Page:A Voyage to Terra Australis Volume 2.djvu/533

On ship-board.] possible that they might have been more different, for the removing of four guns from the quarter deck into the hold, which was done after the northern observations were taken, was likely to make some change in the attraction of the iron in the ship.

In correcting the variations given in this and the preceding volume, to what it is presumed they would have been if observed with the ship's head in the magnetic meridian, the following plan was used. With the dip of the needle, as near as it could be known, and the common multiplier, the radius, or error for eight points was obtained; with this, taken as a distance, and the direction of the ship's head as a course, the correction was found in the departure column of the Traverse Table; and being applied to the observed variation, either to the right or left, according as the dip was north or south and the head on the east or west side of the meridian, it gave the true variation.

Example. The dip being 66° south, and the ship's head W. by S., the variation was observed to be 5° 11′ east; required the true variation?

Dip 66° × ,05 = error for eight points 3°,30 = (3° 18′ =) 198′.

Course 7 points and distance 198′ = departure 194′, or 3° 14′ correction. Then, as in south dip the south end of the needle was drawn forward, or in this case to the West, and the north end went to the East, the east variation observed was too great, and must be reduced 3° 14′; and 5° 11′ observed, −3° 14′ correction, =1° 57′ east, the true variation. Had the north end of the needle dipped, and all other circumstances been the same, the correction 3° 14′ would have been additive; as it would also, had the head been E. by N. or E. by S., instead of W. by S.

To ascertain the proper variation to be allowed on bearings for the survey, I was obliged to go through a double process; unless where variations happened to have been observed with the ship's head in the same direction, or at an equal number of points on the same side of the magnetic