122 ANNUAL OF SCIENTIFIC DISCOVERY. 



earlier recorded observations with those of the present epoch gives 

 reason to believe, that viewed in its generality, the motion of the 

 system of force which produces the secular change has been uniform, 

 or nearly so, in the last two or three centuries. Under favorable 

 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 it too frequently 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 there- 

 fore 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 a 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, are small 

 in comparison 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 agency by which they are produced. To 

 investigate these variations bv suitable instruments and methods, to 



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separate each from the others, and to seek its period, its epochs of 

 maximum and minimum, the laws of its progression, and its mean 

 and numerical value or amount, constituted the chief purposes for 

 which magnetic observatories were established for limited periods at 

 certain stations in Pier Majesty's dominions, selected in the vieAv that 

 by a combination of the results obtained at them a general theory of 

 each at least of the principal periodical variations might be derived, 

 and tests be thus supplied whereby the truth of physical theories pro- 

 pounded for their explanation might be examined. We are just 

 beginning to profit by the collocation and study of the great body of facts 

 which has been collected. Variations corresponding in period to the 

 earth's revolution around the sun, and to its rotation around its own 

 axis, have been ascertained to exist, and their numerical values approx- 

 imately determined in each of the three elements, the Declination, 

 Inclination, aud Magnetic Force. We unhesitatingly refer these 



