68 
MR. HARRIS ON THE TRANSIENT MAGNETIC STATE 
diameter, and about T vth of an inch thick, being delicately balanced on a 
fine point, by means of a transverse bar of wood carrying an agate centre, 
was placed on a firm screen of paper strained tight on a wood frame, and 
covered by a glass shade ; two powerful magnetic bars were caused to revolve 
in a horizontal plane, beneath the screen, with their poles immediately under 
the ring. The ring, as in all the previous experiments of the same kind, soon 
acquired motion in the direction of the revolving bars, which could be arrested 
and reversed at pleasure, by changing the direction of the rotation. It was 
however soon discovered, that a similar effect could be produced when the 
magnets were not present, merely by the action of the rotating apparatus. 
This last consisted of a train of wheels resting on a firm pavement of thick 
stone ; the frame carrying the screen being supported over it on blocks of oak. 
As it was of consequence to ascertain how far small vibratory impulses com- 
municated to the screen, could cause the phenomena just observed, a con- 
tinuous but gentle vibration was induced from one corner of it, by a slight 
tapping, which, after a few minutes, communicated motion to the ring in a 
constant direction ; on transferring the vibrations to the opposite side of the 
frame, the ring was again brought to rest, and caused to revolve in an oppo- 
site direction. A similar result was obtained when a compass needle, and 
light needles of other substances were substituted for the ring of copper. 
3. These results led me to place the copper circle in an exhausted receiver, 
on a strong plate of glass, and to suspend the whole on a convenient frame by 
means of lines passing from firm walls ; the revolving bars being placed as 
before, immediately under it. In this instance, however, I failed at the same 
distance to move the ring, nor could a fine compass needle similarly placed be 
made to deviate more than a few degrees from its meridian, when exposed to 
the influence of a rapidly revolving disc. This result is quite sufficient to 
show how minute a cause may render delicate investigations with a rotating 
body unsatisfactory ; more especially when connected with a heavy apparatus: 
even in the case of screens it does not seem quite clear that certain impulses 
may not be propagated through them, of sufficient magnitude to act on a very 
finely suspended needle or disc ; when the screens are thin and porous, the 
chance of this is greatly increased, and in cases where a screen only is em- 
ployed without a shade; similar results would doubtless ensue from vortices 
