16 
FIFTH REPORT— 1835 . 
case which had first given rise to his researches, the correction of 
the deviations of ship-compasses produced by the “local attrac¬ 
tion and it was this case which suggested to M. Poisson some 
of the problems of his second memoir. Mr. Barlow was soon 
enabled to perceive, from his own experiments, that the guns and 
other iron of a vessel produce the same effect as a small sphere 
of iron in a certain position; and his first idea was to place an¬ 
other iron ball on the opposite side of the compass, so as to coun¬ 
teract this effect. But when a ship moves into various positions, 
she turns round a vertical axis, which does not coincide with the 
axis of magnetic position. Therefore the relative magnetic si¬ 
tuation of the disturbing and the correcting masses would vary 
with the changes of position of the vessel; and the correcting 
ball, in order to discharge its office, must be altered in place or 
size when the vessel turned its head different ways, an inconve¬ 
nience which rendered this device almost nugatory. 
He then proposed to place the ball in a certain fixed position, 
in which it would double the deviation arising from the local 
attraction ; and finally, when he discovered that the attracting 
power of iron resided in the surface, he substituted an iron 
plate for the ball, and thus his apparatus and the mode of using 
it became convenient and easily managed. 
The correcting plate so employed would produce the requisite 
effect if the attraction of the iron in the ship could always be 
referred to the same virtual centre. The attraction of a mass, 
however irregular, is equivalent to a single force acting to a 
single point or “ focus of attraction”. But this focus may be 
different in different positions of the irregular mass; for the mag¬ 
netism which is developed by the earth’s action in any mass 
will depend upon the form and position of its surface; and when 
the position varies, the position of the resulting attraction with 
respect to the mass may also vary. Hence, when a ship’s com¬ 
pass is disturbed by the action of the irregular mass of iron which 
the vessel contains, it may happen that the same plate or ball, 
in the same relative situation, cannot either counteract or dou¬ 
ble the ship’s attraction in all positions. Whether such ef¬ 
fects are possible or not must depend upon calculation. By 
M. Poisson’s investigations it appears that this possibility de¬ 
pends on certain conditions, and that it does not exist gene¬ 
rally*. But it may be observed that the effect of the attraction 
of the vessel is greatest (and therefore the necessity of correction 
greatest) when the dip is considerable, because then the hori¬ 
zontal directive force of terrestrial magnetism is small. Now 
* Memoire, 1822, p. 531. 
