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and in the last, was upon the booms. In order to ascertain clearly what effect this change of place did really produce, I took observations a few days afterward with every compass on board, and Mr. Thistle did the same upon the booms, ten or twelve feet before the main mast, where the compasses were as far removed from any quantity of iron, as they could be placed in any part of the ship. The head was south-west by the steering compass, our latitude was 38° 1′ north, longitude 14° 18′ west, and the results were as under.

Thus a change of place from the binnacle to a little before the centre of the ship, produced an alteration of 4° 37′ in the mean variation, the same way as, but a less quantity than Mr. Thistle had found it off the Start, when the ship's head was west. The true variation I judge to have been 23°, and that the observations on the booms showed 2° too little, and those on the binnacle 2½° too much. The error in excess, upon the binnacle, appeared to continue so long as the ship was in the northern hemisphere and the head to the westward; but it diminished gradually as we approached the equator, and the observations on the binnacle and booms then nearly coincided. This example is sufficient to show the impropriety of allowing a variation upon the ship's course, from observations taken elsewhere than at the binnacle.