Royal Society, 145 



means of a physical theory. I'he attempts which have sometimes 

 been made to explain them by a supposed connexion of the ter- 

 restrial magnetic phaenomena with the distribution of land and sea at 

 the surface of the globe, or with the distribution of heat on that 

 surface, or by electrical currents excited by the rotation of the earth 

 on its axis, contain no provision to meet a systematic variation of 

 this nature ; and break down altogether when the facts of the secu- 

 lar change are duly apprehended. From the phaenomena of a single 

 element at a single station, as here presented, we may assure 

 ourselves that effects proceeding with so much order and regularity, 

 which we cannot ascribe to any other cause than that of the ter- 

 restrial magnetism itself, and cannot therefore separate from its other 

 manifestations, must find a place in any physical theory which pro- 

 fesses to explain the phaenomena of the earth's magnetism. To learn 

 the changes in this and in the other magnetic elements which are 

 simultaneously in progress in other parts of the globe, and to appre- 

 hend their mutual connexion and the general system of secular 

 change which they indicate, it is necessary that the facts should be 

 collected in the same manner as at St. Helena, at a great number of 

 stations distributed over the earth's surface, and that they should be 

 studied both separately and together. This may indeed appear a 

 work of labour ; but it is the most certain, if not the only certain 

 mode of arriving at a correct knowledge of phsenomenal laws, when 

 the laws of their causation are wholly unknown. In this, as in simi- 

 lar studies, however complex the phaenomena may appear at the first 

 aspect, — and it is fully admitted that those of the secular magnetic 

 change do appear extremely complex at the first view, — the mind 

 soon begins to recognize order amidst apparent irregularity, and 

 system amidst incessant variation. The order and regularity with 

 which we are impressed at a single station are soon perceived to cha- 

 racterize, in an equally remarkable manner, a general systematic 

 change taking place connectedly over the whole surface of the globe, 

 and which can everywhere be traced to have been continuously in 

 operation since the earliest epoch of magnetic observation. To those 

 who find pleasure in tracing phaenomena of great apparent com- 

 plexity to laws of comparative simplicity which appear to embrace 

 them all, this study affords its own repayment; and it is indis- 

 pensable towards the acquisition of a knowledge of the laws of 

 terrestrial magnetism. By a comparison of the isogenic lines cor- 

 responding to different epochs (lines of equal Magnetic Declina- 

 tion employed by Halley and since found so useful in generali- 

 sation in this branch of the magnetic phaenomena), we perceive 

 that a secular change of the Declination, almost identical with 

 that at St. Helena, has prevailed at the same time over the greater 

 part of the southern Atlantic ; and that from the /orm of the isogonic 

 lines in that quarter of the globe (which has undergone very little 

 variation in the last 200 years), the regularity of the progression, 

 and its persistence in the same direction, is in accordance with that 

 general progressive motion from east to west, which magneticians 

 have long since recognized as distinguishing the general systematic 

 Phil Mag, S. 4. Vol. 8. No. 50. Aug, 1854. L 



