DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 435 



in some direction, there being no dead point either at the end of the down or 

 up strokes. As a consequence, the curves made by the wing during the clown 

 and up strokes respectively, run into each other to form a continuous waved 

 track, as represented at figs. 13, 14, and 15, pages 342, 344, and 345. A con- 

 tinuous movement begets a continuous buoyancy, and it is quite remarkable to 

 what an extent, wings constructed and applied to the air on the principles 

 explained, elevate and propel — how little power is required, and how little of 

 that power is wasted in slip. 



If the piston, which in the experiment described has been working vertically, 

 be made to work horizontally, a series of essentially similar results are obtained. 

 When the piston is worked horizontally, the anterior and posterior elastic bands 

 require to be of nearly the same strength, whereas the inferior elastic band 

 requires to be much stronger than the superior one, to counteract the very 

 decided tendency the wing has to fly upwards. The power also requires to be 

 somewhat differently applied. Thus the wing must have a violent impulse 

 communicated to it when it begins the stroke from right to left, and also when 

 it begins the stroke from left to right (the heavy parts of the spiral line repre- 

 sented at fig. 8, page 340, indicate the points where the impulse is communi- 

 cated). The wing is then left to itself, the elastic bands and the reaction 

 of the air doing the remainder of the work. When the wing is forced by the 

 piston from right to left, it darts forwards in a double curve, as shown at fig. 

 70, the various inclined surfaces made by the wing with the horizon changing 

 at every stage of the stroke. 



Fig. 70.* • Fig. 71. t 



At the beginning of the stroke from right to left, the angle made by the 

 under surface of the wing with the horizon (x x') is something like 45°, whereas 

 at the middle of the stroke it is reduced to 20° or 25°. At the end of the stroke 

 the angle gradually increases to 45°, then to 90°, after which the wing suddenly 

 turns a somersault, and reverses precisely as the natural wing does at e, f, g of 

 figs. 3 and 5, page 338. The artificial wing reverses with amazing facility, and 

 in the most natural manner possible. The angles made by its under surface 



* Fig. 70. Stroke of artificial wave wing from right to left, x, x', Horizon, in, n, o, Wave track described by 

 wing from right to left, j), Angle made by wing at beginning of stroke, q, Ditto, made at middle of stroke, b, Ditto, 

 towards end of stroke, c, Wing in the act of reversing ; at this stage the wing makes an angle of 90° with the horizon, 

 and its speed is less than at any other part of its course, d, Wing reversed, and in the act of darting up to u, to begin 

 the stroke from left to right (vide u of fig. 71). 



+ Fig. 71. Stroke of artificial wave wing from left to right, x, x', Horizon, u, v, w, Wave track described by 

 wing from left to right, t, Angle made by the wing with the horizon at beginning of stroke, y, Ditto, at middle of 

 stroke, z, Ditto, towards end of stroke, r, Wing in the act of reversing ; at this stage the wing makes an angle of 90° 

 with the horizon, and its speed is less than at any other part of its course, s, Wing reversed, and in the act of darting 

 up to in, to begin the stroke from right to left (vide in of fig. 70). 



VOL. XXVI. PART II. 5 U 



