75 



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 phenomena 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 isogonic 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 phenomena), 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 form 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 

 change in the southern hemisphere from that in the northern, which 

 takes place in the opposite direction ; whilst from the form of the 

 isogonic lines in that quarter, we may further anticipate that, at St. 

 Helena, the secular change of the Declination will continue to take 

 place in the same direction as at present, until the line drawn through 

 the conical summits of the isogonic curves shall in its western pro- 

 gress pass the geographical meridian of that station." 



The author then proceeds to the Variations which are found to 

 take place in periods corresponding to a solar year and a solar day ; 

 a correspondence which, he remarks, " enables us to recognize a phy- 

 sical connexion, although we are still uncertain as to the mode of 

 operation between cause and effect. A correct knowledge of the 

 phenomena themselves is the surest guide to a correct judgement 

 amongst the many theories which have been propounded in anticipa- 

 tion of that knowledge ; and I have therefore taken this opportunity 

 of bringing before the Society a careful analysis of the primary an- 

 nual and diurnal variations at St. Helena attributable to solar influ- 



