Suppose a ball is moving toward G, and rotating in the direction shown by the arrow. Then the two quadrants A and D will move with equal velocities. B and C will also move with equal velocities, but B and C will not move as fast as A and D, because in the case of A and D the velocity due to _rotation_ is to be _added_ to that of _direct forward motion_, and in the case of B and C it is to be _subtracted_. Now the pressure of the air on a moving body varies, as some power of the velocity of the body; that is, the greater the velocity, the greater the pressure. The quadrant A moves against the air with a certain velocity, and the total pressure of the air on that quadrant will be a force, acting toward the center (if we neglect friction), which may be represented by the arrow at A. The quadrant B moves against the air with a less velocity than A; hence the pressure is less. Let it be represented by the arrow at B. The quadrant C moves _away from_ the air with a velocity equal to B; hence the pressure on C must be less than that on B. The quadrant D moves _away from_ the air with a velocity greater than that of C; hence the pressure on D will be less than that on C. Evidently now from this arrangement of forces the resultant force will lie somewhat in the position shown by the arrow R. This will tend to force the ball away from the direct line of flight and to curve it as shown by the dotted line. Thus we see that it is the _pressure_ of the atmosphere that curves the ball, and not the friction. The tendency of the latter is to curve the ball in the opposite direction, but this tendency is unappreciable. This is where the mistake of your correspondent in the February number lies; namely, in considering the friction instead of the pressure. The explanation of F. C. J. in the May number seems to me more correct. The theory of “A Curver” in the May number, that a ball could be curved more easily in a vacuum than in the air, is entirely wrong. It violates Newton’s first law of motion. “A body continues in a state of rest or of uniform motion in a straight line unless acted upon by some external force.” In a vacuum there would be no external force, and the ball would not curve at all. Prof. Wood, in the article referred to, proves that the more slowly a ball is thrown with a given velocity of rotation the more will it curve. Does any one know how this is practically? Hoping that I have not taken up too much of your space, I am, very sincerely yours, J. R. S. * * * * * BRADY CITY, MCCULLOCH, TEXAS.