4-4 THE FLIGHT OF BIRDS 
I will first give somgraccount 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. vi, 
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 
