Magnetism of the Earth. 155 
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 al- 
terations in that respect, 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. 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 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- 
tiania, as their base. The needles should be kept apart from each 
other, and from contact with iron, and particularly with magnetised 
iron. 
I do not attempt in this letter to enter at any length on the 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 679°, 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 703°: 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, nig nearly so, as the 
points of maximum intensity ; and those of minimum occur respec- 
