Change of Variation of the Compass. - The gradual change in the variation must not be forgotten in laying down positions by bearing on
the charts. The magnetic compasses placed on the charts for the purpose of facilitating plotting become in time slightly in error, and in some
cases, such as with small scales, or when the lines are long the displacement of position from neglect of this change may be of
importance. The compasses are re-engraved when the error amounts to a quarter of a point, but the chart plates cannot be corrected more
frequently from the impossibility of making alterations often on one spot in a copper plate.
The geographical change in the variation is in some parts of the world sufficiently rapid to need consideration. For instance, in
approaching Haliofax from Newfoundland the variation changes 10 degrees in less than 500 miles, and in the English Channel about 5 degrees
in less than 400 miles. The variation Chart should be consulted on this head.