EDITOR'S PREFACE. ix 



have begun to emerge from that state in which " facts, 

 though studied with assiduity and sagacity, still appear 

 for the most part unconnected, with little mutual rela- 

 tion, and it may be even in some instances in seeming 

 contradiction with each other." I venture to think that 

 these, the most advanced branches of terrestrial mag- 

 netism, have already in some measure reached the 

 stage at which " observations having multiplied, and 

 having been combined by reflection, more and more 

 points of contact and links of mutual relation are 

 discovered," and the intricacies arising from the " ex- 

 cessive combination of phenomena " are already yield- 

 ing to a " knowledge of the primary laws by which 

 they are regulated," which knowledge is, in its turn, 

 conducting to still " higher and more extensive general- 

 isations" thus preparing the way for that yet more 

 advanced stage, when "a deeper insight into natural 

 forces may be attained." Yet while in regard to these 

 branches it is already becoming " more and more pos- 

 sible to develop general truths with conciseness without 

 superficiality," that task is still one of very great 

 difficulty, and admits of only imperfect and partial 

 realisation. Therefore, inasmuch as it was impossible 

 for me to comprehend within any admissible limits a 

 complete resume of all that has already been gained in 

 terrestrial magnetism, I have, on the present occasion, 



