640 EXPERIMENTAL INVESTIGATION OF THE 
been accurately counterpoised, a process which is 
still further facilitated by the introduction of the 
hand at one side of the box, which is opened for 
that purpose, and afterwards closed by a sliding 
mahogany door. 
When the agitations of the lever have subsided, 
the sliding door is partially opened for the intro- 
duction of the poles of a powerful steel horse-shoe 
magnet, which is made of a long and narrow 
shape for the purpose. This magnet is placed on 
a sliding carriage, by means of which it is made 
to approach the specimen, or recede from it, with 
great facility and in the most gentle manner. In 
consequence of finding decided polarity in some 
specimens in which no magnetism was previously 
known to exist, I have been led to the employ- 
ment of a powerful bar magnet, which I find con- 
venient in those cases where such polarity is 
suspected. I have also employed electro-mag- 
nets, both straight and of the horse-shoe form ; 
but having found that these are troublesome and 
inconvenient, I have abandoned the use of them 
altogether in these inquiries. 
In cases where extreme nicety is required, | 
have found the following mode exceedingly useful. 
