12 On M. Hansteen's recent Magnetic 



Pexperience m'a appris, qu'en faisant choix d'un endroit assez 

 eloigne de toute grande masse de matiere ferrugineuse, on peut 

 atteindre a une precision sufFisante. Dans toutes mes relaches 

 je n'ai jamais manque de comparer les indications des aiguilles 

 a bord, a celles de terre. A Rio et a la Conception les resul- 

 tats ont ete a. peu pres identiques ; ici (at Norfolk Sound) ou 

 l'inclinaison comme la force ont atteint leurs maximum (c'est a 

 dire pour nous) et par consequent ou l'influence du fersurles 

 aiguilles — cscteris paribus — est aussi a son maximum, ici Pin- 

 clinaison a bord ne differoit de celle a terre que d'un petit nom- 

 bre de minutes ; Pintensite de la force indiquee par Paiguille 

 verticale fut precisement la meme, et d'apres Paiguille hori- 

 zontale un peu moindre. En consideVant que Pepreuve fut 

 faite par les circonstances les plus desavantageuses, on con- 

 viendra que les observations faites a bord meritent quelque 

 confiance. Mais il est essentiel de mettre toute Pattention 

 possible au choix du propre endroit; car voulant faire les ob- 

 servations dans ma chambre, je n'etois parvenu qu'a des resul- 

 tats tres fautifs." 



For experiments on the Magnetic Force, it is of the first 

 necessity that the needles employed should retain throughout 

 the same degree of magnetism ; or should undergo merely such 

 slight and gradual alterations in that respect, as admit of cor- 

 rections being applied by interpolation, from experiments made 

 at the same spot before and after the series in which they have 

 been employed. This property of the needles ought always to 

 have been ascertained by previous trial during several months. 

 Those which I send you belonged originally to M. Hansteen, 

 and have been in my possession, and in constant use, for three 

 years past: their magnetism has hitherto undergone a slight 

 but very regular diminution from year to year, well admitting of 

 interpolation. It will be proper, therefore, that observations 

 should be made with them at the port from which the expedi- 

 tion sails, a few days before its departure, and again in the same 

 place, as soon as convenient after its return. It will also be 

 proper that the needles should be then sent back to London, 

 that observations may be repeated with them here, to ensure 

 the connexion of the results obtained by their means, with those 

 of the other experimentors which regard London, Paris, and 



