Note 97 — PROGRESSION IN OR THROUGH THE AIR. (continued 3)

Advantages possessed by long Pinions.—The long narrow wings are most effective as elevators and propellers, from the fact (pointed out by Mr. Wenham) that at high speeds, with very oblique incidences, the supporting effect becomes transferred to the front edge of the pinion. It is in this way “that the effective propelling area of the two-bladed screw is tantamount to its entire circle of revolution.” A similar principle was announced by Sir George Cayley upwards of fifty years ago. “In very acute angles with the current, it appears that the centre of resistance in the sail does not coincide with the centre of its surface, but is considerably in front of it. As the obliquity of the current decreases, these centres approach, and coincide when the current becomes perpendicular to the plane; hence any heel of the machine backwards or forwards removes the centre of support behind or before the point of suspension.”—Nicholson’s Journal, vol. xxv. p. 83. When the speed attained by the bird is greatly accelerated, and the stratum of air passed over in any given time enormously increased, the support afforded by the air to the inclined planes formed by the wings is likewise augmented. This is proved by the rapid flight of skimming or sailing birds when the wings are moved at long intervals and very leisurely. The same principle supports the skater as he rushes impetuously over insecure ice, and the thin flat stone projected along the surface of still water. The velocity of the movement in either case prevents sinking by not giving the supporting particles time to separate.