Drift
'Drift is a ballistic phenomenon that causes a spinning projectile (such as a shell fired from a rifled gun) to be pulled laterally in the direction of the spin as it flies through the air.
Character
The Royal Navy found that for any given gun, the shell would drift a distance that was proportional to the square of the Time-of-flight, and thus was as if a constant force was pushing the shell laterally as it flew. It would calculate the amount of force for the given gun and then create sighting equipment that could correct (exactly, or approximately) this by adding deflection to the gun sights as range was dialed on.
Correction
One lazy (but substantially correct) treatment was to incline the sights as they were fitted onto the gun by cocking them a bit to the left. In this way, as range was dialed on and the sights depressed, they'd also yaw a bit to the right (the direction of drift with the guns rifled rightward). This would require the trainer to train a bit more left. The effect was to bring the shells back on for drift at the given range. This was a fair approximation, and the imperfection was deemed acceptable in most cases. Even so, the presence of a column labelled "Uncorrected Drift" on the Deflection Totaliser created for the Dreyer Fire Control Table seems to suggest that it might have been used to enter in this error so it, too, would be corrected.
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