LINES OF NO VARIATION. 139 



servations, we can have no history of terrestrial magnetism. 

 I here merely reiterate a regret which I have often previous' 

 ly expressed.* 



* At very different periods, once in 1809, in my Recu-il cfObscrv. 

 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 

 endeavored more fully to develop the importance of the proposition 

 advanced in the text (see Report of the Committee of Physics and Me- 

 teor, of the Royal Soc. relative to the Antarctic Exped., 1840, p. 88-91). 

 u 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 observations of inclination or of declination show that the ship 

 has deviated from these points. I am well aware that, in accordance 

 with the comprehensive views of the true basis for a general theory of 

 terrestrial magnetism, which Ave owe to Gauss, a thorough knowledge 

 of the horizontal 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 investi- 

 gations ; but the sum total of the small local attractions, the require- 

 ments of steering ships, the ordinary corrections of the compass, and 

 the safety of navigation, continue to impart special importance to the 

 knowledge of the position, and to the movements of the periodic trans- 

 lation 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, construct- 

 ed 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 observation, which I would here advocate, 

 would, if it led to periodically-repeated determinations, and conse- 

 quently to expeditions prosecuted simultaneous^ by land and sea, in 

 accordance with some preconcerted plan, give the double advantage 

 of, in the first place, yielding a direct practical application, and afford- 

 ing us a correct knowledge of the annual progressive movement of 

 these lines ; and, secondly, of supplying many new data for the fur- 

 ther development of the theory enounced by Gauss (Gauss, § 25). 

 It would, moreover, greatly facilitate the accurate determination of 

 the progression of the two lines of no inclination and no variation, if 

 landmarks could be established at those points where the lines enter 

 or leave continents at stated intervals ; as, for instance, in the years 



1850, 1875, 1900 In expeditions of this kind, which would be 



similar to those undertaken by Halley, many isoclinal and isogonic 

 systems would necessarily be intersected before the lines of no decli- 

 nation and no inclination could be reached, and by this means the hor- 

 izontal and total intensities might be measured alonf the coasts, so 

 that several objects would thus be simultaneously attained. The views 

 which I have here expressed are, I am happy to find, supported by a 

 very great authority in nautical questions, viz., Sir James Ross. (See 

 his Voyage in the Southern and Antarctic Rcg'ons, vol. i., p. 105.) 



