THE MAGNETIC DECLINATION AT ST. HELENA. 
53 
phenomena, whatever they might be, of an intermediate station, whether the inter- 
mediate character should be derived from proximity to the terrestrial equator, to the 
line of no dip, or to that of least force. St. Helena is situated close to the line of 
least magnetic force in a quarter of the globe where that line departs most widely 
both from the terrestrial equator and from the line of no dip ; its latitude being about 
— 16° and the dip about —22°. Should therefore the diurnal variation at St. Helena 
be found to possess the intermediate character, it was considered that it would go 
far to indicate that the character was given by proximity to the line of least intensity, 
rather than to either of the two other lines. A third station, the Cape of Good Hope, 
seemed well-suited to subject this latter point to a still severer scrutiny; although 
somewhat more distant than St. Helena from the line of least intensity (which passes 
between those stations but nearer to St. Helena than to the Cape), the magnetic 
force at the Cape is still so weak as not to exceed in absolute measure the intensity 
on some parts of the line of least force (in the neighbourhood of Singapore for 
example) : it was considered therefore as not improbable that if the intermediate 
character should prove to belong to stations at or near the line of least intensity, the 
Cape of Good Hope might be found to partake of the peculiarities of such a station, 
although the distance of the Cape from the terrestrial equator is not less than 34°, 
and the dip exceeds —53°. 
From the moment when the observations of the first complete year at St. Helena 
and the Cape arrived in England and were examined, their bearing on the solution 
of the problem was perceived : but as opinions had been expressed of the probable 
influence of the cylindrical boxes in which the magnetometers were originally placed 
in generating currents of air at particular hours of the day and seasons of the year, 
and possibly of vitiating to a greater or less degree the diurnal variation thus 
observed, it was judged more prudent to suspend a notice of the inferences to which 
they led, until the observations of subsequent years made with additional precautions 
should have been received. 
The results which will be now communicated to the Society, are founded on the 
observations at St. Helena of five years’ continuance, viz. from 1841 to 1845 inclu- 
sive. From the beginning of 1841 to July 1843, the magnet was inclosed in a 
cylindrical box, corresponding to the description in page 14 of the Report of the 
Committee of Physics of the Royal Society ; and from July 1843 to the end of 1845, 
in a double rectangular casing, which is thus described in a note from Captain 
Smythe of the Royal Artillery, Director of the St. Helena Observatory : — 
“A double rectangular box was made to inclose the magnet and replace the cylin- 
drical one with glass top hitherto used. It is composed of two boxes made of 
mahogany, every way similar to each other, the outer one being about an inch in all 
its dimensions larger than the inner one. The boxes are divided in the middle, and 
the halves fit close when pushed together by being half sunk into each other where 
they join. The interior of the inner box, and the exterior of the outer, are covered 
