OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
81 
ber of vibrations was first determined in the interior ring alone, after which a 
second was added, and the number of vibrations again determined, and so on 
to eight rings ; when the increased effect became almost inappreciable. 
Table V. 
Number of | 
rings / 
1 
2 
3 
4 
5 
6 
7 
8 
Vibrations .... 
44 
30 
24 
21 
19 + 
17 
16 + 
16 
Ratio of force . 
8.54 
13 
16.5 
19 
21 + 
23 
24 + 
25 
The increments in the effect on the bar appear by the above Table conti- 
nually to decrease, so that after a certain number of laminae they seem to be no 
longer appreciable ; hence the numbers expressing the comparative energies 
would become at last equal, and such was found to be the case, the number of 
vibrations with twelve rings being no greater than with ten*. 
22. The quantity of matter in the foregoing experiment being actually di- 
vided into concentric rings, I endeavoured to ascertain how far the above results 
depended on the sum of the whole, or of any number of them ; with this view 
the number of vibrations was determined for each ring, when opposed to the 
bar in the same relative situation which it occupied in the mass, so as to ob- 
tain the separate effect due to any particular ring. These separated results 
were as follow. 
* It is not improbable that some principle of this kind is the immediate cause of the curious re- 
sult arrived at by Professor Barlow in the course of his magnetic inquiries ; who observed that a 
hollow sphere of iron exerted the same influence on a compass needle, as if it were a solid mass. If 
we consider the sphere to be made up of concentric laminae, then, by the foregoing experiment, the 
number of laminae equivalent to exhaust the inductive energy of a small compass needle, will, espe- 
cially in iron, (11) (Exp. /note,) be very few : consequently, all the interior maybe removed. This 
is likewise consistent with a subsequent observation of the same distinguished philosopher ; a certain 
depth of metal being found necessary. If the experiment were accurately tried, it is probable that 
the depth of metal requisite, would vary with the force of the compass bar ; although for small needles 
the differences are so inconsiderable as not to be appreciable. For very powerful magnets, however, 
some further quantity of metal would be required ; but, perhaps, in no instance could we employ a 
magnet of sufficient power to influence the whole mass of a solid iron sphere of large dimensions. 
MDCCCXXXI. MT 
