lii REPORT—1852. 
date, which were submitted to the British Association in the Report on the 
* Variations of the Intensity of the Magnetic Force at different points of 
the Earth’s surface,” which preceded the recommendations of 1838. The 
mnagnetic phenomena, or as it is now customary to call them, the three mag- 
netic elements, appear to be everywhere and in both hemispheres the 
resultants of a duplicate system: of magnetic forces, of which one at least 
undergoes a continuous and progressive translation in geographical space, 
the motion being from west to east in the northern hemisphere, and from 
east to west in the southern. It is to this motion that the secular change in 
all localities is chiefly if not entirely due, affecting systematically and ac- 
cording to their relative positions on the globe, the configurations and geo- 
graphical positions of the magnetic lines, and producing conformable 
changes in the direction and amount of the magnetic elements in every 
part of the globe. The comparison of the earlier recorded observations 
with those of the present epoch gives reason to believe, that viewed in its 
generality, the motion of the system of forces which produces the secular 
change has been uniform, or nearly so, in the last two or three centuries. 
Under favourable conditions the regularity of this movement can be traced 
down to comparatively very minute fractions of time; by the results of 
careful observations continued for several years at the observatory of St. 
Helena, where, in common with the greater part of the district of the South 
Atlantic, the secular change of the declination exceeds eight minutes in the 
year, and from its magnitude therefore may be advantageously studied,— 
every fortnight of the year is found to have its precise aliquot portion of the 
annual amount of the secular change at the station. This phenomenon of 
secular change is undoubtedly one of the most remarkable features of the 
magnetic system, and cannot with propriety be overlooked, as too frequently it 
has been, by those who would connect the phenomena of terrestrial magnetism 
generally, mediately or immediately, with climatic circumstances, relations of 
land and sea, or other causes to which we are assuredly in no degree entitled 
to ascribe secular variation, and who reason therefore as if the great magnetic 
phenomena of the earth were persistent instead of being as they are subject 
to a continual and progressive change. It may confidently be affirmed that 
the secular magnetic variation has no analogy with, or resemblance to, any 
other physical phenomenon with which we are acquainted. We appear at 
present to be without any clue to guide us to its physical causes, but the way 
is preparing for a future secure derivation of its laws to be obtained by a 
repetition, after a sufficient interval, of the steps which we are now taking to 
determine the elements corresponding to a definite epoch. 
The periodical variations in the terrestrial magnetic force, which I have 
before adverted to as distinguished from its secular change, are small in com- 
parison with the force itself, but they are highly deserving of attention on 
account of the probability that by suitable methods of investigation they 
may be made to reveal the sources to which they owe their origin and the 
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