_The Artificial Wing propelled at various degrees of speed during the Down and Up Strokes._--The tendency which the artificial wave wing has to rise again when suddenly and vigorously depressed, explains why the _elevator_ muscles of the wing should be so small when compared with the _depressor_ muscles--the latter being something like seven times larger than the former. That the contraction of the elevator muscles is necessary to the elevation of the wing, is abundantly proved by their presence, and that there should be so great a difference between the volume of the elevator and depressor muscles is not to be wondered at, when we remember that the whole weight of the body is to be elevated by the rapid descent of the wings--the descent of the wing being entirely due to the vigorous contraction of the powerful pectoral muscles. If, however, the wing was elevated with as great a force as it was depressed, no advantage would be gained, as the wing, during its ascent (it acts against gravity) would experience a much greater resistance from the air than it did during its descent. The wing is consequently elevated more slowly than it is depressed; the elevator muscles exercising a controlling and restraining influence. By slowing the wing during the up stroke, the air has an opportunity of reacting on its under surface. _The Artificial Wave Wing as a Propeller._--The wave wing makes an admirable propeller if its tip be directed _vertically downwards_, and the wing lashed from side to side with a sculling figure-of-8 motion, similar to that executed by the tail of the fish. Three wave wings may be made to act in concert, and with a very good result; two of them being made to vibrate figure-of-8 fashion in a more or less horizontal direction with a view to elevating; the third being turned in a downward direction, and made to act vertically for the purpose of propelling. [Illustration: FIG. 130.--Aërial wave screw, whose blades are slightly twisted (_a b_, _c d_; _e f_, _g h_), so that those portions nearest the root (_d h_) make a greater angle with the horizon than those parts nearer the tip (_b f_). The angle is thus adjusted to the speed attained by the different portions of the screw. The angle admits of further adjustment by means of the steel springs _z_, _s_, these exercising a restraining, and to a certain extent a regulating, influence which effectually prevents shock.