FIG. 125.--Flexible _valvular wing_ with india-rubber springs attached to its root.
_a_, _b_, Anterior margin of wing, tapering and elastic. _c_, _d_, Posterior margin of wing, elastic. _f_, _f_, _f_, Segments which open during the up stroke and close during the down, after the manner of valves. These are very narrow, and open and close instantly. _x_, Universal joint. _m_, Superior elastic band. _n_, Ditto inferior. _o_, Ditto anterior. _p_, _q_, Ditto oblique. _r_, Ring into which the elastic bands are fixed.--_Original._]
_How to construct a Wave Wing which shall evade the superimposed Air during the Up Stroke._--To construct a wing which shall elude the air during the up stroke, it is necessary to make it valvular, as shown at fig. 125, p. 241.
This wing, as the figure indicates, is composed of _numerous narrow segments_ (_f f f_), so arranged that the air, when the wing is made to vibrate, opens or separates them at the beginning of the up stroke, and closes or brings them together at the beginning of the down stroke.
The time and power required for opening and closing the segments is comparatively trifling, owing to their extreme narrowness and extreme lightness. The space, moreover, through which they pass in performing their valvular action is exceedingly small. The wing under observation is flexible and elastic throughout, and resembles in its general features the other wings described.
I have also constructed a wing which is self-acting in another sense. This consists of two parts--the one part being made of an elastic reed, which tapers towards the extremity; the other of a flexible sail. To the reed, which corresponds to the anterior margin of the wing, delicate tapering reeds are fixed at right angles; the principal and subordinate reeds being arranged on the same plane. The flexible sail is attached to the under surface of the principal reed, and is stiffer at its insertion than towards its free margin. When the wing is made to ascend, the sail, because of the pressure exercised upon its upper surface by the air, assumes a very oblique position, so that the resistance experienced by it during the _up stroke_ is very slight. When, however, the wing descends, the sail instantly flaps in an upward direction, the subordinate reeds never permitting its posterior or free margin to rise above its anterior or fixed margin. The under surface of the wing consequently descends in such a manner as to present a nearly flat surface to the earth. It experiences much resistance from the air during the _down stroke_, the amount of buoyancy thus furnished being very considerable. The above form of wing is more effective during the down stroke than during the up one. It, however, elevates and propels during both, the forward travel being greatest during the down stroke.