But this is far from being the heavy guns’ only advantage, as will be seen from [Sketch 2]. The heavier the projectile is, the longer it retains its velocity. The angle at which a shot falls from any height depends solely upon its forward velocity while it is falling. [Sketch 2] shows the outline of a ship broadside on to the enemy’s fire, the shell being fired from the right-hand of the sketch. A is the point where the ship’s side meets the water. If the gun were shooting perfectly accurately and was set to 10,000 yards, all the shots would hit at this point. And clearly any shot set at a range greater than this, but one which did not carry the shot over the target, would hit the ship somewhere between the points A and X. Now if a 6-inch shot grazes the point X and falls into the water, it falls at the point B beyond the ship. But the angle at which it is falling is so steep that the difference in range between the point A and the point B is only forty yards. To hit, then, with a 6-inch gun the range must be known within forty yards. This interval is called the “Danger Space.”
Big guns need less accurate range-finding, because the danger space is greater
The 9.2 will fall at a more gradual angle, and the shot grazing on X will fall at C, which is twenty yards beyond B; and a 12-inch shell, falling still more gradually, will fall at D, which is 100 yards from A; and similarly the 13.5 at E, which is 150 yards beyond it. Hence, at any given range, far more accurate knowledge of range is necessary for hitting with a 6-inch gun than with a 9.2, with a 9.2 than with a 12-inch, and with a 12-inch than with a 13.5.
But we have seen from [Sketch 1] that, in proportion as the range gets long, so does the range accuracy of the gun decrease, and that this loss of accuracy is greater in small guns than in bigger. To hit with it at all a more perfect fire control is necessary, and for any given number of rounds a much smaller proportion of hits will be made. The advantage of the big gun over the small, merely as a hitting weapon, is twofold. It does not require such accuracy in setting the sight, and more shots fired within these limits will hit.
FIRE CONTROL
If ships only engaged when they were stationary the range would not change, and it could be found by observation without rangefinders. And even with rangefinders it can never be found at great distances without observation. But ships do not stand still, and when they move the distance between them alters from second to second. If these movements could be (1) ascertained, (2) integrated, and (3) the results impressed upon the sight, change of range would be eliminated, and we should have come back to the conditions in which ships were stationary. Fire control is successful in so far as it succeeds in doing these three things. [Sketches 3 and 4] show the process by which hits are secured, when the conditions are not complicated by changes in the range, that is, if these complications have been eliminated by fire control. The second two illustrate what these complications are. The ships turn away from each other and then turn towards each other.
The [rate graph (6)] shows the effect of these movements on the range and the rate at which it is changing from moment to moment.
The process shown in [Sketches 3 and 4] is called “bracketing.” Two shots are fired at a difference of, say, 800 yards. Observation shows the first to be too short, the second to be too far. The difference is bisected by the third shot. This places the target in one of the halves of the bracket. This half is bisected by the fourth shot, placing the target in a quarter. If an eighth of the bracket is less than the danger space, then the fifth shot must hit.
