90 
FIFTH REPORT— 1835 . 
correspond in date. It is known that the north dip is at present 
diminishing in this part of the world about 3 minutes annually, 
and that it has not differed materially from that rate of diminu- 
tion for several years past : but from a comparison of observa¬ 
tions we learn, that in the Gulf of Guinea the annual diminution 
is little less than ten minutes, if it does not exceed that amount; 
whilst in the China Seas between the years 1700 and 1780 the 
north dip, on the contrary, increased, and at an average annual 
rate which could scarcely have fallen short of fifteen minutes, or 
a quarter of a degree a year. Our knowledge as yet is very far 
from being sufficient to enable us to render justly comparative 
the observations of different years, except in a very few parts 
of the globe. 
In the northern hemisphere we probably now possess the re¬ 
quisite materials for describing the magnetic curves, from obser¬ 
vations greatly to be relied on, and so nearly contemporaneous 
as to occasion but little error in reduction. But it is far other¬ 
wise in the southern hemisphere, particularly in what are usually 
called the high magnetic latitudes, and where an acquaintance 
with the facts would be of principal value towards a knowledge 
of the system of Terrestrial Magnetism. The enterprise of our 
merchant seamen has shown that these latitudes are far more 
accessible, in certain meridians at least, than had been previously 
supposed. The magnetic observations of the voyages of Weddell 
and Biscoe have been confined to those of the variation; these 
fully confirm M. HansteeiTs position of the general westward 
movement of the lines of equal variation, in the southern hemi¬ 
sphere. But it is in the meridians left untouched by those 
vessels,—in those which include and are adjacent to those mag¬ 
netic foci in the southern hemisphere, which M. Hansteen has 
called points of convergence, that observations would be chiefly 
useful; and observations confined to the variation, but including 
also the dip, and intensity of the force. The ice itself, or such 
lands as might be discovered by a vessel coasting the southern ice 
between the meridians of 80° E. and 260° E., "would furnish the 
requisite localities for the observations of the three phaenomena ; 
and would supply what is wanting to complete a map exhibiting 
the arrangement, corresponding to a definite epoch, of the curves 
of equal dip, variation, and intensity, over the whole surface of 
our globe. 
