On ship-board.'] 
appendix. 
513 
it forward ; but there was this remarkable distinction, -in the northern he- 
rn, sphere it was the north end of the needle which was attracted, and in 
the southern hemisphere it was the south end. In the instance off the Sfort 
before cited, when the ship’s head was West, the north end of the needle 
had been drawn forward, or to the left of North, nearly 4°„ and the west 
variation thereby increased to 29 ; with the head East, it would be drawn 
to the right of its natural position, and the variation diminished to about 
2li° ; but at North, the attraction in the ship was in the same line with 
the magnetic poles of the earth, and would therefore produce no change 
The same thing took place at South, for the two attractions were stilAn 
the same continued line, though on opposite sides of the compass ; and 
throughout the voyage I found, that variations taken with the head at North 
and South agreed very nearly in themselves and with the observations on 
shore near the same place, when such observations were not affected by 
local attractions. 
Put although the errors were always the same way in the same he- 
misphere, when the head was at West, and when it was East they were 
always the contrary, yet the quantities varied with the situation of the ship, 
being greater in high, and less in low latitudes ; and yet they did not in- 
crease and diminish in proportion to the latitude. After much examination 
and comparison of the observations, and some thinking on the subject, I 
found that the errors had a close connexion with the dip of the needle 
When the north end of the needle had dipped, it was the north point of 
the compass which had been attracted by the iron in the ship; and as that 
dip diminished, so had the attraction, until, at the magnetic equator, where 
the dipping needle stands horizontal, there seemed to be no attraction. 
After passing some distance into the southern hemisphere, and the south 
end ot the needle dipped, our observations again show ed errors in the com- 
pass ; but the west variation was now too great w hen the ship's head was 
eastward. These errors increased as the dip augmented ; and in Bass’ 
Strait, where the south dip is nearly as great as the north dip j n the 
English Channel, the attraction produced almost as much error as when 
we left England, but it was of an opposite nature. On turning northward 
again, along the east coast of New South Wales, the dip of the south end 
of the needle and the attraction of the iron upon the south point ofthecom- 
3 U 
VOL. ir. 
