66 View of Mr Barlow’s Magnetical 
end in the southern, hemisphere. Now, as in vessels of all 
kinds, the iron which enters into their construction or equipment 
is nearly symmetrically disposed with respect to the axis, or 
longitudinal section, of the ship, when this axis is in the mag- 
netic meridian of any place, the action of the iron is either co- 
incident with, or directly opposed to, the magnetic action of the 
earth, and in either case the lateral direction of the needle is not 
disturbed : but when the ship’s head is on any other point, and 
particularly towards the east or west, the two forces acting on 
the needle being nearly at right-angles to each other, the latter 
is drawn more or less out of its natural position, according to 
the direction of the resultant of these two forces ; and the con- 
sequence of this is, that the course shown on shipboard by the 
compass, differs from the aetual course of the vessel, by a quan- 
tity equal to the whole of this disturbing force, and which, in 
some cases, as we shall see, amounts to two or three points of 
the compass, that is, to 20°, 30° or 40°, but varying with every 
position of the ship’s head, and with every change of terrestrial 
situation, from one pole of the earth to the other, and according 
to laws which, till the present time, seemed to bid defiance to 
every attempt made to unravel them. We have said, that it is 
only within a short period that this disturbing power has been 
observed ; but it should be stated, that some obscure notices of 
such an effect are slightly alluded to by navigators of anterior 
date, as for example by Dampier, by Cook, and one or two 
French navigators. It does not however appear, that, in these 
instances, anv thing more than the mere fact is stated, without 
any idea being thrown out respecting the cause, and much less 
of any remedv for the anomaly in question. r lhe cause is, we 
believe, first distinctly noticed by Mr Downie, Master of His 
Majesty’s ship Glory, in his report to the Admiralty, published 
in Walker’s Treatise on Magnetism, in the year 1794. This 
experienced officer says, “ I am convinced that the quantity 
and vicinity of iron m most ships, has an effect in attracting 
the needle ; for it is found by experience, that it will not point 
in the same direction when placed in different parts of a ship ; 
also it is rarely found that two ships steering the same course 
by their respective compasses, will go exactly parallel to each 
