IN ARRESTING THE PROGRESS OF MAGNETIC ACTION. 
499 
the motion of the disc was restored, and this effect could be obtained as often 
as required. 
In this experiment both the magnet and disc were very completely inclosed 
by glass shades adapted to the nature of the experiment, and were also sup- 
ported on a firmly fixed base. 
( b ) . When a mass of silver or zinc of about the same dimensions was substi- 
tuted for the copper, a similar result ensued ; the motion of the disc was com- 
pletely arrested by the screening power of the intervening mass. 
(c) . The screening property evinced by these substances depended, as in the 
foregoing experiment with the iron plates (2), on the whole mass interposed, as 
subsequently appeared by removing the interior laminae, in which case the 
motion of the disc was no longer impeded. 
6. It may therefore be reasonably inferred that this power of intercepting 
magnetic action is more or less common to every class of substance, and that 
to render it sensible it is only requisite to employ different bodies in masses 
bearing some direct ratio to their respective magnetic energies. 
(d) . Thus in substituting a similar mass of lead in the above-mentioned ex- 
periments a b, the motion of the disc k could not be completely checked, 
and it was subsequently found requisite to increase the quantity of the inter- 
vening mass very considerably before the screening effect became sensible to 
any great extent ; the magnetic energy of lead being so much less than that of 
copper. 
7. The screening influence of substances is best shown by employing a 
powerful magnet, and by placing the disc k just within the limit of the action ; 
thus a sufficient mass may be interposed, and the screening effect made very 
evident. To exemplify the influence of distilled water in this way at about 32° 
of Fahrenheit’s scale, I am led to believe it would be requisite to obtain a 
slight action on the disc, at a distance of rather more than thirty feet, so as to 
interpose, about the same thickness of ice. 
8. This curious property seems to be intimately connected with a principle, 
which may be termed a neutralization of force ; by which the magnetic action 
is, as it were controulled, as in the following experiment. 
(e) . A circular magnetic disc tn, Fig. 2, being put into a state of rapid 
rotation, a light ring of copper k movable on a fine centre was placed imme- 
