OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
77 
and are moveable with friction in a small tube of brass as at ss, figs. 2 ancl 10, 
and may consequently be adjusted with great nicety. The tube in which the 
checks are fixed is sustained in a horizontal direction in the extremity of the 
rod, which passes into the receiver through the centre of the pump-plate. 
14. The substances to be submitted to experiment, are formed into rings as 
at R, fig. 11; in these the bar is caused to oscillate. Each ring is about one 
inch in height, of any convenient thickness, and 4.75 inches interior dia- 
meter, so as to admit of its being accurately adjusted on the cylindrical 
shoulder of the circular base, fig. 10. 
15. In applying the method of an oscillating bar to the investigation of 
minute and transient magnetic forces, it seems essential to keep in view the 
following interesting fact ; viz. the influence of bodies, not susceptible of per- 
manent polarity, on the state of oscillation is such, that the amplitude of the 
vibrations only is diminished, not their duration : that is to say, whether a 
bar vibrate in free space, or otherwise near plates, or in rings of these sub- 
stances, still the number of oscillations in a given time, considered as a unit of 
time, does not vary, although the bar is sooner brought to rest when under the 
influence of such bodies, than when freed from them, whatever substance be 
employed, and at whatever distance the influence be exerted. 
This fact seems materially to distinguish the peculiar influence of non-mag- 
netic substances, from a case of permanent polarity ; by which last, an oscil- 
lating bar is not only brought to rest in less time, but the rate of vibration is 
very sensibly increased. We cannot therefore, as in the latter instance, resort 
to the common law of pendulums, and take the square of the number of oscil- 
lations performed in a given time as a measure of the force in action, since 
the time of each oscillation does not sensibly vary ; we must therefore adopt 
some other method. 
16. In order to arrive at a comparative value of the influence of any sub- 
stance on a vibrating bar, I have been led to employ the following for- 
mula, — 1 ^ r ; which seems to apply in a very remarkable manner to 
the results of experiment ; in which a represents the number of oscillations 
in a given arc in free space, b the number in the same arc, when exposed to the 
influence of a substance not permanently magnetic, and r the retarding force 
