Professor Tait pointed out two very simple ways of finding out whether a golf ball had its two centres approximately coincident, and whether in consequence it ought to and would fly well. The first was by floating it in a bath of brine or mercury and noticing whether it wobbled or turned over. Many golfers are acquainted with this test, and employ it in a cruder and less decisive form by floating the ball in water. While a ball that had a fairly considerable separation between its two centres might not show any wobbling movement when floated in water, and consequently might not completely establish its claim to be properly centered and of good flying capacity so far as this part of its properties was concerned, the presumption would be greatly in its favour. On the other hand, that which did show any perceptible wobble in water would be self-condemned at once, and would undoubtedly be a bad flyer and a danger to the game of the good golfer.

The second test is one of comparison, and is exceedingly simple. You cannot compare the flying capacities of two or more balls by driving them with golf clubs, for however near to exact similarity you may think the strokes to have been, there is certain to have been an appreciable scientific and mathematical difference, such as would make a proper comparison impossible. But you may give practically exactly the same initial impetus in exactly the same circumstances to two or more balls by shooting them in the same direction from a crossbow, when the string is always pulled out to exactly the same point. Here you will have the balls flying under the simplest possible conditions, with no spin to complicate the flight and interfere with the comparison, and anyone who takes the trouble to make this experiment will find that some balls will regularly fly farther than others when shot forward in this manner. If the size and the weights are the same, these balls are better centered and better flyers, and it is an easy matter for any player to establish a standard by this test, and to judge of the perfection of any particular ball at a moment's notice. Of course such a test takes no account of the resiliency of the ball; but then, as every player knows, there is a clear difference between good resiliency and good flying properties. In the old days of the gutta, when so much depended upon the even quality of the material all through the ball - and these were, of course, the days when Professor Tait made his investigations and experiments, and drew his conclusions - the variations between centres were greater than they are now, though not so great as in the early period of the rubber-core, when the winding and covering machinery were imperfect. Rubber - cored balls have lately begun to be covered by winding very thin strips of the covering material round the core in just the same way that the core itself is wound, and this should greatly conduce towards more accurate centering. An understanding of the foregoing will help the player towards an appreciation of some of the chief points of a good ball, and he will see how extreme is the necessity for perfect winding machinery and for the most careful supervision of the process. Nobody calls for a hand-made ball in these days: he wouldn't get it if he did; and it wouldn't be any good if he got it, for the chances would be enormously against its being so well centered as one made by machinery.

V

Now, however good the ball might be, the chances would be against a perfect stillness upon any axis during flight; that is, of course, when no initial spin was imparted to it by the way in which the stroke was played. It would very likely turn just a little upon an axis, and that little would unsteady and injure its flight, inasmuch as the wobble would be from side to side alternately. This difficulty can be got over by imparting an initial spin to the ball which will always be the same all through its flight, and which will thus stop the wobbling. In a word, rotation will steady the flight. This idea was originally at the bottom of the rifling of the barrel through which a bullet is projected; certainly it was the fundamental principle of the rifling of the old 32-pounders. Better to make the ball rotate on an axis that you know about than that it should wobble on one you do not know about, they said; and so the tubes were rifled inside, the balls were made to rotate, and their flight was made steadier and therefore longer.

In the very earliest days of his investigations upon the flight of the golf ball, Professor Tait thought of the application of this idea to the game. A rotation should be given to the ball to steady its flight and make it longer. A moment's thought on the part of those whose rudimentary scientific knowledge is a little rusty, will indicate that there are three definite and clearly distinguished axes of rotation. One is a vertical one, and it is chiefly upon that axis that the golf ball rotates when it is pulled or sliced, or upon an axis that has something of the vertical element in it. Then there is the horizontal axis, which is at right angles to the line of flight; and this is the axis upon which it rotates when either under-spin or top has been applied to it. And, thirdly, there is the horizontal axis, which is parallel with the line of flight. This is the axis upon which the rifle bullet spins. At first Professor Tait was inclined to the idea that the last-named would be the ideal axis for the rotation of the golf ball, but it happens to be the one upon which it is impossible to make it rotate when struck by a golf club. However, in this detail of his preliminary theorising he was wrong, due entirely to his not having then investigated the virtues of under-spin as always given to a ball when well driven, and not having come by the great discovery that this under-spin helped the ball to resist gravity and prolong its flight as nothing else could. How exactly under-spin does this, we have just seen, and readers will now have a very definite perception of the qualities of a ball and the importance of rotation, and though the chief advantage of rotation is in resisting gravity, it is an incidental advantage of it, as will now be understood, that the flight of the ball is thereby steadied, and a very slight inaccuracy in centering made of less consequence than it would otherwise be. But remember that a considerable inaccuracy in centering will interfere with the rotation, 14 and therefore the bad flying ball can still be distinguished, because it will fly badly.