"Let Fig. 3 represent a ball moving through the air in the direction of the arrow, B K, and at the same time revolving about its vertical axis, U, in the direction of the curved arrow, C. Let A A A represent the retarding action of the air acting on different points of the forward half or face of the ball. The rotary motion, C, generates a current of air about the periphery of the ball, a current similar to that caused by the revolving flywheel of a steam engine. "If, now, at a point on the face of the ball we let the arrow, R, represent the direction and intensity of this rotary current of air, and if at the same point we let the arrow, A, represent the direction and intensity of the retarding action of the air, then we will find by constructing a parallelogram of forces that the resultant or combined effect of these two currents acts in the direction indicated by the dotted arrow, T. In other words, we have a sort of compression, or force of air, acting on the face of the ball in the direction indicated by the arrow, T. This force, as we can readily see, tends, when combined with the original impetus given to the ball, to deflect or cause time ball to curve in the direction of the dotted line, B P, instead of maintaining its right line direction, B K. If the ball rotate about its vert axis in the opposite direction, the curve, B N, will be the result." To the above demonstrations it is only necessary to add an explanation of one other feature. The question has arisen why it is that the ball apparently goes a part of its course in a straight line and then turns off abruptly. One might suppose at first thought that the greater speed at the beginning would create the greater resistance and consequently cause the greatest deflection. This, however, is not true. The difference between the resistance upon opposite points of the ball in the circumference of its rotation always remains the same, no matter how great the force of propulsion, and therefore the increased force of the latter at the beginning has no effect on the curve. But while the force of the twist itself is not affected by the rate of the forward movement, its effect upon the ball is greatly nullified. The force of propulsion being so great at first, drives the ball through the air and prevents it from being influenced by the unequal resistance. It is only when the two forces approach one another in strength that the latter begins to have a perceptible effect. As soon, however, as it does, and the course of the ball begins to change, the direction of the dotted arrow, T, begins to change likewise. It follows the course of the ball around, and the more it curves the more this resultant force tends to make it curve, and this continues until the ball has lost either its twist or its forward motion.