DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 441 



blade strikes varies, the angles being always greatest towards the root of the 

 blade and least towards the tip. The angles made by the different portions of the 

 blade are diminished in proportion as the speed with which the screw is driven 

 is increased. The screw in this manner is self-adjusting, and extracts a large 

 percentage of propelling power with very little force and surprisingly little 'slip. 

 A similar result is obtained, if two finely graduated angular-shaped steel 

 plates be placed end to end and applied to the water (vertically or horizontally 

 matters little), with a slight sculling figure of 8 motion, analogous to that 

 performed by the tail of the fish, porpoise, or whale. If the thick margin of the 

 plates be directed forwards, and the thin ones backwards, an unusually effective 

 propellor is produced. This form of propellor is likewise very effective in air. 



EXPLANATION OE THE PLATES. 



Plate XL 



Figures 1, 2, and 3 show how the wing of the gull is elevated and extended towards the termination 

 of the up stroke to prepare it for making the down stroke. At figure 3 the wing is repre- 

 sented as folded upon itself, and in the act of being elevated. It is, therefore, elevated as 

 a short lever, the resistance experienced from the superimposed air being thus greatly 

 diminished. The wing acts as a short lever from the time it leaves the position indicated 

 by 6 of figure 6 until it assumes the position indicated by o of figure 3. At figure 2 the 

 wing is raised higher than in figure 3, and partly extended — the elevation and extension 

 of the wing occurring simultaneously. At figure 1 the wing is fully elevated and fully 

 extended, and, consequently, ready to make the down stroke. It descends as a long lever, 

 Avith great energy, until it assumes the position indicated by 4 of figure 6. The resistance 

 which the wing experiences from the air beneath, is consequently, very great, the buoying 

 power of the wing bearing a fixed relation to the resistance in question. The under sur- 

 face of the wing, Avhen in the position represented at figure 3, makes a very slight angle 

 with the horizon b d. This arises from the fact that the different portions of the wing, 

 when the wing is folded upon itself, are on nearly the same plane. The angle or angles 

 — for they are numerous — made by the under surface of the wing with the horizon become 

 larger when the wing is partly extended, as shown at figure 2 : bd, representing the horizon, 

 and c b d the angle which the root of the wing makes with it. The angles become still 

 larger when the wing is fully extended, as a comparison of c b d of figure 1 with c b <I of 

 figure 2 will show. The under surface of the wing, it will be observed, makes a variety 

 of inclined surfaces with the horizon while the pinion is being extended. The angles of 

 inclination made by the iuclined surfaces in question are increased and diminished by the 

 ascent or descent of the posterior margin of the wing, o p q (the anterior margin acts as an 

 axis to the posterior one), the angles being always greatest when the wing is extended, 

 and least when it is flexed. The angles, moreover, made by the root of the wing are 

 always greater than those made by the tip. The various inclined surfaces made by the 

 under surface of the wing are intimately associated with the power the wing j:>ossesses of 

 alternately seizing and evading the air. The angles are greater at the root of the wing 

 than at the tip, because the portions of the pinion nearer the root travel at a lower speed 

 than portions nearer the tip. The various inclined surfaces made by the wing in flexion 

 and extension are well seen at figures 16 and 17, Plate XIII. At figure 17 the anterior 

 margin of the Aving (xstv w) is nearly on a leA r el with the posterior margin (o, ]>, q)- At figure 

 16, on the other hand, the anterior margin (x, s, t, v, to) is elevated and the posterior margin 

 {op q) depressed. A careful examination of those figures (particularly figure 16) will also 

 show that the angles of inclination made by the several portions of the under surface 



