498 
MR. HARRIS ON THE INFLUENCE OF SCREENS 
3. From this circumstance, it seemed not unreasonable to infer, as was sub- 
sequently proved by the fact, that a screening power might possibly be ob- 
tained in a similar way, by means of substances not containing iron, provided 
such substances were employed in large masses, and were in any degree sus- 
ceptible of a transient magnetic state ; notwithstanding that from the few 
experiments hitherto tried, it was rather to be inferred that such substances 
were deficient in this peculiar property*. 
4. After a few unsuccessful attempts, I succeeded in making the screening 
power of several substances, not supposed to contain iron, very evident ; the 
mechanical arrangements resorted to for the purpose were similar to those 
already described in my paper on the transient magnetic state, of which sub- 
stances are susceptible, and which has been honoured by a place in the Royal 
Society’s Transactions; it will be unnecessary therefore to describe them 
again ; they will however be easily understood in the detail of the following 
experiments. 
(a). A circular magnetic disc m, Plate XIII., fig. 1, being delicately balanced 
on a fine central point, by means of a rim of lead, was put into a state of rota- 
tion, on a small agate cup, at the rate of 600 revolutions in a minute ; and a 
light ring of tinned iron k also finely balanced on a central pivot, placed 
immediately over it, at about four inches distance, by means of a thin plate of 
glass G. 
The glass plate was supported on two sliding bars, and stands F F, by 
which it could be set to any required height ; it was also furnished with four 
levelling screws, passing through two clamps of wood in which the extre- 
mities of the plate were fixed, so as to be further adjusted with the required 
precision : when the iron began to move slowly on its pivot, by the influence 
of the magnet revolving below, a large mass of copper C, about three inches 
thick, and consisting of plates a foot square, was carefully interposed ; it being 
sustained on a convenient carriage r r, moveable on a rail-way R R, in order 
to be easily transferred without deranging the subject of experiment. 
5. The copper thus interposed soon diminished sensibly the motion of the 
disc k , and at length arrested it altogether : on again withdrawing the copper, 
* See Transactions of the Royal Society for the Year 1825. p. 469. 
