But he probed most deeply into the mysteries of the flight of the golf ball when he came suddenly to understand the rotation which was subjected to it by the club, and it is of interest and importance to every golfer that he should understand it also. The starting-point of the wonderful investigations that he made is contained first in the simple fact that when an object is poised in the air there is equal atmospheric pressure upon it at all points; and second, that, as several of the most eminent scientists before him, from Newton onwards, had found out, when a sphere rotates in a current of air the side of the sphere which is advancing to meet the current is subject to greater pressure than is that which is moving in the direction of the current; and a step further in this argument is that, as the result of this extra pressure, if a spherical ball be rotating, and at the same time advancing in still air, it will deviate from a straight path in the same direction as that in which its front side is being carried by the rotation.

Therefore, when a ball is sliced, it is made to spin round so that its front side moves round constantly to the right, and, in accordance with the law just mentioned, there is a greater atmospheric pressure on the left side than on the right, and, consequently, the ball is pushed away to the right - as we see it. When it is pulled, the spin is in the opposite direction, and the extra pressure is from the right, and so it is sent away to the left. When the ball is topped the spin on the front side is downwards, and the ball ducks- the extra pressure this time being in the same direction as gravity; and when under-cut is applied, and under-spin follows, the front side of the ball is spinning upwards, and the extra pressure is from below and against gravity.

When this conclusion was first briefly stated, golfers resisted the suggestion that when driving they imparted any under-spin to the ball; but the Professor stuck to his point, and proved it beyond doubt, and it is in this way that the ball takes six and a half seconds over its flight instead of the three and a half that it would otherwise do; and he proved, moreover, that if there was no under-spin imparted to it when driving it would only travel about half the distance that it usually does. The greater the under-spin the greater the upward pressure, and this conclusion leads to others very interesting.

IV

Golfers are in the way of saying that this ball "flies well" or that the other ball "does not fly well." Sometimes it is imagination born of lack of form; but when great players concur it is not imagination. Some balls are obviously better than others - made of better material, better elastic thread, and more carefully constructed. '1 here is an evident reason why such balls should fly better than others; that is to say, why they should go off the club more quickly, keep their place in the air longer, and travel farther. But then there are many balls of absolutely first-rate quality - and maximum price - that vary considerably in their flying properties, and it very commonly happens that even balls out of the same box, made at the same time and in the same way and of the same stuff, vary also. One frequently finds one or two "bad" balls in a box, and one or two very good ones. The excellent player very soon knows when he has come by the good ones and the bad ones. Now why, under such circumstances, should these balls vary so? What is it that makes them vary ? Golfers in general do not know. Often enough they put it down to "pure cussedness"; others, to an idea that it is due to some accidental flaw in the manufacture. It is neither the one nor the other.

The scientific explanation is really a very simple one, and it was set forth very lucidly by Professor Tait. The perfect ball - using the adjective in its most absolute sense - is that which has its centre of gravity, that is to say its centre of weight, dead in the centre of the ball, the centre of measurement. It is by no means to be assumed that these two centres must necessarily coincide. For them to do so exactly is an ideal state, and while matter and man are what they are, and subject to their constant, even though slight, deviations, it is unattainable. But when a ball is properly cored and properly covered, most carefully and by the most exact machinery, the two centres come very near together, and generally, to all intents and purposes, do coincide. That they do not always do so exactly is merely because the greatest human effort is incapable of achieving the scientific ideal, and it must constantly happen that, despite all that effort, the distances between the two centres vary a little. Practically no effort can prevent it, particularly when the exigencies of circumstances demand that balls should be turned out weekly in tens of thousands, and at a price of not more than two shillings each. Now and again the separation of the centres will be greater than normal - accidental again - and then you get a really bad ball, with much bias upon it. When the centre of weight is not at centre of measurement, it means that the ball in effect is heavier on one side than the other, biassed, and that is practically equal to its being not round. Suppose you inserted a small piece of lead just inside the cover of a ball and closed it up again, shaping it as perfectly as it was before. The effect of this would be to remove the centre of weight very far towards that side, and you would have a great exaggeration of the difference between the two centres that commonly exists. If you laid that ball on a table it would promptly roll round until the weighted or biassed side were underneath. If you floated it in water it would wobble about until eventually it did the same thing; and if you floated it in air it would wobble again, and such wobbling would obviously be detrimental to its straight and even flight. There you have it. The farther the two centres are from each other - from the ideal state of absolute coincidence - the greater must be the tendency towards a wobbly or uneven flight, and diminished rotation, and consequently towards a short flight. In the case of many balls other than golf balls, these variations are very considerable. You have an extreme example when a football is out of shape, and it can be seen to make zigzags in the air. But the flight of footballs, or even cricket balls, is not such a delicate and susceptible thing as the flight of a golf ball at its far greater pace.