382 
LIEUT . -GENERAL SABINE ON TEEEESTEIAL MAGNETISM. 
the Survey. The application of the theory professed, indeed, to be simply a first attempt 
from which we were entitled to expect little more than a rough approximation. Viewed 
in this light (and it must be remembered that this was all that the author himself claimed 
for it), its publication could only be regarded as strengthening the grounds on which the 
desire was felt for more complete and trustworthy data than were possessed by M. Gauss 
in 1839. The failure of the theory, as provisionally applied, to reproduce in the Southern 
Hemisphere the peculiar and characteristic features of the Terrestrial Magnetism which 
were so well established by observation and represented by the theory in the Northern 
Hemisphere, and the strong objection to receiving on less than conclusive evidence so 
improbable an anomaly as this diversity in the two hemispheres, tended, without 
doubt, greatly to stimulate the endeavours of those who desired to render the facts of 
observation in the Southern Hemisphere more commensurate with the theory in which 
they were to be employed. Accordingly in the VI. th Number of the Contributions (Phi- 
losophical Transactions, 1844, Art. VII.) I exhibited in Plate XIII. the lines of equal 
intensity deduced from the observations in the first two years of Sir James Ross’s Survey, 
in comparison with Gauss’s theoretical lines taken from Plates XVIII. and XIX. of the 
‘Atlas des Erdmagnetismus,’ showing the difference between the two. The immediate 
object of this comparison was to strengthen the application which was then being made 
for the additional employment of the ‘ Pagoda ; ’ whilst the two following sentences 
which I venture to extract from the paper accompanying the Plate, are sufficient to 
manifest that a full respect for the theory itself and for its illustrious author was not 
wanting. 
“ The very imperfect resemblance between the two systems of lines in the southern 
hemisphere is of course no impeachment of the sufficiency of the theory, with corrected 
numerical elements, to represent the natural phenomena in parts of the globe which 
observation may not have reached. The degree of approximation to which it will do this 
must depend upon the extent and correctness of the observation-basis from whence the 
numerical elements are derived, and upon the order of the magnitudes comprehended in 
the calculation. 
“ The evidence which the Plate affords that the calculation in the elaborate work 
referred to differs so widely from the facts in the southern latitudes, shows how much 
observations were wanting in those latitudes for the purpose of perfecting the theory, 
and is an ample justification (if indeed any justification were necessary) of the exertions 
which the last few years have witnessed to obtain them.” 
I am greatly indebted to the Hydrographer, Captain Richards, R.N., F.R.S., for his 
kind permission to have the maps which accompany this Number of the Contributions 
prepared at the Hydrographic Office, and am particularly obliged to the Assistant 
Hydrographer, Captain Frederick John Evans, R.N., F.R.S., for the very valuable 
superintendence which he has kindly given to their preparation and execution. 
It may be desirable that I should add a few words in explanation of the sense in 
which, in the present and in earlier papers, I have employed certain of the technical ex- 
