1853.] OF THE ROYAL INSTITUTION. 231 
of the screws before spoken of; the motion to the given distance 
from c, by the other four. 
Supposing the distance from c towards m to be adjusted to 
0.6 of an inch, when the beam was loaded above, and no object 
before the magnet (the beam having been of course previously 
adjusted to its normal position and the torsion index placed at 
zero), it then remained to determine the return of the beam to its 
place when the object had been suspended on it and repelled: 
this was done in the following manner. A small plane reflector 
is fixed on the beam, near its middle part, under the point of suspen- 
sion: a small telescope associated with a divided scale is placed 
about 6 feet from the reflector, and in such a position that when 
the beam is in its right place, a given degree in the scale coin- 
cides with the fine wire in the telescope. Of course the scale ap- 
pears to pass by the wire as the beam itself moves, and with a 
double angular velocity, because of the reflexion. As it is easy to 
read to the fiftieth and even to the hundredth of an inch in this way, 
and as each degree occupies apparently 2.4 inches with the radius 
of 6 feet, so an angular motion, or difference of 51, of a degree 
could be observed; and as the radius of the arm of the beam 
carrying the object was 6 inches, such a quantity there would be 
less than 5,4; of an inch: 7. e. the return of the beam to its first 
or normal position by the torsion force put on to counteract the 
repulsion, could be ascertained to within that amount. When an 
object was put on the adjusted beam, if diamagnetic it was re- 
pelled ; and then, as the observer sat at the telescope, he, by means 
of a long handle, a wheel, and pinion, put on torsion until the 
place of the beam was restored; and afterwards the amount of torsion 
read off on the graduated scale became the measure in degrees of 
the repulsive force exerted. At the time of real observations, the 
magnet, balance, and telescope were all fixed in a basement room, 
upon a stone floor. But it is unnecessary to describe here the nu- 
merous precautions required in relation to the time of an obser- 
vation, the set of the suspension wire by a high torsion, the 
possible electricity of the object or beam by touch, the effect of 
feeble currents of air within the box, the shape of the object, the 
precaution against capillary action when fluids were employed as 
media, and other circumstances; or the use of certain stops, and 
the mode of procedure in the cases of paramagnetic action; — the 
object being at present to present only an intelligible view of the 
principles of action. 
When a body is submitted to the power of a magnet, it is affected, 
as to the result, not merely by the magnet, but also by the medium 
surrounding it; and even if that medium be changed for a vacuum, 
the vacuum and the body still are in like relation to each other. In fact 
the result is always differential; any change in the medium changes 
the action on the object, and there are abundance of substances 
which when surrounded by air are repelled, and when by water, are 
attracted, upon the approach of a magnet. When a certain small 
