xlii NOTES. 



( W6 ) p. 146. At very different periods, face in 1809, in my Recueil d'Ob- 

 serv. astron. vol. i. p. 368, and again in 1839, in a letter to Lord Minto, then 

 First Lord of the British Admiralty, a few days after the departure of Sir James 

 Ross on the Antarctic Expedition, I have described more fully the importance 

 which I attach to the proposal alluded to in the text. (Compare Report of the 

 Committee of Physics and Meteor, of the Royal Soc. relative to the Antarctic 

 Exped. 1840, p. 88 91.) " Suivre les traces de 1'e'quateur magne'tique ou 

 celles des lignes sans de'clinaison, c'est gouverner (diriger la route du vaisseau) 

 de maniere a couper les lignes zero dans les intervalles les plus petits, en chan- 

 geant de rumb chaque fois que les observations d'inclinaison ou de de'clinaison 

 prouveht qu'on a de'vie*. Je n'ignore pas que d'apres de grandes vues sur les 

 ve'ritables fondements d'une Theorie generate du Magnetisme tewestre, dues a 

 M. Gauss, la connaissance approfondie de Yintensite horizontal, le choix des 

 points ou les trois elements de de'clinaison, d'inclinaison et d'intensite' totale ont 



y 



e'te* mesures simultanement, suffisent pour trouver la valeur de (Gauss, 4 



B 



:t 27), et que ce sont la les points vitaux des recherches futures; mais la somme 

 des petites attractions locales, les besoins du pilotage, les corrections habituelles 

 du rumb et la securite des routes continuent k donner une importance speciale a 

 la connaissance de la position et des mouvements de translation peYiodique des 

 lignes sans dcclinaison. Je plaide ici leur cause, qui est lie'e aux inte'rSts de la 

 ge'ographie physique." Many years must pass away before declination-charts, 

 constructed from the theory of terrestrial magnetism, can be such as to afford 

 adequate guidance to navigation (Sabine, in the Phil. Trans, for 1849, Pt. II. 

 p. 204); and the entirely objective view which I here advocate, if it led to 

 periodically repeated determinations by sea and land expeditions undertaken for 

 the purpose, and made, therefore, at the same time, would afford the twofold 

 advantage of immediate practical use, and of an exact knowledge of the progres- 

 sive secular movement of the lines, as well as of obtaining a large supply of 

 fresh data as bases of calculation for the theory of terrestrial magnetism founded 

 by Gauss (Gauss, 25). It would be highly expedient, for the sake of the 

 exact determination of the movement of the lines of no inclination and no decli- 

 nation, to establish substantial and conspicuous land-marks at the points where 

 these lines enter or quit the coasts of continents for the years 1850, 1875, 1900. 



In such expeditions, similar to the ancient ones of Halley, many 



other isoclinal and isogonic lines, besides those of no inclination and no declina- 

 tion, would be crossed and their position determined ; and the horizontal and 

 total magnetic force could also be measured (at least on shore); and thus several 

 objects would be attained at the same time. I find the wish which I have thus 





