found roughly by the bearing of two well-defined heads or points set in a line from opposite directions. If, after the proper corrections are made according to the ship's head, the bearing be not the same, the difference will be seen.
These memoranda are mostly relative to a compass fixed on the binnacle; but the trouble of correction may be saved if a place can be found near the taffrel, where the attraction of the iron at the stern will counteract, by its greater vicinity, the more powerful attraction in the centre and fore parts of the ship; and should the after attraction be too weak, it may be increased by fixing one or more upright stanchions or bars of iron in the stern. If a neutral station can be found or made, exactly amidships, and of a convenient height for taking azimuths and bearings, let a stand be there set up for the compass; and if the stand must of necessity be moveable, make permanent marks, that the exact place and elevation may always be known. Observations taken here should never undergo any change from altering the direction of the ship's head, at any dip of the needle; but it will be proper to verify. occasionally, and to compare the azimuths and bearings with others taken on the binnacle. The course should also be marked from this compass, though the ship be steered by one before the wheel; a quarter or half point being allowed to the right or left, according as the two may be found to differ.
These precautions are not intended to supersede the taking of angles with a sextant or circle, from the sun to any chosen object, and from thence to others; but in using the compass on ship-board such are those I would employ, in order to arrive at the true variation and to know what should be allowed on each set of bearings. In surveying with a theodolite or circumferentor on shore, my memorandum is,—To observe azimuths with the same instrument, and in the same spot where each set of bearings is taken; unless where the back bearing can be had of some former station or of the ship, where the variation has been observed.
Had I been more early aware of the necessity of these precautions in the use of the magnetic needle, both on ship-board and on shore, much perplexing labour would have been saved; and although every existing datum has been employed to remedy the deficiences, the charts which accompany this work would then have presented a more correct delineation of the coasts of Terra Australis.
