The tendency of a ship from her true course; the departure of the magnetic needle from its true bearing, when influenced by iron or the local attraction of the mass. In artillery, the deviation of a shot from the direction in which it is fired. The term is usually reserved to lateral deviations, especially those resulting from irregular causes those constant ones due to the regular motion of rifled projectiles coming under either of the designations "constant deflection," "derivation," borrowed from the French, or "drift," from the Americans. These latter, according to the direction usually given to the rifling in the present day, all tend away to the right, though they include some subordinate curves not yet distinctly determined.