MAGNETISM. 
When common carbonate of magnesia is 
exposed to a moderate heat, it is decom- 
posed: its carbonic acid disengaged. It 
loses about half its weight, and the magne- 
sia remains nearly pure. 
Under the magnesian genus of fossils are 
comprehended, not only those in which 
magnesia is the ingredient which is present 
in largest proportion, but those also in which, 
though in a smaller proportion, there exist 
the characters in some measure peculiar to 
this genus. These are softness, unctnosity, 
and being in general destitute of hardness, 
lustre, and transparency, which are conspi- 
cuous in many of those which belong to the 
silicious and argillaceous genera. Magne- 
sian fossils have usually a green colour more 
or less deep. 
MAGNET. Sec Magnetism. 
MAGNETISM is supposed to have been 
first rendered useful about the end of the 
twelfth, or at least very early in the thir- 
teenth century, by John de Gioja, a handi- 
craft of Naples, who noticed the peculiar 
attraction of metals, iron in particular, 
towards certain masses of rude ore; the 
touch of which communicated to other 
substances of a ferruginous nature, especi- 
ally iron or steel bars, the same property 
of attraction: these touched bars he ob- 
served to have a peculiar and similar ten- 
dency towards one particular point ; that 
when suspended in equilibrio, by means of 
threads around their centres, they inva- 
riably indicated the same point ; and that, 
when placed in a row, however adversely 
directed ; they .soon disposed themselves in 
perfectly parallel order. In this instance 
he improved upon the property long known 
to, but not comprehended or applied to 
use by the ancients, who considered the 
load-stone simply as a rude species of iron 
ore, and curious only so far as it might 
serve to amuse. Gioja being possessed of 
a quick understanding, and of a strong 
mind, was not long in further ascertaining 
the more sensible purposes to which the 
magnet might be appropriated. He ac- 
cordingly fixed various magnets upon pivots, 
supporting their centres in such manner as 
allowed the bars to traverse freely. Find- 
ing that, however situated within the reach 
of observation and comparison, they all 
had the same tendency, he naturally con- 
cluded them to be governed by some at- 
traction which might be ultimately ascer- 
tained and acted upon. He therefore re- 
moved into various parts of Italy, to satisfy 
himself whether or not the extraordinary 
VOL. IV. 
impulse which agitated these bars that had 
been magnetised by friction, existed only id 
the vicinity of Naples, or was general. 
The result of his researches appears to be, 
that the influence was general, but that the 
magnets were rendered extremely variable, 
and fluctuated much,, when near large, 
masses of iron. The experiments of Gioja 
gave birth to many others, and at length to 
a trial of the magnetic influence on the sur- 
face of the water. To establish this, a ves- 
sel was moored out at sea, in a direction 
corresponding with that of the magnet; and 
a boat, having a magnet equipoised on a 
pivot at its centre, was sent out at night in 
the exact line indicated thereby ; which, 
being duly followed, carried them close to 
the vessel that was at anchor. Thus tlie 
active power of attraction appeared to be 
established on both elements, and in the 
course of time the magnet was fixed to 
a card, marked Vith thirty-two points, 
whereby the mariner’s compass was pre- 
sented to us. The points to which the 
magnet always turned itself, being gene- 
rally in correspondence With the meridian 
of the place where it acted, occasioned the 
extremities of the bars to be called poles. 
Succeeding experiments proved, that the 
magnetic bar never retained an exactly 
horizontal position ; but that one of its poles 
invariably formed an angle with any perfect 
level, over which it was placed ; this was 
not so very measurable in a short bar, but 
in one of a yard in length was found to give 
several degrees of inclination. This, which 
is called “The Dip of the Needle,” (or 
magnet) seems to indicate that the attract- 
ing power is placed within the earth. What 
that attracting power is we cannot deter- 
mine ; some consider it to be a fluid, while 
others conjecture it to be an immense mass 
of load-stone situated somewhere about the 
north pole. The difficulty is, however, 
considerably increased by the known fact of 
the needles of compasses not always point- 
ing due north ; but in many places varying 
greatly from the meridional lines respec- 
tively; and from each other at different 
times and places. The facility with which 
a meridional line may be drawn by solar 
observation, and especially by taking an 
azimuth, fortunately enables navigators to 
establish the variation between the true 
northern direction, and that indicated by 
the magnet attached to the card of the 
compass. Nevertheless, we have great 
reason to believe, that, for want either of 
accurate knowledge of the prevalent varia- 
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