Page:A Voyage to Terra Australis Volume 2.djvu/529

On ship-board.] five different vessels; and the general results, so far as they are necessary to the present explanation, were these.

1st. At or near the binnacle, the north point of the compass was attracted forward in all the ships; but the quantity of error produced, on one side when the head was East, and on the other when West, varied from 6½° to 0° 21′.

2nd. When the compass was placed in other parts of the different ships, the attraction was sometimes forward and sometimes aft; but always aft from the forecastle. The error at some of the stations was greater than at the binnacle, and at others less.

3rd. The errors were least when the ship's head was at, or near to North or South, and greatest at, or near to East or West; and as the head was made to deviate from the points of least error towards the greatest, the increase of error was found to be in proportion to the sines of the angles of deviation.

This last was the particular subject of my anxiety; and being then satisfied that the law before deduced from analogy was certain, I employed it to find a standard correction for all my observations in the Investigator. For this purpose a selection of them was made where the ship's head was in the most opposite points and furthest from the meridian, and where the true variation could be ascertained within a small quantity; the difference between the observed and true variations gave the errors, and when the head had not been at East or West, they were proportioned to eight points or radius by the sines of the angles. These observations were collected into tables, one for the north, and another for the southern magnetic hemisphere, and classed according to the dips of the needle; and the error for eight points at each dip being reduced to parts of that dip, a medium of the whole was taken, and considered to be the standard radius applicable to all situations. The two tables are here inserted for the satisfaction of naval and philosophical readers; and no further explanation of them seems requisite, than that when the errors of observation were to the right they are marked +, and − when to the left.