How birds fly, and what is the force employed to sustain and propel them, has been so often discussed that most would think the subject exhaustively explained and thoroughly understood. Yet it is doubtful whether such is the case, seeing that many now are inclined to ascribe the phenonemena of forward flight to the curvature of the surface of the wing. Another view, and a nearly opposite one, is this, that the wing's chief function is that of sustaining, and has but slight propelling property, and what propelling power the curvature of the under surface endows the wing with, is exerted in giving a forward course to the wing itself in its descent; this is the view that will be sought to be worked out in this paper.