M A G 
$1 A G 
^rd (fig. 14), is a circular piece of paper, 
which is fastened upon the needle, and 
moves with it. Sometimes there is a slender 
rim of brass, which is fastened to the ex- 
tremities of the needle, and serves to keep 
the card stretched. The outer edge of this 
card is divided into 36.0 equal parts or de- 
grees, and within the circle of those divisions 
it is again divided into 32 equal parts, or 
' arcs, which are called the points of the com- 
pass, or rhumbs, each pf which is often sub- 
divided into quarters. 'The initial letters N, 
| A Is, occ. are annexed to those rhumbs, to 
denote the north, north-east, &c The mid- 
dlemost part of the card is generally painted 
[I with a sort of star, whose rays terminate in 
I the above-mentioned divisions. To avoid 
I confusion those letters, &c. are not drawn in 
the figure. 
The azimuth compass is nothing more than 
the above-mentioned compass, to which two 
- sights are adapted, through which the sun is 
I t\) be seen, in order to find its azimuth, and 
from thence to ascertain the declination of 
the magnetic needle at the place of observa- 
tion ; see tig. 15. The particulars in which 
it differs from the usual compass, are the 
sights!'’, G ; in one of which, G, there is an 
oblong aperture with a perpendicular thread 
or wire stretched through its middle; and 
in the other sight !, there is a narrow per- 
l pendicular slit. The thread or wire H 1 is 
I stretched from one edge of the box to the 
I opposite. The ring A B of the gimbalds rests 
| with its pivots on the semicircle C D, the 
; | foot E of which turns in a socket, so that 
j whilst the box KLM is kept steady, the 
; compass may he turned round, in order to 
place the sights !, G, in the direction of the 
sun. 
The pivots of the gimbalds of this, as well 
ji as &t the common sort of compasses, should 
lie iu the same plane with the‘ point of sus- 
j pension of the needle, in order to avoid as 
; much as possible the irregularity of the vi- 
brations. 
There are, on the inside of the box, two 
lines drawn perpendicularly along the sides 
| of the box, just from the points where the 
thread H 1 touches the edge of the box. 
I I hese lines serve to shew how many degrees 
| the north or south pole of the needle is dis- 
tant from the azimuth of the sun ; for which 
i purpose, the middle of the apertures of the 
[sights F, G, the thread HI, and the said 
; hues, must be exactly in the same vertical 
J plane. 1 he use of the thread H 1, which is 
; olten omitted in instruments of this sort, is 
likewise to shew the degrees between the 
magnetic meridian and the azimuth, when 
the eye of the observer stands perpendicu- 
I Hilly over it. On the side of the box of this 
port of compasses, there generally is a nut or 
ptop, which, when pushed in, bears against the 
It. tid and stops it, in order that the divisions 
jot the card which coincide with the lines in 
tiie box, may be more commodiously read 
The dipping-needle, though of late much 
improved, is however still far from perfec- 
tion. Tiie general mode of constructing 
it is to pass an axis quite through the nee- 
dle, to let the extremities of this axis, like 
those of the beam of a balance, rest upon 
its supports, so that the needle may move 
tselt vertically round, and when situated in 
lie magnetic meridian, it may place itself 
Vol. 11. ' 1 
M A G 
in the magnetic line. The. degrees of incli- 
nation are shewn upon a divided circle, in 
the centre of which the needle is suspend- 
ed. Fig. Id represents a dipping-needle of 
the simplest construction; A B is the needle, 
the axis of which F E rests upon the middle 
of two lateral bars C I), Cl D, which are 
made fast to the frame that contains the di- 
vided circle A I B K. This machine is fixed 
on a. stand G; but, when used at sea, it is 
suspended by a ring II, so as to hang per- 
pendicularly. When the instrument is fur- 
nished with a'stand, a spirit-level O is gene- 
rally annexed to it, and the stand has three 
screws, by which the instrument is situated 
so that the centre of motion of the needle, 
and the division of 90° on the lower part of 
the divided circle, may be exactly in the 
same line, perpendicular to the horizon. See 
Level. 
The few experiments which follow, are 
principally intended to illustrate the theory. 
Ex. J. 1 he method of discovering whether 
a body is attractable by the magnet or not, 
and whether it lias any polarity or not, or 
which is its south, and which its north pole, 
is so easily performed as not to require many 
words; for by approaching a magnet to the 
body in question (which, if necessary, may 
be set to sw im upon water), or by presenting 
the body in question to either extremity of 
a suspended magnetic needle, the desired 
object may be obtained. 
Ex. 2. Tie two pieces of soft iron wire, 
AH, AB, fig. 17 and 18, each to a separate 
thread, z\ C, A C, which join at top, and 
forming- them into a loop, suspend them so 
as to hang freely. Then bring the marked 
end D fig. 19, which is the north, of a mag- 
netic bar just under them, and the wires w ill 
immediately repel each other, as shewn in 
fig. 18; and this divergency will increase to 
a certain limit, according as the magnet is 
brought nearer, and vice versa. The rea- 
son of this phenomenon is, that by the action 
of the north magnetic pole D, both the ex- 
1 ‘tremities B, B, of the wires, acquire the same, 
viz. the south polarity ; consequently they 
repel each other; and the extremities^ A, A, 
acquire the north polarity, in consequence 
of which they also repel each other, 
If instead of the north pole D, you present 
the south pole of the magnetic bar, the re- 
pulsion will take place as before ; but now 
the extremities B, B, acquire the north, and 
the extremities A, A, acquire the south po- 
larity. 
On removing the magnet, the wires, if of 
soft iron, will soon collapse, having lost all 
their magnetic power; but if steel wires, 
or common sewing-needles be used, they 
will continue to repel each other after the 
removal of the magnet ; the magnetic power- 
being retained by steel. 
Ex. 3. Lay a sheet of paper fiat upon a 
table, strew some iron filings upon the paper, 
place a small magnet among them ; then give 
a few gentle knocks to the table, so as to 
shake the tilings, and you will find that they 
dispose themselves about the magnet N S, 
as shewn iu tig. 20 ; the particles of iron 
clinging to one another, and forming them- 
selves into lines, which at the very poles N, S, 
are in the same direction with the axis of 
the magnet ; a little sideway of the poles 
they begin to bend, and then they form 
.07 
complete arches, reaching from some point 
in the northern half of the magnet, to some 
other point in the southern halt. 
Ex. 4. Place a magnetic bar A B, fig. 21, 
so that one of its poles inay project a short 
way beyond the table, and apply an iron 
weight C to it; then take another magnetic 
bar, D E, like the former, and bring it pa- 
rallel to, and just over the other, at a little 
distance, and with the contrary poles towards 
each other; iu consequence of which the 
attraction of B will be diminished, and the 
iron C, if sufficiently heavy, will drop off, 
the magnet A B being then only able to sup- 
port a smaller piece of iron. By bringing 
the magnets still nearer to each other, the 
attraction of B will be diminished still farther ; 
and, when the two magnets come quite into 
contact (provided they are equal in power), 
the attraction between B and C will vanish 
entirely ; but if the experiment he repeated 
with this difference, viz. that the homologous 
poles of the magnets be brought towards each 
other, then the attraction between B and C, in- 
stead ot being diminished, w ill be increased. 
MAGN 11 UDE, whatever is made up of 
parts locally extended, or that has sev. ral 
dimensions; as a line, surface, solid, See. 
1 he apparent magnitude of a body is that 
measured by the visual angle, formed by 
rays drawn from its extremes to'the centre 
ot the eye ; so that whatever things are seen 
under the same or equal angles, appear equal ; 
and vice versa. 
MAGNOLIA, a genus of the polygynia 
order, belonging to the poiyandria class of 
plants ; and in the natural method ranking 
under the 52nd order, coadnatce. The calvx 
is triphyllous ; there are nine petals; the 
capsules bivalved and imbricated ; the seeds 
pendulous, and in the form of a berry. 
1 here are seven species : the principal are, 
1. File glauca, or small magnolia, a native 
of Virginia, Carolina, and other parts of 
North America. In moist places it rises 
fron seven or eight to fifteen or sixteen feet 
high, with a slender stem. The wood is 
white and spongy, the flowers are produced 
at the extremities of the brandies, are white, 
composed of six concave petals, and have an 
agreeable scent. 2. The grandiflora, or 
great magnolia, is a native of Florida and 
South Carolina. It rises, to the height 
ot eighty feet or more, with a straight trunk 
upwards of two feet diameter, having a 
regular head. The leaves resemble those 
of the laurel, but are larger, and continue 
green throughout the year. The flowers 
are produced at the ends of the brandies, 
and are of a purplish-white colour. 3. The 
tripetala, or umbrella tree, is a native of 
Carolina; it rises, with a slender trunk, to 
the height of sixteen or twenty feet; the* 
wood is soft and spongy ; the leaves remark- 
ably large, and produced in horizontal cir- 
cles, somewhat resembling an umbrella, 
whence the inhabitants of those countries ■ 
have given it this name. The flowers are 
composed of ten or eleven white petals, that 
hang down without any order. The leaves 
drop off at the beginning of winter. 4, The 
acuminata, with oval, spear-shaped, pointed 
leaves, is a native of the inland parts of 
North America. The leaves are near eight 
inches long, and five broad, ending in a 
point. The flowers come out early in the 
spring, and are composed of twelve white 
