LINES OF NO VARIATION. 143 



individual portions of lines of no variation in Northern 

 Asia, in the Indian Archipelago and the Atlantic O^ean, 

 we have still to regret, that in this department of o?ir 

 knowledge, where the necessity of cosmical elucidation is 

 strongly felt, the progress has been tardy and the results 

 deficient in generalization. I am not ignorant that a large 

 number of observations of accidental crossings of lines of no 

 variation have been noted down in the logs of various ships, 

 but sve are deficient in a comparison and co-ordination of 

 the materials, which cannot acquire any importance in re- 

 ference to this object or in respect to the position of the 

 magnetic equator, until individual ships shall be despatched 

 to different seas for the sole purpose of uninterruptedly fol- 

 lowing these lines throughout their course. Without a 

 simultaneity in the observations, we can have no history of 

 terrestrial magnetism. I here merely reiterate a regret 

 which I have often previously expressed. 86 



86 At very different periods, once in 1809, in my Recueil d'Observ. 

 Astron. vol. i, p. 368, and again, in 1839, when, in a letter addressed to 

 the Earl of Minto, then First Lord of the Admiralty, a few days before 

 the departure of Sir James Ross on his Antarctic expedition, I endea- 

 voured more fully to develope the importance of the proposition ad- 

 vanced in the text (see Report of the Committee of Physics and Meteor, of 

 the Royal Soc. relative to the Antarctic Exped. 1840, pp. 88 91). " In 

 order to follow the indications of the magnetic equator or those of the 

 lines of no variation, the ship's course must be made to cross the lines 

 at very small distances, the bearings being changed each time that obser- 

 vations of inclination or of declination show that the ship has deviated 

 from these points. I am well aware that, in accordance with the com- 

 prehensive views of the true basis for a general theory of terrestrial mag- 

 netism, which we owe to Gauss, a thorough knowledge of the horizon- 

 tal intensity, and the choice of the points at which the three elements of 

 declination, inclination, and total intensity have all been simultaneously 

 measured, suffice for finding the value of ^- (Gauss, 4 and 27), and 

 that these are the essential points for future investigations; but the 

 sum total of the small local attractions, the requirements of steering 

 ships, the ordinary corrections of the compass, and the safety of navi- 

 gation continue to impart special importance to the knowledge of the 

 position, and to the movements of the periodic translation of lines of no 

 variation. I here plead the cause of these various requirements, which 

 are intimately connected with the interests of physical geography." 

 Many years must still pass before seamen can be enabled, to guide the 

 ship's course by charts of variation, constructed in accordance with the 

 theory of terrestrial magnetism (Sabine, in the Phil. Transact, for 1849, 

 pt. ii, p. 204), and the wholly objective view directed to actual observa- 



