ADDRESS OF THE PRESIDENT OF THE BRITISH ASSOCIATION. 



[1852. 



Lm" corresponding to tlie present epoch. Tlie magnetic phe- 

 nomena, or as it is now ciistoniary to call them, the three magnetic 

 elements, api>ear to he everywhere and in both hemlsplieies the 

 resultants of a duplicate system of mag-netic 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 west to east in the southern. It 

 is to this motion that the secuhu- change in all localities is chiefly 

 if not entirely, due; affecting systematically and according to 

 their relative positions on tlie globe the config-urations and j^eo- 

 graphical positions of the magnetic lines, and producing conform- 

 able changes in the direction and amount of the magnetic 

 elements in every part of the globe. The comparison of the 

 earher recorded observations with those of the present epoch 

 gives reason to believe, that ^■iewed in its generality, the motion 

 of the system of forces which produces the secular change has 

 been uniform, or neai'ly so, in the last two or three centuries. 

 Under fiivouvable conditions the regularity of this movement can 

 be traced down to comparatively very minute fractions of time. 

 By the results of carefid observations continued fir several yeare 

 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 ahcjuot 

 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 mag-netic 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 assur- 

 edly in no degree entitled to ascribe secular variation, — and wlio 

 reason therefore as if the great magnetic phenomena of the earth 

 were pereistent, instead of being, as they are, subject to a continual 

 and progressi\'e change. It may confidently be affirmed that the 

 seculai- 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 laivs to be obtained by a repetition, after a suffi- 

 cient 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 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 tlieir origin and the agency by which 

 they are produced. They formed accordingly the subject of a 

 distinct recommendation from the British Association, which met 

 with an equally favourable reception. To mvestigate these vari- 

 ations by suitable instruments and methods, to separate eajh from 

 the others, and to seek its period, its epochs of maximum and 

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

 ■value or amount, constituted the chief purposes for which mag- 

 netic observatories were established for limited periods at certain 

 stations in Her Majesty's dominions, selected in a view that by 

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

 each at least of the principal periodical variations might be 

 dc-i-ixc'l, and tests be thus su]i|ilied whereby the truth of physical 

 thi'ories propounded for their ox[ilanation might be examined. 

 We are just beginning to jirofit by the collocation and study of 

 the great body of facts which has been collected. Variations 

 coiTcsponding in period to the earth's revolution around the sun, 

 snd to its roUition around its own axis, ha\e been asccrLiinod to 

 exist, and their numerical values approximately d(.'tcrmincd in 



each of the three elements, the Declination, Inclination, and Mag- 

 netic Force. We unhesitatingly refer these variations to the sun 

 as their primary source, since we find that in whatever prn-t of 

 the globe the phenomena are observed, the solstices and equinoxes 

 are ihe critical epoclis of the variations whose period is a year, 

 whilst the diurnal variation follows in all meridians nearly the 

 same law of local solar houre. To these unquestionable e\Tidences 

 of solar influence in the magnetic afl'ections of the earth, we have 

 now to add the recently ascertained fact, that the magnetic storms, 

 or distm'bances, which in the absence of more correct knowledge 

 were supposed to be wholly irregnilar in then' occuixence, ai'e 

 strictly periodical phenomena, conforming with systematic regu- 

 lai'ity to laws in which the influence of local solar hom's is 

 distinctly traced. 



But whilst we recognize the sun as the j>rimary cause of vari- 

 ations whose periods attest the source from whence they dei-ive 

 their origin, the mode or modes in which the eft'ects are produced 

 constitute a question which has been and may still be open to a 

 yariety of opinions; the direct action of the sun as being itself a 

 magnet — its calorific agency in occasioning thermo-electric and 

 gahauic currents, or in alternately exalting and depressing the 

 magnetic condition of substances near the surface of the earth ca- 

 in one of the constituents of its atmosjjhere, — have been severally 

 adduced as hypotheses affording plausible explanations. Of each 

 and all such hypotheses the facts are the only true criteria; but 

 it is right that we should bear in mind that in the present state of 

 our knowledge, the evidence which may give a decided counte- 

 nance to one hypothesis m preference to othere does not preclude 

 their possible co-existence. The analysis of the collected materials, 

 and the disentanglement of the various effects which ai'e compre- 

 hended in them, is far from being yet complete. The corres- 

 pondence of the critical epoelis of the annual variation with the 

 solstices and equinoxes rather than with the epochs of maximum 

 and minimum temperature, which at the surface of the earth, in 

 the subsoil beneath the surface, or in the atmosphere above the 

 surface, are sepai'ated by a wide interval from the solstitial epochs, 

 appears to favour the hypothesis of a direct action ; as does also the 

 remarkable fact which has been established, that the magnetic force 

 ii greater in both the northern and southei'n hemispiheres in the 

 ii'.onths of December, January, and February, when the sun is 

 neaj'cst to the earth, than in those of May, June, and July, when 

 he is most distant from it: whereas if the effect were due to 

 temperature, the two hemispheres should be oppositely instead 

 of similarly affected in each of the two periods refeiTed to. Still, 

 tliere are doubtless minor periodical irregular variations which 

 have yet to be made out by suitable analj'tical processes, which, 

 possible accordance with the epochs of maxinnmi and minimum 

 temperature, may support in a more limited sense, not as a sole 

 but as a co-ordinate cause, the hypothesis of caloiific agency so 

 generally received, and so ably advocated of late in eomiexion 

 with the discovery by our great chemist and philosopher of the 

 magnetic properties of oxygen and of the manner in which they 

 are modified and affected by differences of temperature. It may 

 indeed be diflieult to suppose that the magnetic phenomena which 

 we measure at the surface of the globe should not be in any 

 degree influenced by the v.iriations in the magnetic conditions of 

 the oxygen of the atmos]ihere in difl'erent seasons and at different 

 hours of the day and night; but whether that influence be sen- 

 sible or not, whether it be ap]irociable by our instruments or 

 inajipreciableby them, is a question which yet remains for solution 

 by the more minute siftnig of the accumulated facts which are 

 now undergoing examination in so many quartere. 



To justify the anticipation that conclusions of the most striking 

 character, and •wholly unforeseen, may yet be derivable from the 

 materials in our ])ossession, we need only to recall the experience 

 of the last few months, which have brought to our knowledge 



