Magnetism of the Earth. 155 



For experiments ou 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 al- 

 terations in that res]3ect, as admit of corrections being applied by in- 

 terpolation, 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. Ilansteen, and 

 have been in my possession and in constant use for three years past ; 

 their magnetism has hitherto undergone a slight but very regular dim- 

 inution, 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 expedition sails, a few days before its 

 departure, and again in the same place, as soon as convenient after its 

 return. It will then be proper, that the needles should be sent back 

 to London, that observations may be repeated with them here, to en- 

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

 of the other experimenters, which regard London, Paris and Chris- 

 tianla, as their base. The needles should be kept apart from each 

 otlier, and from contact with iron, and particularly with magnetised 

 iron. 



I do not attempt in this letter to enter at any length on llie consid- 

 eration of the curves of Dip and of Variation. M. Hansteen has shewn, 

 in the treatise already named, the general conformity of these phe- 

 nomena to such an arrangement of magnetic attraction, as is indicated 

 by the course of the isodynamic curves. His observations in Siberia, 

 in as far as they go, confirm this view. Thus for example, in the 

 parallel of 55° north, the Dip, which in tracing the parallel to the 

 eastward progressively decreases, from Labrador, where it exceeds 

 80'°, is found by M. Hansteen to attain a maximum 67f °, about the 

 42° of longitude east of Greenwich ; from thence it increases, until 

 the intersection of the parallel with the meridian of the Siberian 

 maximum of intensity, (102° east,) where its amount is 70|° : from 

 that meridian it again decreases to a second maximum, by the obser- 

 vations of Russian Officers, in the meridian of Kamtchatka, (163° 

 east.) Hence as regards the dip in the parallel of 55° north, there 

 are two points of maximum and two of minimum ; those of maxi- 

 mum are in the same geographical meridians, or nearly so, as the 

 points of maximum intensity ; andjhose of minimum occur respec- 



