What makes a ball swing?: Atherton had the answer, but didn't know it, William Bown and Rabi Mehta reveal

WHEN England took the field at Headingley in the current Test Match, all eyes were on Mike Atherton, the captain. Would he repeat his actions at Lord's, where some observers thought he used something from his pocket to alter the condition of the ball? As always, commentators were ready with their analysis of what can be achieved by successful ball-tampering and, in particular, with expositions of how bowlers can confound the batsman with reverse swing.

The trouble is that nearly all of them have it wrong. Cricketers all over the world are earnestly devoting themselves to changing the nature of the ball in ways which almost certainly have no effect on swing. Yet the scope for successful ball-tampering is much greater than even the most underhand bowlers have yet realised.

The key to reverse swing, the professionals suggest, is to make one side of the ball heavier than the other. This is why teams with swing bowlers expend so much effort on soaking one side of the leather with sweat and saliva. But they are wasting their time. Our experiments - involving spinning cricket balls in wind tunnels - show that there is only one thing that can cause a flying ball to swerve: a difference in the air pressure on the two sides of the ball.

The air pressure depends on the flow of air over each side of the ball. Swing is generated when bowlers, by accident or design, disrupt the flow of air over one side of the ball.

So what happens when the bowler delivers a conventional outswinger, which swings away from the right-handed batsman? One side of the ball is polished smooth and the delivery is bowled with this face to the fore and the seam pointing at the slips, about 15 degrees to the side of the line of flight of the ball.

The leading part of the ball is covered by a film of fast-moving air, called the boundary layer. About halfway round the ball, the boundary layer separates from the surface. On the non-seam side the boundary layer peels away before the halfway mark. But on the seam side the flow is disrupted by the protuberance of the seam, the boundary layer is tripped into a chaotic turbulence and peels away after the halfway mark. The effect is to make the air pressure on the seam side of the ball lower. The pressure difference pushes the ball towards the seam side, away from the batsman.

Reverse swing - into the right-handed batsman - is quite different. The bowler has the rough side of the ball to the fore. If the ball is bowled quickly enough, the air flow will become turbulent on both sides before the flow reaches the seam. Suddenly the effect of the seam is reversed. It now acts as a ramp, pushing the turbulent air away from the ball and causing the boundary layer to peel away sooner. That makes the air pressure on that side higher, forcing the ball to swing towards the batsman.

The smart bowler should aim for a ball which is smooth on one side, rough on the other, with a prominent seam in between. He can then swing the ball both into and away from the batsman (normally a very rare talent) depending solely on which side of the ball is delivered at the front. Because the bowler needs to change neither grip nor action, the batsman will have no clue as to which way the ball is likely to move.

Getting reverse swing depends on the speed of the ball, as well as the roughness of the surface. Our experiments show that, with a new ball, smooth on both sides, the bowler has to reach 80-90 miles an hour to get appreciable reverse swing. Only a dozen or so swing bowlers have ever managed such speed consistently. But a scuffed ball can generate substantial reverse swing at speeds well within the capabilities of any medium-paced bowler.

This is what provides the incentive for ball-tampering. Left to deteriorate naturally, most balls will become knocked about enough for reverse swing after about 40 overs. But bowlers can get reverse swing much earlier if they roughen the surface artifically. They can do this in two ways. One is to pit the surface - with a bottle top or thumbnail, for example. The other is to stick irregularities on to the surface of the ball. The obvious candidates are dirt or sawdust, with sweat or saliva as the glue. This method has the added advantage that the evidence of tampering disappears as soon as the ball hits the bat - or the wicket.

If this is what Atherton was doing with the dirt that he admitted was in his pocket at Lord's, it would explain why the umpires found the ball unchanged. But it is impossible to believe that he was doing any such thing: it would suggest that he knew more about reverse swing than any other cricketer on the planet.

Should cricket's rulers change the laws? If the objective is to prevent tampering which makes it easier to gain reverse swing then it is probably impossible to rule out every conceivable method; the essential point is that the ball's surface must not be made rougher in any way and this is what would have to be excluded by the laws.

But cricket's governing authorities might first ask themselves what sort of game they want. If they want to favour batsmen, they can eliminate swing altogether by removing the seam from the ball. If they want to eliminate reverse swing only, while preserving the conventional outswing, they can provide new balls every 10 or 20 overs. If they want to favour bowlers, they can encourage reverse swing by allowing some roughening in the same way as some polishing is allowed. The balance between bat and ball in cricket is a delicate one; remember the fuss a few years ago when a new type of ball resulted in a surfeit of runs in county matches. But cricket's lawmakers can only make the right decisions if they have a proper grasp of what can and cannot be achieved by ball-tampering.

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