1893.] The Flight of Birds. 21 
bird, as thick as the perpendicular distance between the front 
and rear edges of the wings, and say fifty feet long, is driven 
bie a FOP 
tor Np A 
out of the way in one second of time. This is done, as above 
stated, in both cases, but the way of doing it is the vital - 
matter. It is driven out by gnand not by f. Suppose f should 
stop acting. Motion of bird would then be downward on the 
normal line, in which motion gn would do the same work in 
one second as before. Motion both ways goes on simultan- 
eously, neither having the slightest effect upon the other, for 
the reason that they are 90° apart, and rectangular forces do 
not affect each other, the bird moving under each force as if. 
the other were not acting. The wav in which the air is driven 
out is determined by f, but gn does all the work. The law of 
meer ee eee ere 
om> 
fluid reactions throws all the air resistance to the bird’s motion 
around, normal to the plane of its wings; gn then forces it 
down, while f keeps the bird level. 
There is yet a very important point to be understood. In 
methods sanctioned by mechanical authority weight and air 
resistance are added in pressure. Both are assumed to be 
resistance to the soaring force. They should be subtracted, 
