213 
employed in the Observatory. 
amount of declination at that moment; as the quantity 
that it is east or west of what was previously ascertained 
to be the zero, or magnetic axis of the bar, has only to 
be converted into angular quantity, and applied ac¬ 
cordingly to the constant angle between the line of 
collimation of the telescope and the true meridian. 
In adjusting this instrument for observation, after 
correct levels, &c. have been obtained, the principal 
difficulty consists in overcoming or destroying the force 
of torsion, or the force by which the suspending thread 
resists an effort to twist it, or by which, when twisted, it 
tends to untwist. This force, although slight, exists in 
the finest fibre, and, if not overcome, must, of course, 
greatly oppose such a delicate force as that of magnetism. 
The object is, therefore, to place the plane of detorsion 
—or, in other words, the line of no twist—exactly in the 
plane of the magnetic meridian. The suspension ap¬ 
paratus is provided with a graduated circle for the 
purpose, and an unmagnetic bar, similar in form to the 
other. When the unmagnetic bar is suspended to the 
thread, and takes up a position of repose in the magnetic 
meridian, with the same point of the scale corresponding 
to the vertical wire of the telescope, as was before ascer¬ 
tained to lie in the magnetic axis, it is evident that the 
plane of detorsion then coincides with the magnetic 
meridian; but, as any movement of the magnet from 
that position must in some slight degree twist the fibre 
by which it is suspended, it becomes necessary to ascer¬ 
tain that amount, or the ratio of the force of torsion to 
that of magnetism, before the true changes of declina¬ 
tion can be ascertained. 
The mode of observation is simple : 
As the magnet, from its extreme sensibility, i& seldom 
at rest, it is, when seen through the telescope, usually 
performing a small arc of vibration: it becomes, there- 
