distribution , &c* of the magnetic intensity in ships of war. 323 
been anticipated, from the change which the whole system of 
local attractions was found to undergo, in the different posi- 
tions of the vessel. It may be interesting, however, to trace 
the course of its variation. 
When the head of the ship was north, the centre of force 
was found at the point O, fig. 2, Plate XV., at the distance of 
0.02 feet from the middle section, on the larboard side of the 
vessel. As the ship however moved eastward, it crossed 
the middle section, and was finally found at Q, on the star- 
board side, distant from that section 0.13 feet. When the 
vessel passed from north to north-west, the centre receded 
from O to P, on the larboard side, distant from the middle 
section 0.11 feet. The distance of the centre of force there- 
fore from the middle section was a minimum when the vessel's 
head was north ; and a maximum when east. 
With reference to the transverse plane, the same point 
varied its position considerably, being found, when the ship's 
head was north, at O, fig. 2, Plate XV., or fig. 1, Plate XVI., 
distant 49.23 feet from it. A change of position in the vessel 
also, to the east or the west, caused the centre of force to 
advance towards the bow. When, for instance, the head bore 
north-west, the distance of this point from the transverse 
plane was 49.84 feet, as at P ; and when east, 50.84 feet, as 
at Q ; — the former position producing an increase of its dis- 
tance 0.61 feet from the plane, and the latter 1.61 feet. The 
centre of force is therefore situated at its minimum distance 
from the bow, when the vessel's head is east, and attains its 
maximum, when due north. 
Nor is the distance of the same point from the horizontal 
co-ordinate plane of the same constant magnitude ; for when 
