ae 
ON THE MAGNETIC INTENSITY OF THE EARTH. 11 
pleted and published his elaborate exposition of the theory of 
the earth’s magnetism, to which he had been conducted by the 
study of the phenomena of the variation and dip as far as they 
were then known, entered into the field of experimental re- 
search, in which he has since rendered such important practical 
services to his favourite science. His exceedingly portable ap- 
paratus for determining the intensity by horizontal needles is too 
well known to need description here; and his good fortune in 
possessing a needle of remarkably steady magnetism, supplied 
by Mr. Dollond, renders little more necessary to be said in re- 
gard to his determinations, than to refer to the publications in 
which they may be found, and to enter them in the general table. 
From 1819 to 1824 his observations were confined to Norway 
and the shores of the Baltic, and were published in the iiird 
vol. of the Ann. der Physik, the intensity stations being 37. In 
1825 he extended them round the shores of the Gulf of Bothnia; 
and the determinations of that year, being 30 in number, were 
published, first, in the ixth vol. of the Ann. der Physik, and, 
secondly, with corrections, in the Astro. Nach., No. 146. 
Lirichsen, 1824; Keilhau and Boeck, 1825-1827; Erman, 
1826.—I have classed these observations together, because 
they were all made, I believe, at the instance and with the ap- 
paratus of M. Hansteen, and were communicated to the public 
through him in the Astro. Nach., No. 146. Captain Erichsen’s 
consist of 3 stations on the shores of the Baltic, and in Ger- 
many; Messrs. Keilhau and Boeck’s of 9 stations in Germany ; 
and M. Erman’s of 2 stations in Germany. They were all 
connected with Paris through Christiania, and are entered in 
the general table. 
Sabine, 1822-1823.—These observations were made during 
two voyages, in which I was furnished by the British Govern- 
ment with a vessel for my conveyance to stations’ at remote 
latitudes from each other, for the purpose of determining the 
_ amount of the ellipticity of the earth by means of the pendu- 
lum. The first voyage was to the equatorial shores of the Afri- - 
- ean and American continents, and the second to the north of 
_ Europe, Greenland, and Spitzbergen. For these voyages I sup- 
_ plied myself with as many as six horizontal needles, in anti- 
_ cipation that some amongst them might prove unsteady in their 
_ magnetism. The observations with all the needles, and at all 
_the stations visited, were published in 1825, with the account of 
the pendulum experiments. 
One of the needles, No. 2, lost so much of its magnetism.in 
