The albatross (fig. 105, p. 200) and pelican afford instances of the third class, embracing the heavy-bodied, long-winged birds. The duck (fig. 107, p. 204), pigeon (fig. 106, p. 203), crow and thrush, are intermediate, both as regards the size of the wing and the rapidity with which it is made to oscillate. These constitute the fourth class. The albatross (fig. 105, p. 200), swallow, eagle, and hawk, provide instances of sailing or gliding birds, where the wing is ample, elongated, and more or less pointed, and where advantage is taken of the weight of the body and the shape of the pinion to utilize the air as a supporting medium. In these the pinion acts as a long lever,[97] and is wielded with great precision and power, particularly at the shoulder. [97] _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.