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° (_p_), whereas at the middle of the stroke it is reduced to 20° or 25° (_q_). At the end of the stroke the angle gradually increases to 45° (_b_), then to 90° (c), after which the wing suddenly turns a somersault (_d_), and reverses precisely as the natural wing does at _e_, _f_, _g_ of figs. 67 and 69, p. 141. The artificial wing reverses with amazing facility, and in the most natural manner possible. The angles made by its under surface with the horizon depend chiefly upon the speed with which the wing is urged at different stages of the stroke; the angle always decreasing as the speed increases, and _vice versâ_. As a consequence, the angle is greatest when the speed is least. When the wing reaches the point _b_ its speed is much less than it was at _q_. The wing is, in fact, preparing to reverse. At _c_ the wing is in the act of reversing (compare _c_ of figs. 84 and 85, p. 160), and, as a consequence, its speed is at a minimum, and the angle which it makes with the horizon at a maximum. At _d_ the wing is reversed, its speed being increased, and the angle which it makes with the horizon diminished. Between the letters _d_ and _u_ the wing darts suddenly up like a kite, and at _u_ it is in a position to commence the stroke from left to right, as indicated at _u_ of fig. 128, p. 250. The course described and the angles made by the wing with the horizon during the stroke from left to right are represented at fig. 128 (compare with figs. 68 and 70, p. 141). The stroke from left to right is in every respect the converse of the stroke from right to left, so that a separate description is unnecessary.