20 MR. BROOKE ON THE AUTOMATIC REGISTRATION OF MAGNETOMETERS, 
accomplished, if by any mechanical agency either of the quantities W, a, or h could 
be simultaneously influenced by change of temperature, and proportionally to the 
altered value of m. 
As it would be desirable that the correction should involve the small coefficient of 
the second power of the temperature, a very definite value of which has been deter- 
mined*, it was at first pi-oposed to act on W by means of a hollow glass globe 
attached to the suspension frame of the magnet, with a vertical tubular stem dipping 
into a cup of mercury. It is clear that as the elastic force of the air contained in the 
bulb is diminished by heat, a column of mercury in the stem would fall, and the 
diminution of the suspended weight thus occasioned, might by a due adjustment of 
the capacity of the bulb be rendered equivalent to the constant part of the loss of 
power in the bar, and would thus represent the coefficient of the first power of the 
temperature, while an equivalent to the coefficient of the second power might be 
obtained by a due adjustment of the diameter of the stem. It is evident that the 
smaller tlie diameter of the stem, or in other words, the longer the space occupied by 
the mercury depressed during a large interval of temperature, the greater would be 
the difference of the spaces corresponding to successive small intervals. But this 
arrangement would be liable to several small sources of error ; first, an alteration of 
the suspended weight by hygrometric changes in the length of the suspension skein ; 
secondly, a change in the bulk of the air in the bulb corresponding to barometric 
changes; and thirdly, a small and uncertain secular variation, from the evaporation 
of mercury in the cistern in which the stem would be immersed. 
From these considerations, it has been deemed advisable to abandon the attempt 
to introduce in the compensation an equivalent for the small coefficient of and to 
rest satisfied with acting on a or h by means which would effect an equal change for 
equal intervals of temperature, and would therefore represent the coefficient of t only. 
Metallic expansion naturally suggested itself as the means of accomplishing this 
object, and h the lower interval of the skeins as the most appropriate point of appli- 
cation, inasmuch as the compensating apparatus would then be in close proximity 
with the magnet, and being isolated from the surrounding atmosphere by enclosure 
in the same case or cases, might be presumed to vary in temperature and 
simultaneously with the magnet ; and if the changes in temperature were not suffi- 
ciently simultaneous, the radiating capacity of the magnet might be increased by par- 
tially coating it with dead-black varnish, or diminished by gilding and burnishing it, 
to any required extent. 
The compensating apparatus consists of a glass rod clamped at its middle point to 
the centre of magnet, the axes of the rod and bar being parallel : the free ends of the 
rod are enclosed in two zinc tubes, at the inner ends of which, where they nearly 
meet in the centre, and to their upper surface, two hooks are attached : two loops at 
the ends of the suspension skein are attached to these hooks, the skein passing over 
* See a paper. No. III. by the Author in the Phil. Trans., Part I. for 1850. 
