78 
MR, HARRIS ON THE TRANSIENT MAGNETIC STATE 
by which the bar tends to rest in free space ; the oscillations being supposed 
to take place in an exhausted receiver. Thus, if in free space 420 oscil- 
lations are performed before the arc of vibration is reduced from 45° to 10°, 
and the number of vibrations in the same arc, are 30 and 20 respectively, 
when the bar vibrates under the influence of two given substances taken in 
succession, and whose magnetic energies we propose to compare with each 
other ; then the energy of the one may be expressed by — 1^ r, and that 
of the other by — l^r; that is to say, their energies will be to each 
other as 13 : 20 *. 
17. Previously to examining the susceptibility of substances generally to 
magnetic influence, the number of vibrations was first determined between 
45° and 10° in air, under a glass-shade; then the number in the same arc 
also in air, when the bar was surrounded by a ring of wood of one fifth of an 
inch thick, and an inch deep ; each pole being ^jths of an inch distant from 
the interior of the ring : this being carefully ascertained, the same was repeated 
in an exhausted receiver ; the results are as follow : 
Table II. 
Number of Vibrations from 45° to 10°. 
In air. 
In exhausted receiver. 
Vibrations 
in space. 
Vibrations in 
ring of wood. 
Vibrations 
in space. 
Vibrations in 
ring of wood. 
232 
210 
420 
308 
It may be seen in this experiment that the ring of wood exerted a very 
sensible retarding power on the vibrations of the bar, which is more apparent 
when the impediment to motion arising from a resisting medium is removed. 
I submitted to experiment in this way, in a pneumatic vacuum, a few rings of 
* A magnetic bar whilst vibrating under the influence of any substance, must be considered to be 
operated on by two retarding forces ; one by which it would be eventually brought to rest, supposing 
the given substance away ; and another resulting from induction. The number of vibrations in a 
given arc, or their differences, therefore do not seem to be a measure of the latter force, since some 
portion of the former will always enter into the result of the experiment. 
