138 COSMOS. 



of the magnetic meridian, in which the needle has vibrated 

 equally to either side, from one solstice to another, the com- 

 parison of the angles which the magnetic meridian describes 

 at different parallels with the geographical meridian has led, 

 in the first place, to the knowledge of lines of variation of 

 strikingly heterogeneous value (Andrea Bianco in 1436, and 

 Alonzo de Santa Cruz, cosmographer to the Emperor Charles 

 V., even attempted to lay down these lines upon charts) ; 

 and, more recently, to the successful generalization of isogonic 

 curves, lines of equal variation, which British seamen have 

 long been in the habit of gratefully designating by the his- 

 torical name of Halley's lines. Among the variously curved 

 and differently arranged closed systems of isogonic lines, 

 which are sometimes almost parallel, and more rarely re-en- 

 ter themselves so as to form oval closed systems, the great- 

 est attention, in a physical point of view, is due to those 

 lines on which the variation is null, and on both sides of 

 which variations of opposite denominations prevail, which in- 

 crease unequally with the distance.* I have already else- 

 where shown how the first discovery made by Columbus, on 

 the 13th of September, 1492, of a line of no variation in the 

 Atlantic Ocean, gave an impetus" to the study of terrestrial 

 magnetism, which, however, continued for two centuries and 

 a half to be directed solely to the discovery of better meth- 

 ods for obtaining the ship's reckoning. 



However much the higher scientific education of mariners 

 in recent times, and the improvement of instruments and 

 methods of observation, have extended our knowledge of in- 

 dividual portions of lines of no variation in Northern Asia, 

 in the Indian Archipelago, and the Atlantic Ocean, we have 

 still to regret that in this department of our knowledge, 

 where the necessity of cosmical elucidation is strongly felt, 

 the progress has been tardy and the results deficient in gen- 

 eralization. I am not ignorant that a large number of ob- 

 servations of accidental crossings of lines of no variation have 

 been noted down in the logs of various ships, but we are de- 

 ficient in a comparison and co-ordination of the materials, 

 which can not acquire any importance in reference to this 

 object or in respect to the position of the magnetic equator, 

 until individual ships shall be dispatched to different seas 

 for the sole purpose of uninterruptedly following these lines 

 throughout their course. Without a simultaneity in the ob- 



* Op. cit., vol. i., p. 187-189 ; vol. ii., p. 657-659, and p. 54-60 of 

 the present volume. 



