By an error in filing, several letters concerning curve-pitching were overlooked two months ago, and so they failed to appear with the others in the August “Letter-box.” But as the friendly correspondents who sent them have taken great pains to explain their theories, it would be unjust to withhold the letters and diagrams from the thousands of boy-readers who are interested in the vexed questions of how and why a ball curves. Some of these letters, therefore, are presented here; the remaining two or three will appear in next month’s “Letter-box.” * * * * * FRANKLIN, PA. DEAR ST. NICHOLAS: May I have a word with your readers on that vexed subject, curve-pitching? Though I am not one of your subscribers, I have a younger brother who has been one for several years, and who also pitches for an amateur club here. Through him I have verified for myself the fact that a ball will curve in the direction in which it is rotating; _i. e._, it will curve to the right, or “in,” if it rotate in the direction of the hands of a watch, and _vice versa_. In this, I think, any careful observer of a curving ball will agree with me. Now, all will admit, I think, that if it were possible to throw a ball in a straight line _without any rotation_ whatever, there would be a cushion of air of greater density than the surrounding atmosphere _exactly_ in front of the ball, and a partial vacuum behind it. Nor would this cushion of air have definite limits, but it would thin out gradually as it streams over the sides of the ball, thus (Fig. 1): [Illustration: FIG. 1.] But now, suppose the ball be rotating rapidly to the right, in the direction of the hands of a watch. The sides of the ball, as they rotate, must carry by friction some of the surrounding air with them. That is, the point b (Fig. 2), as the ball rotates, will tend to carry air from its present location around to d, and so with any other point on the ball in proportion as it is on or near the equator of rotation. [Illustration: FIG. 2.] But when each point on the ball’s equator reaches the point c with its load of air, it meets with a resistance produced by this cushion of air in front of the ball, and, in order to pass on, must leave its load behind it. In other words the air carried around in the direction b c d becomes massed against the cushion in front, and the cushion is thickened at and around the point c. And, on the other hand, each point on the equator tends to carry the air from the right-hand side of the cushion, the point c, and consequently, to decrease the density or thickness of the cushion at that point. So that we soon have the cushion of air not exactly in front of the ball, but somewhat to the left of front; thus (Fig. 3):