120 On the Cosmical Features of Terrestrial Magnetism. 



netic lines in the arctic and antarctic regions, — foremost in diffi- 

 culty, indeed, but also foremost in value. Time will not allow 

 me to specify the numerous and valuable data obtained by our 

 naval and military officers, or by our travellers, or by those of 

 other nations. The coordination of this large mass of materials 

 is necessarily a work of time ; but much has already been accom- 

 plished*, and the work is advancing towards completion. We 

 shall by these means possess a general representation of the mag- 

 netic state of the globe corresponding to a particular epoch (1840) 

 based wholly on observations made within a few years of each 

 other, and admitting of easy reduction to a common epoch. A 

 repetition of such a work, after a sufficient interval shall have 

 elapsed, will give a second representation, comparable with the 

 first, and furnishing by that comparison far more extensive and 

 accurate data than we have hitherto possessed for the secular 

 changes in all parts of the globe. Already the survey of Scot- 

 land and England, executed for the year 1837, has been repeated 

 for the year 1 860 ; and the result has shown the sufficiency of 

 an interval of even less than a quarter of a century to determine 

 with great precision the changes which have taken place in the 

 values and directions of the magnetic lines, making it clear that 

 perseverance in the systematic mode of investigation which has 

 been commenced will put us ultimately in possession of the laws 

 of the phenomena of our own globe. 



It may be that our knowledge may have its limit there ; and 

 that the cosmical or other physical causes in which the secular 

 change originates may not be discernible. Without the discovery 

 of the decennial variation of the solar spots, the cosmical rela- 

 tion of the magnetic variations having the same period would 

 have remained unknown to us ; and it is quite conceivable that 

 cosmical relations may exist in the case of the secular changes 

 which may not reveal themselves to us by any external manifes- 

 tations. 



We cannot perhaps close this brief and imperfect notice of the 

 interest attaching to the secular magnetic variations more appro- 

 priately, than by recalling the remarkable words with which Dr. 

 Halley terminated his own discussion of the subject in his first 

 paper, written nearly two centuries ago : — " Whether the mag- 

 netical poles move with one motion or with several, — whether 

 equally or unequally, — whether circular or libratory, — if circular, 

 about what centre, — if libratory, after what manner, — are secrets 

 as yet unknown to mankind, and are reserved for the industry 

 of future ages." 



* "Contributions to Terrestrial Magnetism," Nos. I. to IX. (Phil. Trans. 

 1840, Art. IV.; 1841, Art. II.; 1842, Art. II.; 1843, Art. IX.; 1843, 

 Art.X.; 1844, Art. VII.; 1846, Arts. XVII. and XVIII. ; 1849, Art.XIL). 



