and longer in proportion to the speed attained. The more vertical direction of the loops formed by the wing of the bat and bird will readily be understood by referring to figs. 74 and 75 (p. 145), which represent the wing of the bird making the down and up strokes, and in the act of being extended and flexed. (Compare with figs. 64, 65, and 66, p. 139; and figs. 67, 68, 69, and 70, p. 141.) [Illustration: FIG. 74. FIG. 75. FIGS. 74 and 75 show the more or less perpendicular direction of the stroke of the wing in the flight of the bird (gull)--how the wing is gradually extended as it is elevated (_e f g_ of fig. 74)--how it descends as a long lever until it assumes the position indicated by _h_ of fig. 75--how it is flexed towards the termination of the down stroke, as shown at _h i j_ of fig. 75, to convert it into a short lever (_a b_), and prepare it for making the up stroke. The difference in the length of the wing during flexion and extension is indicated by the short and long levers _a b_ and _c d_ of fig. 75. The sudden conversion of the wing from a long into a short lever at the end of the down stroke is of great importance, as it robs the wing of its momentum, and prepares it for reversing its movements. Compare with figs. 82 and 83, p. 158.--_Original._] The down and up strokes are compound movements,--the termination of the down stroke embracing the beginning of the up stroke; the termination of the up stroke including the beginning of the down stroke. This is necessary in order that the down and up strokes may glide into each other in such a manner as to prevent jerking and unnecessary retardation.