PARAMAGNETIC AND DIAMAQNETIC FORCES. 247 
undisturbed by the influence of the latter. The helix 
being 12 inches high, a cylinder of soft iron 6 inches long, 
suspended from a string and passing over a pulley, can be 
raised or lowered within the helix. When it is so far sunk 
that its lower end rests upon the table, the upper end 
finds itself between the poles N' s' of the astatic system. 
The iron cylinder is thus converted into a strong magnet, 
attracting one of the poles, and repelling the other, and 
consequently deflecting the entire astatic system. When 
the cylinder is raised so that the upper end is at the level 
of the top of the helix, its lower end conies between the 
poles N' s'; and a deflection opposed in direction to the 
former one is the immediate consequence. To render 
these deflections more easily visible, a mirror m is attached 
to the system of magnets.; a beam of light thrown upon the 
mirror being reflected and projected as a bright disk 
against the wall. The distance of this image from the 
mirror being considerable, and its angular motion double 
that of the latter, a very slight motion of the magnet is 
sufficient to produce a displacement of the image through 
several yards. 
This, then, is the principle of the beautiful apparatus* by 
which the investigation was conducted. It is manifest 
that if a second helix be placed between the poles SN with 
a cylinder within it, the action upon the astatic magnet 
may be exalted. This was the arrangement made use of 
in the actual inquiry. Thus to intensify the feeble action, 
which it is here our object to seek, we have in the first 
place neutralized the action of the earth upon the magnets, 
by placing them astatically. Secondly, by making use of 
two cylinders, and permitting them to act simultaneously 
on the four poles of the magnets, we have rendered the 
deflecting force four times what it would be, if only a 
single pole were used. Finally the whole apparatus was 
enclosed in a suitable case which protected the magnets 
from air-currents, and the deflections were read off 
through a glass plate in the case, by means of a telescope 
and scale placed at a considerable distance from the instru- 
ment. 
A pair of bismuth cylinders was first examined. Sending 
* Devised by Prof. W. Weber, and constructed by M. Leyser, of 
Leipsic. 
