482 Mr. Babbage and Mr. Herschel's account of the 
perform its six revolutions in 57‘.3, that is to say, very nearly 
in the same time as when entire. This is the more remark- 
able, since tin, as we have seen, is not above half so ener- 
getic as copper when acting directly. This indirect mode of 
action therefore affords us a means of magnifying small 
magnetic susceptibilities which may hereafter prove ■ very 
valuable. 
21. To illustrate this more strongly, we suspended a brass 
disc of £^“.25 in diameter, and o‘°.i5 in thickness, as in the 
last case, and noted the time of its performing successive 
revolutions, as follows : 
1 rev. 2 rev. s rev. 4 rev. 5 rev. 
20».2 29.1 35.2 40.8 45.7 
It was now cut, as in the last case but it being necessary for 
this purpose to use a saw, the abraded portions, which were 
pretty copious, were strewed over it with the intervention of 
a piece of thin paper, to obviate the effect of loss of weight, 
as nearly as might be. The times were now found increased 
as follows : 
1 rev. 2 rev. 3 rev. 4 rev. 5 rev. 
41.1 57.9 71.0 83.0 93.7 
being almost exactly doubled, and of course the force was 
reduced in the ratio of about 4 to 1 . 
The cuts were now cleanly soldered with bismuth ; and 
though, as we have seen, the direct force of bismuth is so 
small as to be scarce perceptible, yet its indirect effect in 
restoring the magnetism of the brass was such as to cause 
the same arcs to be described in the following numbers of 
seconds* 
