66 MR. BROOKE ON THE AUTOMATIC REGISTRATION OF MAGNETOMETERS 
amplitude of the scale. This may be observed by a comparison of figs. 1 to 20, 
Plate VII., in which the scale is 0'05 inch to 1', with fig. 5, in which it is 0* 1 inch to l' : 
the only difference is in the darkness of the line, depending on the intensity of the 
point of light by which it is described. 
In order to prevent the paper being darkened by the influence of stray light, a rec- 
tangular cover is placed over the cylinders, with a slit in the side, in the same hori- 
zontal plane as the axis of the cylinders, the slit being just wide enough to allow the 
point of light to pass through it: the paper is thus protected from the rays dispersed 
by the water lens. The time-piece, cylinders and lens, are placed on a tripod stand 
with the usual vertical adjustment (see fig. 1, Plate VI.), by which the whole apparatus 
may be so placed that the lens may receive the brightest part of the image. The 
surface of the stand, and all parts of the apparatus from which light could be reflected 
on to the paper are blackened over, and the whole is covered by a second case ; in 
the side of which, towards the reflected pencil, is an aperture guarded by a tube 
about one foot long, and sufficiently large to admit the reflected pencil in any posi- 
tion that it may assume. The box in which the magnet is inclosed, to protect it 
from being disturbed by currents of air, and all other objects visible through the 
tube by an eye placed at the paper, except the mirror, are also rendered black ; and 
so complete is this protection, that not the slightest difference can be perceived in 
the paper, whether bright daylight is freely admitted through three large windows, 
or wholly excluded. This apparatus has hitherto been applied to register the varia- 
tions of the declination magnet only, but it may be considered equally applicable, 
with appropriate modifications, to record the variations of the horizontal and vertical 
elements of magnetic force. 
The experiments have been conducted under the unfavourable influence of that 
constant tremor which exists in a London thoroughfare: this has been as far as pos- 
sible counteracted by attaching to the magnet, about half-way between the point of 
suspension and its extremity, a piece of copper wire, the end of which dips into a 
glass vessel of oil ; this does not appear to have interfered with the movements of the 
magnet*. The effect of the damper, in arresting the influence of local causes on the 
vibrations of the magnet, may be best appreciated by a reference to fig. 19, Plate VII., 
a register taken during a maximum local disturbance, namely, a quadrille party in 
the next house, from the party-wall of which the magnet has been suspended. The 
unsteady movement of the magnet during the period of the dance (the termination 
of which is very well-marked) is strikingly contrasted with those periodically aug- 
mented and diminished vibrations of the magnet about its mean place, the existence 
* The use of oil as a damper is liable to the objection that it may congeal at low temperatures, and thus im- 
pede the movement of the magnet ; and on this account a slip of wood attached to the magnet, and dipping into 
a glass vessel of mercury, has been substituted. The use of mercury seems moreover desirable on account of 
its gravity when so large a concave surface as that of a mirror of 5 inches aperture is exposed to the influence 
of currents of air, which would in all probability produce more or less vibration in the magnet. — May 1847. 
