64 MR. BROOKE ON THE AUTOMATIC REGISTRATION OF MAGNETOMETERS, 
The following arrangements have been adopted to fulfil the third indication, that 
of magnifying sufficiently the movements of the magnet. A concave mirror of ten 
inches focus, and three inches aperture, is attached to a brass stem intervening be- 
tween the frame which supports the magnet and the suspension chord. In order to 
reduce as much as possible the influence of torsion on the position of the magnet, 
the suspension skein consists of six equal bundles of untwisted silk fibres about six 
feet long, being portions of the same skein of flos silk ; these, after being boiled in a 
solution of bichloride of mercury to render them less hygrometric, were stretched by 
six equal weights, the sum of which was very nearly that which the whole would subse- 
quently have to support, and after having been left for three or four days to find their 
position of rest, were firmly bound together : by these means the probable equality of 
tension and freedom from torsion in the entire chord will, it is believed, be consider- 
ably augmented. The mirror may be moved round with the stem through any angle 
of azimuth, that the reflected pencil of light may proceed in a convenient direction : 
and the lamp must be placed at such a distance from the mirror that the conjugate 
focus may be at any required distance : and on this distance depends the amplitude 
of the scale, on which the variations of the magnet are represented. If the image of 
the slit is formed at a distance of 7 feet 2 inches from the line of suspension, each 
minute of angular motion of the magnet will be represented by a change of position 
of the line on the paper, amounting to xoth of an inch ; if formed at 9 feet 6^ inches, 
l' will be equivalent to xg-th of an inch ; if at 11 feet 1 1^ inches, to xath of an inch ; 
and if at 14 feet 4 inches, x^h °f an inch will represent l'. The support of the lamp 
by which its adjustments are effected will be readily understood by an inspection of 
Plate I. fig. 1 *. 
The pencil of light forming the image of the slit is intercepted by a cylindrical 
lens, the axis of which is horizontal, and perpendicular to the vertical plane passing 
through the image and the centre of the mirror; the lens being placed at its focal 
distance from the image. By this arrangement, that portion of the pencil which 
passes through the lens is condensed vertically into a narrow space, without at all 
interfering with its horizontal movement. The most suitable focal length of the 
cylindrical lens depends upon the length of range : at the shortest distance above- 
mentioned, a focal length of T5 inch will be found to answer very well ; at the next, 
two inches ; and at the longest, three inches. 
As the amount of spherical aberration increases considerably as the eccentricity 
of the reflected pencil is increased, it is desirable that this eccentricity should be 
the least possible; with this view, the lamp must be so adjusted in the horizontal 
separate lamp placed about 9 or 10 inches from the cylinder ; a small pencil, the axis of which is perpendicular to 
the axis of the cylinder, is received by a lens placed at less than its focal distance from the paper, a small por- 
tion of the pencil thus condensed is transmitted through a narrow vertical fixed slit, and marks the paper (see 
fig. 8, Plate IX.). In the combined register of the balanced magnetometer and the barometer, the base-line is 
described by the barometer lamp, as mentioned in the Supplement. — May 1847. 
* The lamp now stands on a fixed pillar to which the fine slit is attached. — May 1847. 
