Let _A B_ be a ball moving toward _D_, and rotating in the direction shown by the arrows. As the ball flies along, it pushes the air aside, as shown by the lines _F E_; the air thus rubs against the sides of the ball. But as the half of the ball called _B_ is revolving with the air as shown by the arrow, there is much less friction on that side than on the half called _A_, which is revolving against the air. Therefore, as motion is always in the direction of least resistance, the ball must curve in the direction _C_. If any of the readers of the ST. NICHOLAS can not understand my explanation, let them imagine, for example (instead of a regular ball), a ball covered with paddles, and they will then readily understand it. If they think it not a good simile, let them look at the surface of a ball through a microscope and their doubts will not last long. Your friend, W. H. Finally, from Boston, comes this rather formidable-looking explanation, which will interest those of our boy-readers who are accustomed to take their recreation and fun in the form of mathematical problems. [But doesn’t it make the ball curve in the wrong direction?] BOSTON, MASS. DEAR ST. NICHOLAS: Let the line _A B_, Fig. 1, represent in length and direction a given force acting on the point _B_. Draw any parallelogram, as _A C B D_, having _A B_ for a diagonal; then the lines _C B_ and _D B_ represent in length and direction two forces acting on _B_, which, together, are equivalent to _A B_, _A B_ is the resultant of _C B_ and _D B_, _C B_ and _D B_ are components of _A B_. [Illustration: Fig. 1] [Illustration: Fig. 2] If the parallelogram, _A C B D_, Fig. 2, is a rectangle, the components _C B_ and _D B_ are known as right components. The force represented by _D B_ has no effect in the direction _C B_, and the force represented by _C B_ has no effect in the direction _D B_.