It will thus be seen that Professor Thomson's explanation in this matter is incorrect and misleading. This is about the most unscientific explanation which could be given of this matter, and it is one which is calculated to mislead people who would otherwise understand the matter quite clearly, so we shall drop Professor Thomson's idea of giving the ball a "nose " which is always in the front of it, but which is also supposed to be continually travelling sideways. It is obvious that Professor Thomson cannot have it both ways.

It is very clear indeed that Professor Thomson is not well acquainted with the method of applying spin to balls which are used in playing games. He says:

A lawn-tennis player avails himself of the effect of spin when he puts "top-spin" on his drives, i.e. hits the ball on the top so as to make it spin about a horizontal axis, the nose of the ball travelling downwards as in figure 4; this makes the ball fall more quickly than it otherwise would, and thus tends to prevent it going out of the court.

I have played lawn-tennis for more than twenty years, and I am the author of three books on the game, one of which is supposed to be the standard work on the subject, and I can assure Professor Thomson that no lawn-tennis player would dream of doing anything so silly as to hit a lawn-tennis ball "on the top" in an attempt to obtain "top-spin."

The scientific method of obtaining top-spin is to hit the lawn-tennis ball on what Professor Thomson, if he were driving the ball over the net to me, would call its nose - that is to say, I should hit the ball on the spot which was farthest from Professor Thomson. I should hit it there with a racket whose face was practically vertical, but I should hit it an upward, forwardly glancing blow which would impart, as Professor Thomson expresses it, "spin about a horizontal axis to the ball."

Professor Thomson goes so far as to show by diagram the travel of a ball which has been hit so as to impart top-spin to it, but even in this diagram he is absolutely wrong, for he shows that immediately the ball has been hit with top-spin it begins to fall, but this is not so. In lawn-tennis the ball travels for a long distance before the spin begins to assert itself, and to overcome the force of the blow which set up the spin.

Professor Tait makes this same error in his article on "Long Driving," and it is quite evident to me that Professor Thomson is following, in many respects, the errors of his eminent predecessor.

Professor Thomson also says:

Excellent examples of the effect of spin on the flight of a ball in the air are afforded in the game of base-ball. An expert pitcher, by putting on the proper spin, can make the ball curve either to the right or the left, upwards or downwards; for the side-way curves the spin must be about a vertical axis; for the upward or downward ones, about a horizontal axis.

There are no particular laws with regard to the curves of a base-ball. The same laws regulate the curves in the air of every ball from a ping-pong ball to a cricket ball, and Professor Thomson, in saying that "for the side-way curves the spin must be about a vertical axis," is absolutely wrong. Every lawn-tennis player who knows anything whatever about the American service, will know that Professor Thomson is utterly wrong in this respect, for the whole essence of the swerve and break of the American service, which has a large amount of side-swerve, is that the axis of rotation shall be approximately at an angle of fifty degrees, and any expert base-ball pitcher will know quite well that he can get his side-curve much better if he will, instead of keeping his axis of rotation perfectly vertical, tilt it a little so that it will have the assistance of gravitation at the end of its flight instead of fighting gravitation, as it must do if he trusts entirely to horizontal spin about a vertical axis for his swerve.

Professor Thomson says:

If the ball were spinning about an axis along the line of flight, the axis of spin would pass through the nose of the ball, and the spin would not affect the motion of the nose; the ball, following its nose, would thus move on without deviation.

The spin which Professor Thomson is describing here is that which a rifle bullet has during its flight, for it is obvious that the rifle bullet is spinning "about an axis along the line of flight," and that the axis of spin does pass through the nose of the bullet, but we know quite well that in the flight of a rifle bullet there is a very considerable amount of what is called drift. It is, of course, an impossibility to impart to a golf ball during the drive any such spin as that of the rifle bullet, although in cut mashie strokes, and in cutting round a stymie, we do produce a spin which is, in effect, the same spin, but this is the question which Professor Thomson should set himself to answer. He states distinctly that a ball with this spin would not swerve. If this is so, can Professor Thomson explain to us why the rifle bullet drifts? As a matter of fact, a ball with this spin would swerve, but not to anything like the same extent as would a ball with one of the well-recognised spins which are used for the purpose of obtaining swerve.

PLATE XI.

JAMES BRAID Finish of drive, showing clearly how Braid's weight goes on to the left leg.

Professor Thomson proceeded to prove by the most elaborate experiments the truth of those matters stated by Newton centuries ago, but it will not be necessary for me to follow him in these, because these principles have been recognised for ages past.

It is curious to note that in the reference to Newton, who was aware of this principle of swerve so long ago, we are shown that Newton himself did not quite grasp the method of production of the stroke, although he analysed the result in a perfectly sound manner. Writing to Oldenburg in 1671 about the Dispersion of Light, he said in the course of his letter: "I remembered that I had often seen a tennis ball struck with an oblique racket describe such a curved line." The effect of striking a tennis ball with an oblique racket is, generally speaking, to push it away to one side. The curve, to be of a sufficiently pronounced nature to be visible, must be produced by the passage of the racket across the intended line of flight of the ball.