On and near land .] 
APPENDIX. 
529 
or South, to be divested of the error which the attraction of the iron in 
the ship may produce. In making them on laud, it should be done on 
the open shore, so as that no attraction, purely local, may interfere; and if 
the direction of that shore be North and South, the experiment would be 
more satisfactory. 
In an investigation of the cause why the attraction of the iron in a 
ship, and in some cases of the land, should decrease with the dip of the 
needle, and cease at the magnetic equator, the position of the dipping needle 
must again be consulted. At the equator it is horizontal ; and therefore 
the line connecting the north and south polarities in each piece of iron in 
the ship, if it still possess magnetism, will also be horizontal, and the two 
attracting parts equally near to the level of the binnacle ; and it should fol- 
low, that the attractions on the north and south points of the compass 
would be equal, and counteract each other. But it seems not improbable 
that stanchions. arid other upright pieces of iron, and perhaps the whole lose 
most, if not all their magnetism at the equator ; from the rotatory motion 
of the ship not allowing any piece to have one end directed to the North, 
and the other end to the South, a sufficiently long time to acquire or retain 
magnetism. This was not the case where the dipping needle approached 
the perpendicular; for there, however the ship were turned, the upper 
part or end of each fixed piece of iron still remained the Upper part; and 
the more nearly the needle stood to the perpendicular, the more strongly 
would the magnetism of the iron be concentrated at the upper and lower 
extremities, and consequently the more strong would be the attractive 
power on the compass. This I take to be the true cause of the errors in- 
creasing and decreasing in close connexion with the dip of the needle. 
With respect to land near the magnetic equator, the analogy should 
not hold, because the magnetic vein or mass is not, like the iron in a ship, 
subject to a rotat ory motion. Suppose that in the upper part of an island 
near the equator there be a mass of iron ore, or other stone possessing 
magnetism ; the north end of this mass will have a power of attracting the 
south point of the compass, and the south end, the north point; and it 
should follow, that when the centre of thi island bears S. W. or N. E., at 
a little distance, the west variation hould be less than when it bears S. E. 
or N. W. At the small island Trinidad, where the south end of the needle 
3 Y 
VOL. II. 
