MAGNETISM, 
arid the direction is east and west, the in- 
tended course of the vessel is exactly perpen- 
dicular' to the position of the magnet. A 
little reflection will shew how the vessel may 
be steered in any other direction. 
An artificial magnet fitted up in a proper 
box, for the purpose of guiding the direction 
of a traveller, is called a magnetic -needle, 
and the whole together is called the mariner’s 
compass. 
Although the north pole of the magnet in 
every part of the world, when suspended, 
points towards the northern parts, and the 
south pole towards the southern parts, yet 
its ends seldom point exactly towards the 
poles of the earth. The angle in yhich it 
deviates from due north and south, is called 
the angle of declination, or the declination 
of the magnetic needle, or the variation of 
the compass ; and this declination is said to 
be east or west, according as the north pole 
of the needle is eastward or westward of the 
astronomical meridian of the place. 
This deviation from the meridian is not 
the same in all parts of the world, but is dif- 
ferent in different places, and it is even con- 
tinually varying in the same place. For in- 
stance, tins declination is not the same in 
London as at Paris, or as in India ; and the 
declination in London, or in any other place, 
is not the same at this time as it was some 
years ago. This declination from the me- 
ridian is so variable, that it may be observed 
to change, even in one or two hours time ; 
j and this is not owing to the construction of 
| the magnetic needle ; for in the same place, 
! and at the same time, all true magnetic 
needles point the same way. 
The declination from the meridian, and 
| the variation of this in different parts of the 
: world, are very uncertain, and cannot be fore- 
j told; actual trial is the only method of ascer- 
I taining them. This circumstance forms a great 
i impediment to the improvement of navfoa- 
| tion. It is true, that great pains have been 
j taken by navigators and other observers, to 
ascertain the declination in various parts of 
; the world, and such declinations have been 
marked in maps, charts," books, &c. ; but 
still, on account of the constant change to 
which this variation is liable, these carfonlv 
t serve for a few years; nor has the law of this 
variation or lluctuation been yet discovered, 
though various hypotheses have been formed 
for that purpose. When the variation was 
first observed, the north pole of tjie magnetic 
needle declined eastward of the meridian of 
London; but it has since that time been 
changing continually towards tire west; so 
that in the year 1657 the magnetic needle 
pointed due north and south. At present, it 
declines about 24|° westward, and it seems 
to be still advancing towards the west. 
Before volcanic eruptions and earthquakes, 
the magnetic needle is often subject to very 
extraordinary movements. 
It is also agitated before and after the ap- 
pearance of the aurora borealis. 
The magnetic inclination, or dip of the 
needle .- — If a needle which is accurately ba- 
lanced, and suspended so as to turn freely in 
a vertical plane, is rendered magnetical, 
the north pole will be depressed, and the 
south pole elevated above the horizon : this 
* property is called the inclination, or dip of 
the needle, and was discovered by Robert, 
Norman, about the year 1576. 
Take a globular magnet, or, which is more 
easily procured, an oblong one, like S N, 
fig. 4; the extremity N of which is the north 
pole, the other extremity S is the south pole, 
and A is its middle or equator ; place it hori- 
zontally upon a table C 1): then take another 
small oblong magnet u s (viz. a bit of steel 
wire, or a small sewing-needle magnetized) 
and suspend it by means of a fine thread tied 
to its middle, so as to remain in an horizontal 
position, when not disturbed by the vicinity 
of iron, or other magnet. Now if the same 
small magnet, being held by the upper part 
of the thread, be brought just over the mid- 
dle of the large magnet, within two or three 
inches of it, the former will turn its south 
pole s, towards the north pole, N, of the 
large magnet ; and its north pole n, towards 
the south pole, S, of the large one. It "will 
be farther observed, that the small magnet, 
whilst kept just over the middle A of the 
large one, will remain parallel to it ; for 
since the poles of the small magnet are 
equally distant from the contrary poles of the 
large magnet, they are equally attracted. 
But if the small magnet be moved a little 
nearer to one end than to the other of the 
large magnet, then one of its poles, namely, 
that which is nearest to the contrary pole of 
the large magnet, will be inclined down- 
wards, and of course the other pole will be 
elevated above the horizon. It is evident 
that this inclination must increase according 
as the small magnet is placed nearer to one 
of the poles of the large one, because the at- 
traction of the nearest pole will have more 
power upon it. If the small magnet be 
brought just opposite to one of the poles of 
the large magnet, it will turn the contrary 
pole towards it, and will place itself in the 
same straight line with the axis of the large 
magnet. 
This simple experiment will enable the 
reader to comprehend easily the phenomena 
of the magnetic inclination, or of the dipping 
needle, upon the surface of the earth; for 
it is only necessary to imagine that the earth 
is a large magnet fas in fact in appears to be), 
and that any magnet, or magnetic needle, 
commonly used, is the small magnet employ- 
ed in the above-mentioned experiment ; for, 
supposing that the north pole of the earth is 
possessed of a south magnetic polarity, and 
that the opposite pole is possessed of a north 
magnetic polarity, it appears evident, and it 
is confirmed by actual experience, that when 
a magnet, or magnetic needle, properly 
shaped and suspended, is kept near the equa- 
tor of the earth (since neither the magnetic 
equator, nor the magnetic poles of the earth, 
coincide with its real equator and poles), it 
must remain in a horizontal situation : if 
the magnet is removed nearer to one of the 
magnetic poles of the earth, it must incline 
to one of its extremities, namely, that which 
is possessed of the contrary polarity ; and 
this inclination must increase in proportion 
as the needle recedes from the magnetic 
equator of the earth. Lastly, when the nee- 
dle is brought exactly over one of the mag- 
netic poles of the earth, it must stand per- 
pendicular to the horizon of that place. 
A magnetic needle constructed for the 
purpose of shewjng this property, is called a 
dipping-needle, and its direction in any place 
is called the magnetical line. When it was 
saidj that the .north pole of the earth 
§5 
possessed south polarity, it was only meant 
that it had a polarity contrary to that end of 
the magnetic needle which 'is directed to- 
wards it. 
If the geographical poles of the earth (that 
is, the ends ot its axis), coincided with its 
magnetic poles; or even if the magnetic 
poles were constantly at the same distance 
from them ; the inclination of the needle, as 
well as its declination, would always be the 
same ; and hence, by observing the direc- 
tion of the magnetic needle in any particular 
place, the latitude and longitude of that 
place might be ascertained; but this is not 
the case, for the magnetic poles of the earth 
do not coincide with its real poles, and they 
are also constantly shifting their situations ; 
hence the magnetic needle changes continu- 
ally and irregularly, not only in its horizontal 
direction, but likewise in its inclination, ac- 
cording as it is removed from one place to> 
another, and also while it remains in the very 
same place. 
This change of the dip in the sarn« 
place, however, is very small. In London, 
about 1576, the north pole of the dipping 
needle stood 71° 50 r below the horizon; and 
in 1775, it stood at 72° 3'; the whole change 
of inclination, during so many years, amount- 
ing to less than a quarter of a degree. 
There are various methods of giving the 
magnetic property to steel or iron. In some 
cases, it appears to be acquired without the 
use of another magnet. 
If you take a bar of iron three or four 
feet long, and hold it in a vertical po- 
sition, you will find that the bar' is mag- 
netic, and will act upon another magnet ; 
the lower extremity of the bar attracting the 
south pole, and repelling the north pole. 
IB you invert the bar, the polarity will be 
instantly reversed; the extremity which is- 
now lowest, will be found to be a north pole,, 
and the other extremity will be a south pole. 
A bar of hard iron, or steel, will not an- 
swer for the above experiment, the magnet- 
ism ot the earth not being sufficient to mag- 
netise it. 
Bars of iron that have stood in a perpen- 
dicular position, are generally found to be 
magnetical ; as fire-irons, bars of windows* 
&c. 
If a long piece of hard iron is made red- 
hot, and then left to cool in the direction of 
the magnetical line, it becomes magnetical. 
Striking an iron bar with a hammer, or 
rubbing it with a file, while held in this di- 
rection, likewise renders it magnetical. An 
electric shock produces the same effect ; and 
lightning often renders iron magnetic. 
A magnet cannot communicate a degree 
of magnetism stronger than that which itself 
possesses ; but two or more magnets, joined 
together, may communicate a greater power 
to a piece of steel, than either of them pos- 
sesses singly: hence we have a method of 
constructing very powerful magnets, by first 
constructing several weak artificial magnets, 
and then joining them together to form a 
compound magnet, and to act more power- 
fully upon a piece of steel. 
1. Place two magnetic bars. A, B, fig. 5. 
in a line with the north, or marked end of 
one, opposed to the south, or unmarked end 
ot the other; but at such a distance from each 
other, that the magnet to he touched naav 
rest with its marked end, on the unmarked 
2 
