500 MR. HARRIS ON THE INFLUENCE OF SCREENS ON MAGNETIC ACTION. 
diately over it as in Fig. I, on a thin plate of glass gg, and at such a distance 
as to be just within the limit of the action ; when the copper disc began to move 
slowly on its pivot, a mass of iron i about half an inch thick was carefully 
raised immediately under the revolving magnet, by means of a nut n and 
screw ri affixed to the block s on which the whole was sustained ; when the 
iron i was near the under surface of the magnet, the action on the disc above 
began to diminish, so that the motion of the disc was finally checked alto- 
gether : by depressing the mass of iron i the motion of the disc was again 
restored, and this result could be obtained as often as required. 
9. It does not therefore appear essential to place the iron i immediately be- 
tween the magnet and disc in order to screen the action of the former ; since 
the same effect is produced when the iron is placed immediately beneath the 
magnet. 
10. This subject of screens seems to possess great scientific interest, and if 
fully investigated is not unlikely to bring us further acquainted with one of 
those wonderful agencies, on which the phenomenon of attraction may be sup- 
posed to depend, the more complete elucidation of which is of the utmost con- 
sequence in the present state of physical science. 
Ply mouthy April 3, 1831. 
