BIRD-FLYING. 
57 
difcover that their flying conflfts mainly of failing. The 
weight of the bird’s body that impels it down the inclined 
plane of the Iky fends it up again by the momentum thus 
acquired, aided generally by a few vigorous flrokes of its 
wings; and the rufh of the wing-furface of the bird over 
the air-fpaces it traverfes, amounts pra&ically to the fame 
thing as a wind or air-current rufhing under the bird. 
Swallows are fplendid examples of projedtile-force flying ; 
the momentum acquired in their flight enabling them 
oftentimes to rife perpendicularly for feveral feet; the 
fhape of the bird, and its wings, being for this mode of 
flight marvelloufly adapted. Its power too, to increafe 
or leflen the tendency of its body to fall, by the opening 
and doling of its wings and tail is wonderful; its 
movements in doing this are like flalhes of lightning. 
What the fwallow is able to perform in flill air, by the 
celerity of its movements on fmall wings, the albatrofs, 
the condor, and the eagle accomplifh amid powerful 
wind-forces with their greater weight and fpread of wings 
by flow and ealily fuftained movements and at almoft no 
expenditure of their motive-power. 
No better illuftration than the following can be given 
of the ufe made by birds of the force-denlity of the 
atmofphere, and of its neceflity to them under certain 
conditions. 
When a flock of geefe, feeding on the commons (a 
cuftomary thing in New England, U.S.A.) feel an incli- 
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