Fig. 1
(2) Backward rotation, retrograde, or screw, is obtained by striking the ball on C B below the centre.
(3) Right side,[[15]] or rotation round the vertical axis H B from left to right, is attained by striking on the line C D; whilst
(4) Left side,[[15]] or rotation from right to left, results from striking on C G.
And these can manifestly be combined; thus, the ball struck in the sector C H D has both follow and right side; struck in C D B the combination is screw and right side; in C G B screw and left side; and in C G H follow and left side. These are the practical divisions for purposes of play, but it must be borne in mind that so long as the cue is delivered horizontally the path travelled is the prolongation of its axis, of a line parallel to that axis, and the effect of the rotation communicated does not show itself, save to a very minute extent, till after impact with a ball or cushion. Then it becomes immediately apparent and often bewildering in the strangeness of its results. Who, for example, has forgotten the feeling of awe with which he first contemplated the result of a well-executed screw, ball 1 striking ball 2 smartly and thence returning to the point of the cue? And to this day the most consummate masters cannot explain some of the strange results whose practical effects are sufficiently well known.
Before passing from this figure it may be as well to explain that the maximum of rotation can best be effected, or most side given, by striking at the ends of the diameters H B and G D, on the principle of the lever being longest at those ends; but practically the limit is reached at the point on either line beyond which a miss-cue would result. Each player will in time find out this point for himself, and it is remarkable how practice improves the power. With it a man can hit clean and sharp further out on the arm of the lever—that is, further away from the centre—than is possible for an untrained person, and it will be found, moreover, that in time and by practice a delicacy of touch and increase of effect are acquired.
But what is this rotation, what causes it, and how is it regulated?
The main factor, or at any rate the main reason whereby its effects become visible and are regulated, is friction with the cloth or bed of the table. If balls were perfectly smooth, the bed also being equally hard and smooth, and if they were unaffected by the resistance of the air and the force of gravity, once set in motion they would continue to slide along for ever within the limit of the length of the bed. They would not roll, but would slide as a curling stone does on smooth ice. But the practical condition of affairs is different. In the first place, the surfaces of the balls, be they never so finely finished, instead of being smooth, are if examined under a microscope found to be palpably rough. The cloth, too, no matter how well stretched or of how fine a texture, is both soft and rough. A ball at rest on it is standing in a little cup, whilst one travelling forms a narrow groove, along which, it is plain, resistance will vary according to its direction and that of the nap of the cloth. If with the nap the friction will be less, if against it more.
Fig. 2