636 EXPERIMENTAL INVESTIGATION OF THE 
electro-magnets, which have afforded different 
modes of assailing those substances that were the 
objects of investigation. 
15. In all cases where delicate magnetic 
needles are employed, especially when the sus- 
pension is by means of a fibre, and the system 
astatic, the experiments are exceedingly tedious, 
and much time is required to allow of the system’s 
repose from its agitations before an attempt can 
be made to approach it with the specimen to be 
examined. When, however, a single needle is 
employed, whose support is a finely pointed pivot, 
the experiments are less subject to delay than by 
the other mode, though much caution and some 
dexterity are still required to enable the experi- 
menter toarrive at satisfactory results. Butin which- 
ever way the magnetic needle may be suspended 
in these delicate investigations, the bodies under 
examination must either be held immediately in 
the hand, or indirectly, by means of some other 
body previously ascertained to have no influence 
on the needle. If held in the hand, thermo- 
electric currents are to be suspected; and if 
attached to the end of a wooden rod, by means of 
sealing wax, or resinous cement, other electric 
actions may interfere with the results; or may, 
iti 
