344 Proceedings of the Poyal Society 
A diving bird which flies under the water is lighter than the 
zuater, and flies downwards. A bird which flies in the air is 
heavier than the air , and flies upwards. Relative levity and weight 
are therefore necessary to the diving and flying bird as at pre- 
sent constituted. 
Weight, when associated with or operating upon wings, con- 
tributes to horizontal flight. A flying animal, when it drops from 
a height with expanded motionless wings, does not fall vertically 
downwards, but downwards and forwards , the wings converting 
what would otherwise be a vertical fall of the body partly into 
forward travel. The weight of the body thus to a certain extent 
relieves the muscular system from excessive exertion. If a suffi- 
cient breeze be blowing, the weight of the trunk and the breeze 
upon the wings operating conjointly are sufficient to keep the 
body of the animal in the air for protracted periods. This is well 
seen in the case of the albatross, which can sail about for an hour 
at a time when there is wind without once flapping its wings. 
The wing, as a rule, is more flattened in the insect than in the 
bat and bird. It is, moreover, driven at a higher speed, those 
animals which fly the quickest having for the most part the 
flattest wings. The dragon fly furnishes a good example. 
The greater the concavity of the wing, the greater the elevating 
power ; the flatter the wing, the greater the propelling power. 
The wings in living animals are thoroughly under control both 
during the down and up strokes ; the wing, consequently, is not 
simply an elastic apparatus, which derives the movements of its 
separate parts from the air ; on the contrary, it directs and 
controls the air in such a manner as to extract the maximum of 
support and propulsion from it. 
The wings of bats and birds are moved by direct muscular action 
in combination with certain elastic ligaments, and the same holds 
true of the dragon fly and some other insects. The elasticity of 
the wing and the resiliency and reaction of the air, however, assist 
the muscles and ligaments. 
The great speed attained by the tip and body of the wing is due 
to the fact that the wing is articulated or jointed at its root, any 
movement communicated at the root being quickened in propor- 
tion to the distance from the root. In other words, a compara- 
