f20 Terrestrial Magnetism. 
quitting them ; the one imparting northern, and the other 
southern, polarity, and which have been respectively 
denominated the “ Boreal” and “ Austral fluids. 
The attractive power of the magnet, or loadstone, 
appears to have been known from the remotest antiquity ; 
and it has even been supposed that the Chinese were 
acquainted with its directive power, or polarity, at a very 
early period. There is, however, every reason to believe 
that this is an error, and that the polarity of the magnet 
is a discovery of comparatively modern times. During 
the 12th and 13th centuries, the mariner’s compass was 
evidently known; and several curious old documents 
state, that by it navigators were guided in their course, 
and that it was of indispensable utility to all who 
travelled by sea, showing how fully they were aware of 
its properties. From that time until the latter end of the 
15th century, it was generally supposed that the needle 
pointed towards the true north and south poles of the 
earth ; and Columbus, in his memorable voyage, appears 
to have been the first to notice its variation from the pole- 
star—“ a phenomenon which,” says his biographer, 
.Washington Irving, “ had never before been remarked.” 
The well-known anecdote of the manner in which he 
allayed the fears of his superstitious companions, by a 
timely and satisfactory explanation of the reason for this 
deviation, need not here be repeated. No further notice 
appears to have been taken of this phenomenon until a 
century afterwards, when, in 157G, the dip of the needle 
was first observed by Robert Norman; and, in 1724, the 
fact of a daily change in the amount of the declination 
or variation of the needle appears to have been dis¬ 
covered by Mr. George Graham, and has been con¬ 
firmed repeatedly by subsequent observers. 
The instruments used in those days were rude, and of 
imperfect construction : the observations therefore taken 
