505 
copper and other substances during rotation. 
From my experiments t being i, I should obtain the fol 
■j • I n Sin. 6 
i owing values oi — 
j c » [c 
c e 
O / 
3-5 * 39 
3-0 3 H 
2.5 6 10 
2.0 14 10 
1.5 29 20 
Although there is a considerable difference in the num- 
bers, especially the last, yet as the parts whose action is not 
considered have here the greatest effect, and all the observa- 
tions are liable to errors arising from the difficulty of making 
the copper revolve with the same velocity in all cases, I 
think the agreement is sufficiently near to indicate that the 
copper acts as I have supposed. A thick copper ring would 
be best adapted for obtaining results for comparison ; and 
when I have leisure I propose making use of one. 
For the purpose of determining the law according to which 
magnets act upon a copper disk at different distances, I sus- 
pended, successively, two copper disks over the bar magnets 
placed horizontally by the side of each other, with their poles 
of the same name adjacent. The magnets were made to re- 
volve until the torsion of the wire caused the disk to return 
in the contrary direction, when I considered that the force of 
torsion would be double the force with which the magnets 
urged the disk. The time in which this took place was 
noted, and also the degree of torsion. After this the magnets 
were made to revolve again with the same velocity, and the 
sin. 0 
(c + ty 
2.3316' 
2.6341 
2.6409 VMean 2,505 
2.8144 J 
2 , 1 040 J 
