217 
employed in the Observatory . 
vertical line, the two threads will no longer be vertical, 
nor will they be in one plane, and the body itself will be 
slightly raised : this, consequently, gives a tendency to the 
body to return to its former position with a moment of 
rotation, which may be regarded as proportional to the 
sine of the angle of deviation from the position of rest, 
and is therefore greatest when it amounts to 90°. The 
force which retains the body in equilibrium by this inode 
of suspension may be termed the u directive force of sus¬ 
pension and its magnitude is dependent, 1st, upon the 
length of the suspending threads; 2nd, on their distance 
apart; and, 3rd, on the weight of the body. 
Suppose then a horizontal magnetic bar to form part 
of the suspended body; a second directive force is exerted, 
and the equilibrium depends upon a combination of the 
two forces. In this point of view, there are three cases 
in which the body would be in a state of equilibrium,— 
when the direction of the two forces coincide, are oppo¬ 
site to, or form an angle with each other; for it is 
manifest that the difference between these cases depends 
upon the relation of the two angles which the straight 
line joining the two lower points of connection of the 
threads forms with the magnetic bar, and which the straight 
line joining the two upper points of suspension forms 
with the magnetic meridian. When it is so arranged 
that the two forces shall form an angle with each other, 
the conflict of these two forces will end in an interme¬ 
diate position, where, on the one hand, the magnet will not 
be in the meridian, and, on the other, a straight line 
through the lower points of connection of the threads will 
not be parallel to a straight line through the upper 
points. It is therefore most advantageous to arrange the 
apparatus so that the magnet in its mean position of 
equilibrium shall form a right angle with the meridian ; 
because, under such circumstances, the deviation of the 
