154 Magnetism, of the Earth. 



but too likely to impair the accuracy of the results. Still, when we 

 consider how large a portion of the southern hemisphere is covered 

 by the ocean, it does appear desirable to make the endeavor to ob- 

 tain the best results, that circumstances will permit, over such extensive 

 portions of the globe ; and particularly as in the opinion of those, who 

 from experience are most competent to judge, it is possible by great 

 care, to obtain results worthy of confidence. M. Humboldt has re- 

 corded several observations which he made himself, at sea in the 

 nortliern Atlantic, both of the dip and of tlie intensity ; the latter of 

 which accord well with the curves of intensity traced in the accom- 

 panying sketch. 



M. Hansteen beheves, that by giving a dipping needle the sort of 

 suspension used in Captain Cook's 3d voyage — by choosing tliose 

 times for observation where a calm sea and moderate wind allow the 

 ship to keep a steady course — by confining the use of the instrument 

 always to the same place on the ship's deck — and by comparing the 

 results on land and on shore on all occasions when in harbor — obser- 

 vations on board ship, might become very valuable. I will venture 

 to add an extract on this subject, from Captain Liitkes' letter, whose 

 remarks are the more encouraging, as when he quitted Europe he 

 was by no means sanguine of success in the use of delicate magnetic 

 instruments at sea : — " Je dois pourtant faire quelques reraarques sur 

 les observations faites abord. J'avais ete, comme vous, endoute qu' 

 elles puissent donner des resultats dignes de confiance, mais I'experi- 

 ence m'a appris, qu'en faisant choix d'un endroit assez eloigne de 

 toute grande masse de matiere ferrugineuse, on pent 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 resultats ont ete a pen pres identiques ; ici 

 (at Norfolk Sound) oii I'inclinaison comme la force ont atteint leurs 

 maximum (c. a. d. pour nous,) et par consequent, ou I'influence du 

 fer sur les aiguilles — ceteris paribus — est aussi a son maximum, ici 

 I'inclinaison a bord ne differoit de celle a terre que d'un petit nombre 

 de minutes ; I'intensite de la force indiquee par I'aiguille verticalle 

 iut precisement la meme, et d'apres I'aiguille — horizontalle un pen 

 moindre. En considerant que I'epreuve fut faite par les circonstan- 

 ces les plus desavantageuses, on conviendra que les observations 

 faites a bord meritent quelque confiance. Mais il est essentiel de 

 mettre toute I'attention possible au choix du propre endroit ; car 

 voulant faire les observations dans ma chambre, je n'etois parvenu 

 qn'a des resultats tros fuutifs." 



