Page:A Voyage to Terra Australis Volume 2.djvu/545

How to be obviated.] In some parts of this little discussion upon the attraction of land, I feel to have stepped out of my sphere; but if the hints thrown out should aid the philosopher in developing a system of magnetism applicable to the whole earth, or even be the means of stimulating inquiry, the digression will not have been useless. I conclude this article with copying some precautionary memoranda upon the use of the compass in marine surveying; they were made for myself, in case of being hereafter called upon to sail in another Investigator, and may not be without their use to other officers.

1. So soon as the guns are on board and the ship ready for sea, to nail small cleats on the binnacle for showing the place where the azimuth and surveying compass is to stand, when in use. To ascertain by repeated observations whether it be at North and South, or at what other opposite directions near them, that this compass gives exactly the same variation; and to note these as the Points of no difference.

2. Ascertain what the difference in variation is, when the head is placed at right angles to the points of no difference, on each side. Half this difference is the Error for eight points; which being divided by the dip, will give the common multiplier for that hemisphere, and perhaps for both.

3. To try the accuracy of the common multiplier as often as can conveniently be done, by observations taken at various ports where the dip of the needle is different; and more especially to ascertain whether observations in the southern hemisphere give the same multiplier as in the north.

4. No change to be made in the disposition of the iron work or guns during the voyage; but if a change be indispensable, to ascertain as soon after as may be, what alteration it may have produced in the points of no difference, and in the multiplier.

5. The direction of the ship's head, by compass, to be noted to the nearest quarter point when the variation is observed, or bearings of land are taken; this to be considered an indispensable part of such observations, since without it the true variation cannot be known, nor the proper allowance made to the bearings.

6. On arriving upon the coast to be surveyed, to miss no opportunity of observing the variation, by azimuth if possible; and on passing from one side of a projecting cape or island to the other side, to remark if any difference arise in the compass. This is best done by azimuth; but it may be