Inqiiwies concerning the Magnetisn of the Earth. 1S5 
general use, the easterly deviation of the needle was recognised 
to be, not an accidental error, but a universal property of the 
instrument ; another century was required to discover the dip^ 
and the variable quantity of both these observations. Dr Gil- 
bert’s work in the year 1600, first pointed out the source of 
these phenomena, in the magnetic influence of the earth itself ; 
and Halley’s theory, which followed at the distance of more 
than a century, may be regarded as the earliest attempt to con- 
nect the various facts together by any thing like a general prin- 
ciple. The four poles of his magnetic terrella^ situated in the 
interior of the earth; the motion of this terrella.^ less rapid than 
that of the shell which enveloped it, and therefore causing the 
poles to be carried slowly round from east to west, had the me- 
rit of aflPording a simple and beautiful expression for a very 
complicated class of appearances ; and though the hypothesis 
did not embrace all the circumstances of the case, it shewed the 
track by which the solution of them was to be obtained. Euler’s 
attempt to explain the variation and dip, by means of two mag- 
netic poles, appears to be generally abandoned. The author 
had studied but an inconsiderable part of the phenomena, and 
his theory does not at all represent the facts observed in many 
portions of the globe, concerning which he possessed only defec- 
tive or erroneous information. 
That part of the science which regards the mechanical pro- 
perties of natural and artificial magnets, and which must evi- 
dently form the basis of whatever system aims at accounting for 
the magnetical phenomena of the earth, has not been more 
third discoverer of this island, departed from Rogaland in Norway, to seek Ga- 
darsholm, (Iceland), some time in the year 868. He took three ravens with him, to 
serve as guides ; and in order to consecrate them to this purpose, he offered up a 
great sacrifice in Smb’rsund, where his ship lay ready to sail ; for in those times 
SEAMEN HAD NO LOADSTOlilE IN THE NORTHERN COUNTRIES, ( Puiat pa kofdo hofisig- 
lingarmenn enger Leidarstein i pan pinna d nordorlondum ). In Icelandic, Leid sig- 
nifies region, and on this account the Pole star is named Leidstjerna ; consequent- 
ly Leidarstein signifies guiding-stone. According to the testimony of Snorro Stur- 
leson. Are Frode was born in the year 1068 ; therefore his book was probably 
written about the end of the 1 1th century, at which time, consequently, the polari- 
ty of the magnet must have been known in Norway. From the expression, how- 
ever, it may be inferred, that they were not acquainted w ith the compass, but 
merely suspended the natural magnet by a thread,” 
1 
