20 The American Naturalist. (January, 
THE FLIGHT OF BIRDS. 
By I. LANCASTER. 
I have been asked what effect the application of soaring 
methods has upon active wing flight. If soaring goes on 80 
easily where gravity is the motive power, why do not all birds 
soar? o 
The soaring activity is not understood when such a question 
is put, obscurity arising, doubtless, from misconception. 
The usual statement of soaring held by everybody, and 
especially by mechanical authority, is diagrammed in 1, 
where g represents weight, w, horizontal air resistance, and p, 
Pate Sree E 
én A? (1) 
normal pressure. When a horizontal force, f, is applied, of 
sufficient magnitude to cause p to equal both g and w, the — 
plane moves on the horizontal path of soaring flight. Soar- 
ing, or indeed any sort of bird flight, would be impossible — 
thus stated, which may be called the formula method. | 
The soaring statement presented by nature, is seen in 2, 8 
where the horizontal force, f, and the horizontal resistance, W, a 
are stricken out. Vertical g is replaced by gn and gp, while p- 4 
remains the same in direction but not in magnitude. The — 
pressure plane of air throws g out of vertical, making gn and — 
gp out of it; p is equal to gn; fis equal to gp, and the plane ‘ 
soars; f pushes gp up as fast as gn pushes p down, producing — 
a horizontal resultant. a 
Compare both statements with 3. In either case a prism of 
air as wide as the distance across the extended wings of the — 
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