_3d_, The nature of the powder employed. _4th_, The necessity for adapting certain elastic substances to the root of the wing if in one piece, and to the root and the body of the wing if in several pieces. And, first, as to the manner of construction. Borelli, Durckheim, and Marey maintain that _the anterior margin of the wing_ should be _rigid_; I, on the other hand, believe that no part of the wing whatever should be rigid, _not even the anterior margin_, and that the pinion should be flexible and elastic throughout. That the anterior margin of the wing should not be composed of a rigid rod may, I think, be demonstrated in a variety of ways. If a rigid rod be made to vibrate by the hand the vibration is not smooth and continuous; on the contrary, it is irregular and jerky, and characterized by two halts or pauses (dead points), the one occurring at the end of the _up stroke_, the other at the end of the _down stroke_. This mechanical impediment is followed by serious consequences as far as power and speed are concerned--the slowing of the wing at the end of the down and up strokes involving a great expenditure of power and a disastrous waste of time. The wing, to be effective as an elevating and propelling organ, should have no dead points, and should be characterized by a rapid winnowing or fanning motion. It should reverse and reciprocate with the utmost steadiness and smoothness--in fact, the motions should appear as continuous as those of a fly-wheel in rapid motion: they are so in the insect (figs. 64, 65, and 66, p. 139). To obviate the difficulty in question, it is necessary, in my opinion, to employ _a tapering elastic rod_ or _series of rods_ bound together for the anterior margin of the wing. If a longitudinal section of bamboo cane, ten feet in length, and one inch in breadth (fig. 117), be taken by the extremity and made to vibrate, it will be found that a wavy serpentine motion is produced, the waves being greatest when the vibration is slowest (fig. 118), and least when it is most rapid (fig. 119). It will further be found that at the extremity of the cane where the impulse is communicated there is _a steady reciprocating movement devoid of dead points_. The continuous movement in question is no doubt due to the fact that the different portions of the cane reverse at different periods--the undulations induced being to an interrupted or vibratory movement very much what the continuous play of a fly-wheel is to a rotatory motion.