Flinders bar

from The Century Dictionary.

• noun Nautical, an appliance for correcting a part of the local deviation of the compass-needle on shipboard, consisting of a soft iron cylinder, generally two or three inches in diameter, placed vertically in front or in the rear of the compass-binnacle at such a distance as may be required. Besides helping to correct the semicircular deviation, it tends to lessen the heeling-error.