223 
employed in the Observatory: 
but as magnetism is frequently induced in ferruginous 
matter, and not permanent, it becomes necessary to 
ascertain whether such exists or not. If it were uni¬ 
formly distributed throughout the limb, it could produce 
no disturbance in the position of a needle which divides 
the limb symmetrically. If not distributed uniformly, 
by turning* the instrument in various azimuths, the points 
where the magnetism in the limb is concentrated are 
brought into every relative position as regards the needle, 
and the disturbance it may cause will disappear from the 
mean result. 
6th. This is the most fruitful source of error; and it 
is easy to perceive that if the axis of the needle is not 
truly cylindrical, but is, on the contrary, a little flattened 
at any part, the imperfection of shape will cause a 
resistance to its movements. It is also obviated in the 
same manner as the former, by turning the instrument 
through various azimuths; because, if a needle is forced 
to move in a vertical plane out of the magnetic meri¬ 
dian, it no longer assumes the same position with respect 
to the horizon, but the more the plane of its motion is 
turned from the meridian, the more it becomes inclined, 
until, having arrived at right angles to the meridian, it 
hangs vertically. Now, in arriving at this position, it 
must bear upon several points of the arc of its axle; so 
that by going through a complete series of observations, 
in various azimuths, the whole circumference of the axle 
may be brought under trial. A simple formula* deduces 
the true inclination from these observations in various 
azimuths, and so computed will be as free as possible 
from all error arising from imperfection in the axle. 
* Cotan 2 I = cotan 2 h cotan 2 h'. 
I representing the True Inclination, and h and h being the ob¬ 
served angles of inclination in two planes at right angles to one 
another. 
