amount of the dip of the magnetic needle, in London, &c. 5 
grooves in the thicker part of the axis receive the Y’s, which 
raise and lower the needle on its supports, and ensure that the 
same parts of the axis rest in each observation on the 
planes. 
A small brass sphere traverses on a steel screw, inserted 
in the lower edge of the needle, as nearly as possible in the 
perpendicular to the index line passing through the axis of 
motion ; by this mechanism, the centre of gravity of the nee- 
dle, with the screw and sphere, may be made to fall more or 
less below the axis of motion, according as the sphere is 
screwed nearer or more distant from the needle, and ac- 
cording as spheres of greater or less diameter are em- 
ployed. 
The object proposed in thus separating the centres of 
motion and gravity, is to give to the needle a force arising 
from its own weight to assist that of magnetism in overcom- 
ing the inequalities of the axis, and thus to cause the needle 
to return, after oscillation, with more certainty to the same 
point of the divided limb, than it would do were the centres 
strictly coincident. 
The centres of motion and of gravity not coinciding, the 
position which the needle assumes, when placed in the mag- 
netic meridian, is not that of the dip : but the dip is deducible, 
by an easy calculation, from observations made with such a 
needle, according to the following directions. 
If the needle has been carefully made, and the screw in- 
serted truly as described, the centres of motion and of gravity 
will be disposed as in the lever of a balance, when a right 
line joining them will be a perpendicular to the horizontal 
passing through the extremities (or to the index line) ; this 
