OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
73 
substance not permanently magnetic ; nor could motion be induced by the 
action of such substances on each other, however near they could be placed 
with an intervening screen ; although the rapidity of the rotating body was 
caused to exceed 1000 revolutions in a minute. 
(h) A needle of soft iron was not influenced by a copper disc revolving at 
the rate of 600 times in a minute ; but rotated freely when surrounded by a 
helix transmitting an electro-magnetic current. 
(*') A disc of tempered steel not magnetic, after simple contact with a power- 
ful magnet acquired sufficient force to induce motion in metallic rings. 
(k) A needle of tempered steel was not influenced by a rotating disc of soft 
iron at the distance of one fifth of an inch, and revolving at the rate of 600 
times in a minute ; but the needle revolved rapidly after simple contact 
with a magnet. 
(/) Magnetic needles delicately mounted on an horizontal axis so as to admit 
of motion in a vertical plane, remained at rest when the axis was perpendicular 
to the radius of the revolving disc ; but the needles appeared to be carried in 
the direction of its motion when the axis was turned in the same plane. 
10. The above facts seem to show very clearly, that the presence of permanent 
magnetism, and a susceptibility of magnetic induction are essential conditions 
in the phenomena of rotation hitherto observed : at least this deduction is fair 
for all distances between the bodies at which they can be well placed with an 
intervening screen : without a screen it seems extremely difficult ever to arrive 
at a satisfactory result ; since even in a very rare medium (Exp ts (a), ( b ), (c), 
( d )) vortices are produced by the action of a rapidly revolving body, although 
with perfectly rounded and smooth edges, capable of carrying round light sub- 
stances ; and although separated from such substances by a considerable in- 
terval. So far therefore as this method of examining the influence which bodies 
not permanently magnetic can exert on each other extends, it seems not sus- 
ceptible of the requisite precision ; and is, therefore, in a great degree inade- 
quate to detect any very minute and delicate force, which can be supposed to 
arise from such an influence. 
1 1 . The law according to which metallic or other discs in a state of rotation 
influence a permanent magnet seems to be directly as the rapidity of the rota- 
tion, and inversely as the squares of the distances between the attracting 
MDCCCXXXI. L 
