502 
MR. HARRIS ON THE POWER OF MASSES OF IRON 
4. To regulate the distance between a magnet or other bodies, and the sus- 
pended iron, when placed immediately under the latter, either vertically or 
horizontally, I employed a simple screw and nut, attached to a brass frame simi- 
lar to that shown in Fig. 2 : thus any required altitude could be obtained, 
the distance between the magnet and the suspended iron being estimated by 
a graduated scale resting on the magnet. 
5. Wherever magnetic attraction is exerted between two bodies, it seems to 
be accompanied by a sort of neutralization of the same force in respect of a 
third substance. 
(a) . Thus if the magnet A, Fig. 1, be attracting the suspended iron b, the 
vicinity of a mass of iron C will diminish the apparent force of A upon b : if, 
therefore, when the force of A upon b is exerted so as just to depress the beam, 
the force being measured by the inclination of the index (3), we place a mass 
of iron C close to A, the beam will immediately tend to recover its previous 
position. 
( b ) . This power of the iron C to controul the attractive force of the magnet 
upon b , seems only to extend to a given point within the magnet, the distance 
between the magnet and iron remaining the same ; for if the small iron b be 
suspended over a point at some distance from the. extremity, as at w, then the 
action of C will not be felt at that point w , except by decreasing the distance 
between the iron and magnet, or otherwise by increasing the neutralizing 
power of the iron C. 
6. That this depends on a sort of action which is not inappropriately termed 
a neutralization of force, in regard to the suspended iron b, is evident from the 
following experiments. 
(c) . The mass of iron C may be placed immediately below A, as in Fig. 3, the 
effect will be the most apparent when the thickness of A is not considerable ; 
hut if the magnet A be very thick, then the neutralizing effect of C is not so 
evident, on account of the intervening mass of which the magnet is composed, 
as also on account of the iron C being kept as it were at a greater distance 
from the immediate surface of attraction. 
{d). Conversely, the mass of iron C may be placed above, immediately be- 
tween b and A as in Fig. 4 ; but in this case we have to take into account the 
inductive effect on C, by which it becomes itself a temporary magnet, and con- 
