76 
MR. HARRIS ON THE TRANSIENT MAGNETIC STATE 
nothing more than the presence of a temporary magnetic development induced 
in successive points of a body by the action of a permanent magnet, the body 
being supposed in motion. If the body therefore be at rest, this development 
will restrain the motion of a bar vibrating near it, and so diminish the ampli- 
tude of its oscillations ; and thus by determining the number of vibrations per- 
formed in a given arc, we may from thence arrive at a comparative value of 
the force in action. 
To this effect the bar already mentioned (12) was suspended, and exposed to 
the influence of different substances, in the following manner. 
Two perpendicular rods of glass r r, r' r v , fig. 1, furnished with foot- and cap- 
pieces are fixed in a solid block of mahogany r r', fig. 1 ; these rods by the in- 
tervention of a short wood cylinder /, and two horizontal rods, also of glass, 
rl, rl, sustain a glass tube t f which slides with friction through the cylinder. 
This tube is furnished with a cap-piece at t , through which passes a brass rod : 
the magnetic bar is suspended from this rod by a filament of silk, and is finally 
raised or depressed to the required altitude by a micrometer-screw at t : the 
altitude of the glass tube l being previously fixed. The centre of the maho- 
gany block is hollowed into a cylindrical cavity as represented in fig. 10, and 
is firmly fixed upon the pump-plate by means of the screw and shoulder pro- 
jecting through the brass collars (4). Its outer part is also depressed, leaving 
a cylindrical projection of about one fourth of an inch deep, 4.7 inches diameter, 
and 1 .25 inch wide. 
The graduated circle of stout card-board D already mentioned (6) slides 
with friction between the glass rods, so as to be easily adjusted at any given 
point. The bar is retained at the given angle from its meridian, and again set 
free, when the exhaustion is sufficiently complete, by means of the lever and 
double stop before explained (4), and which moves in the interior circular 
cavity of the block, fig. 10. 
This method of arresting the bar seems to be of some consequence to 
the experiment ; for if one pole only be checked, the force operating on the 
other, will for an obvious reason, give the bar a swinging motion, which is very 
undesirable, but which is here effectually prevented ; so that when set free it 
will appear to oscillate as if mounted on a fixed centre. The checks by which 
the bar is thus arrested, are so contrived as to be independent of each other, 
