LIEUT. -G-ENEKAL SABINE ON TEEEESTEIAL MAGNETISM. 
381 
accordance of the results of the Declination on successive days, or on the return in suc- 
cessive years to the same localities, as satisfactory to a degree which might scarcely have 
been anticipated, supposing the existence of the above-noticed fact to have been known 
and duly borne in mind, viz. that the observed Declinations were subject to variations 
extending to 100 degrees and upwards, according to the direction of the ship’s head at 
the moment when the compass was observed*. 
Tables in -which all the required corrections have been applied to the individual 
observations have been prepared, and will accompany this paper with a view to their 
being deposited in the Archives of the Royal Society, in case a reference to them should 
be hereafter desired : and from these, General Tables have been prepared showing the 
mean Geographical Positions and the mean Magnetical Values after the application of 
the Corrections (including the latest of these), together with the number of observations 
from which each result has been obtained. These General Tables are subjoined at the 
close of the paper. 
The intensities of the magnetic Force are expressed in Absolute Value, in British 
units, depending upon the determinations of these values at the Magnetic Observatories 
of Hobarton and the Cape of Good Hope, regarded as the base-stations of the Survey 
(Contribution X., Philosophical Transactions, 1866, Art. XX. pp. 463, 464). 
The mean results have been inserted in the Maps of Declination, Inclination, and 
Intensity of the Force, respectively, and constitute the authorities upon which the 
endeavour has been made to trace the general course of the isogonic, isoclinal, and 
isodynamic lines, in conformity with the observations of the Survey. The maps with 
the lines thus traced upon them have supplied the groundwork for the next step, viz. 
the assignment, as far as could safely be done, of the approximate values of each of the 
three elements at the intersections of every fifth degree of latitude between — 40° and 
— 90°, and of every 10th degree of longitude from 0° to 360°, thus supplying the nume- 
rical coefficients which may be required, in whole or in part, in a revision of Gauss’s 
‘ Allgemeine Theorie des Erdmagnetismus.’ 
We learn from the original publication of that important work, in the ‘ Resultate aus 
den Beobachtungen des Magnetischen Vereins,’ im Jahre 1838, and the ‘ Atlas des Erd- 
magnetismus,’ 1840, that the numerical data employed in the calculations were for 
“twelve meridional points on each of seven parallels of latitude” — the greater part of 
the parallels being taken north of the equator, and none south of —20°. To those who 
duly considered the possible influence of this incompleteness of the numerical coefficients, 
it will not have been matter of surprise that where these were wanting the result of the 
calculations should have been found to differ widely from the facts made known by 
* No record appears to have been made of the amount of the ship’s “ heeling ” at the times of observation ; 
in reference to this Mr. Archibald Smith remarks in a note to myself, that in the ‘Erebus,’ on North aDd 
South courses, when the effect produced is a maximum, it would only be 30" tan 0 for every degree of heel ; 
which at a dip of 88° (nearly the highest observed) would give a correction of only about 15' for each degree of 
heel, and may therefore be neglected. 
3 g 2 
