[Illustration: FIG. 3.] Now, since action is always equal to reaction, the cushion of air must push back against the ball just as hard as the ball pushes against it. And since the cushion is thickest where the combined forces of the ball are greatest, the cushion must push back hardest in the direction of a line drawn through the thickest part of the cushion and the center of the ball; _i. e._, in the line a, b, Fig. 3, a direction against that of the onward motion of the ball, and to the right of it. [Illustration: FIG. 4.] Finally, if at any instant we represent the force of the ball and its direction by the line A, B (Fig. 4), and the backward push of the cushion and its direction by the line A, C, then, according to the law of physics, that, “if two forces act on a point, and if lines be drawn from that point, representing the forces in magnitude and direction, and on these lines, as sides, a parallelogram be constructed, their _resultant_ will be represented in magnitude and direction by the diagonal which passes through that point,” the line A, D will represent the actual force and direction of the ball at the given instant. But both the forces, A, B and A, C, are constantly decreasing from the moment the ball leaves the pitcher’s hand, and, moreover, the force A, B decreases more rapidly than the force A, C, inasmuch as A, B is acting against the resistance of the air plus the attraction of gravity, while A, C is acting against the resistance of the air alone. Consequently, the direction of A, D is being constantly changed and away from A, B, and becomes a curve, instead of a straight or broken line. This curve will obviously have the direction, A, D (Fig. 5), or to the right, which is the direction in which a ball rotating to the right will curve. [Illustration: FIG. 5.] This reasoning will hold for a ball rotating in any direction, so long as the axis of rotation is, or tends to be, at right angles to the line of progression. The problem of the “drift” of a projectile from a rifle or rifled cannon is entirely different, and one I should like to see discussed after this one of “curving” is settled to your readers’ satisfaction. I think the above explanation solves the problem as well as explains all the fallacies of your former correspondents. Very respectfully, S. P. E. * * * * * ANNAPOLIS.