of the Earth’s magnetic Force in absolute Measure. 13 
3 inches, and their diameter 34; of inch. The suspended bar was hung by 
two fibres of untwisted silk, and inclosed in a small wooden box with glazed 
front. The deflections were observed by means of a mirror attached below the 
magnet, which reflected the divisions of a scale placed at a distance of nearly 
six feet from it. As the utmost precaution was required in the experiments, 
the use of a copper ring or metallic box was dispensed with, and the arc of 
vibration reduced by means of a magnet, which was always replaced carefully 
in the same position after use. The deflecting magnet was placed on the east 
and west sides of the suspended one, at distances varying from 15 to 30 inches. 
The distances were observed by the help of a standard scale, a line at the 
middle of the magnet being made to coincide with the image of the division of 
the scale, reflected from its polished side. The observations were made beginning 
with the longest, and proceeding to the shortest distance at one side, and back 
again in the reverse order at the other, so that the two observations at the same 
distance were taken at times equally remote from the middle epoch. 
The angles of deflection were calculated by the formula 
tan 2u=43(n,—n,) k;3 
where n, and 7,, denote the observed readings of the scale, with the marked end 
of the deflecting bar to the east and to the west respectively. The value of the 
constant / is given by the formula 
a H 
—_ (i =) = 
d H F/’ 
a denoting the length of one division of the scale, d its distance from the mirror, 
H : : : 
and = the ratio of the torsion force to the magnetic force. In the present 
instance, a = *038935 of inch ; d = 68°52 inches; and - = :000345. Hence 
log k = 6°75467 ; and the angle corresponding to one division of the scale was 
consequently 
1 tan7 k = 58-623. 
The following Tables exhibit the results of the observations. The first 
column of each contains the distances of the magnets, in feet; the second, the 
values of } (2, — 7,,), these values being the means of those obtained with the de- 
flecting magnet on the east and on the west side of the suspended magnet ; 
