PINOLE, CAL., 1886. DEAR ST. NICHOLAS: The explanation of “curving” suggested in your April number is ingenious, but unfortunately it makes the ball curve the wrong way. The anonymous explanation in February has the same defect, though it narrowly misses being correct. I ask your readers to look over that explanation again, and they will see that the ball, under the extraordinary conditions imposed upon it, instead of “curving,” as the writer says, to the right, will not advance at all, but will revolve around the point D to the left. There can be no doubt about the possibility of “curving” a base-ball. The subject is an old one in gunnery, and has been thoroughly investigated by various experimenters. When spherical projectiles were the only ones in vogue, the matter was one of vital interest to artillerists. When such a ball is thrown, it is acted upon by two forces—the resistance of the air and the force of gravity. The normal path for a given velocity and angle of projection is a plane curve, because both these forces act in the vertical plane of fire—the force of gravity vertically and the resistance of the air tangentially to the curve. When, in addition to the motion of translation, the ball is given a motion of rotation about its center of inertia, the latter will no longer describe the normal path, but will deviate from it by “curving” to the right or left, up or down, depending upon the direction of the rotation. This is because the resultant of the air-resistances will no longer act in the line of the tangent in the general case, but at an angle with it. The general effect will be an increase or diminution of the normal range or deviation to the right or left of the plane of fire.