upper-arm bone) (see. Chap, vn, 

 fig. 21). Its pull, therefore, tends to lower the front 

 of the wing relatively to the hinder part by rotating 

 the bone. But the air, acting on the feathers that 

 spread out rearward, greatly aids the muscle, lifts 

 the hinder part of the wing, and encourages the rota- 

 tion. But the work of the air does not end here. 

 As soon as the wing has an upward incline from front 

 to back, it cannot but move forward ; the mere 

 action of the air on a surface so inclined cannot but 

 bring this about. Thus the upward incline from 



