MAD 
MAG 
M AG 
92 
the other side of the spot' seems to have va- 
nished in its turn, being hid from the view by 
the upper edge of the excavation, from the 
oblique position of its sloping sides with re- 
spect to the eve. But as the spot advances 
on the sun’s disc, this umbra, or side of the 
cavity, comes in sight ; at first appearing nar- 
row, but afterwards gradually increasing in 
breadth, as the spot moves towards the mid- 
dle of the disc. These appearances per- 
fectly agree with the phases that are exhibited 
by an excavation in a spherical body, re- 
volving on its axis ; the bottom of the cavity 
being painted black, and the sides lightly 
shaded. 
Dr. Herschel supposes that the spots in 
the sun are mountains on its surface, which 
considering the great attraction exerted by 
the sun upon bodies placed at its surface, and 
the slow revolution it has about its axis, he 
thinks may be more than 300 miles high. 
He says, that in August 1792 he examined 
the sun with several powers, from 90 to 500; 
and it appeared that the black spots are the 
opaque ground or body of the sun, and that 
the luminous part is an atmosphere which 
being broken, gives a glimpse of the sun it- 
self. 
MADDER. See Rubia. 
MADNESS. See Medicine. 
MADREPORA, in natural historv, 
lie discovered that its head or centre was 
lifted up occasionally above the surface, and 
often contracted and dilated itself like the 
pupil of the eye: he saw all its claws moved, 
as well as its head or centre. When the 
animals of the madrepora are destroyed, its 
extremities become white. In the madrepora, 
lie says, the animal occupies the extremity, 
and the substance is of a stony but more loose 
texture than the coral. This is formed, like 
other substances of the same nature, of a 
liquor which (he animal discharges: and lie 
farther adds, that there are some species of 
the polvpe of the madrepora which are pro- 
duced singly, and others in clusters. See 
Plate Nat. Hist. figs. 256 and 257; and Zoo- 
phytes. 
MADREPORITE, a mineral found in the 
valley of Russback in Salzburg, and which ob- 
tained its name from its resemblance to ma- 
drepore. Colour in some parts black, in others 
dark-grey. Found in large round masses. 
Fracture even passing to the conchoidal. 
Lustre greasy, passing to the silky. Brittle: 
moderately 'heavy. Streak grey it is com- 
posed of 93.00 carbonat of lime 
0.50 carbonat of magnesia 
7.25 carbonat of iron 
0.50 charcoal 
4.50 silica in sand. 
the 
name of a genus of submarine substances, the 
characters of which are, that they are almost 
of a stony hardness, resembling the corals, 
and are usually divided into branches, and 
pervious bv many holes or cavities, which are 
frequently of a stellar figure. 
I 11 the Linnaean system, this is a genus of 
lithophyta: the animal that inhabits it is a 
medusa ; it comprehends 39 speeies. Ac - 
cording to Donati, the madrepora is like the 
coral as to its hardness, which is equal to bone 
or marble; the colour is white when polished; 
its surface is lightly wrinkled, and the wrinkles 
run lengthwise of the branches*; in the centre 
there is a sort of cylinder, which is often 
pierced through its whole length by two or 
three holes. Erom this cylinder are detached 
about 1 7 lamina*, which run to the circum- 
ference in straight lines; and are transversely 
intersected by other laminae, forming many 
irregular cavities; the cellules, which are com- 
posed of these lam. na: ranged info a circle, 
are the habitations of little polypes, which are 
extremely tender animals, generally transpa- 
rent, and variegat *d with beautiful colours. 
M. de Peyssonel observes, that those writers 
who only considered the figures of submarine 
substances, denominated that class of them 
which seemed pierced with holes, pora; and 
those the holes of which were large they call- 
-ed madrepora. He delines them to be all 
these marine bodies which are of a stony 
substance, without either bark or crust, and 
which have but one apparent opening at each 
extremity, furnished with rays that proceed 
fro 11 tip centre to the circumference. He 
observes that the body of the animal of the 
madrepora. whose flesh is so soft that it di- 
vides upon the gentlest touelp tills the centre ; 
the head is placed in the middle, and sur- 
rounded bv several feet or claws, which till 
the into u als of the partitions observed in 
this sub-tmnje, and are at pleasure brought to 
its head, and are furnished with yellow papillae. 
99.75 
MADRIER, in the military art, a long 
and broad plank of wood, used for support- 
ing the earth in mining and carrying on a 
sap, and in making coffers, caponiers, galle- 
ries, and for many other uses at a siege. Ma- 
driers are also used to cover the mouths of 
petards after they are loaded, and are fixed 
with the petards to the gates or other places 
designed to be forced open. 
MiEMACTERION, the fourth month of 
the Athenian year, consisting of only 29 days, 
and answering to the latter part of September 
and the beginning of October, 
MAGAZINE, a place in which stores are 
kept, or arms, ammunition, provisions, &c. 
Every fortified town ought to be furnished 
with a large magazine, which should contain 
stores of all kinds, sufficient to enable the 
garrison and inhabitants to hold out a long 
siege, and in which smiths, carpenters, wheel- 
wrights, bakers, &c. may be employed in 
making every thing belonging to the artillery, 
as carriages, waggons, &c. 
Magazine, powder, is that p’ace where 
the powder is kept in very large quantities 
Authors differ greatly both in regard to situa- 
tion and construction; but all agree, that 
they ought to he arched, and bomb-proof. 
In fortifications they are frequently placed in 
the rampart; but of late they have been built 
iu different parts of the town. The tirst 
haunches, even with a straight horizontal ex- | 
trados ; and still much more so iu powder- j 
magazines, whose outside at top is formed 1 
like the roof of a house, by two inclined. | 
planes joining in an angle over the top of I 
the arch, to give a proper descent to the rain ; I 
which effects are exactly what might be ex- I 
nee ted agreeable to the true theory ofarches. I 
Now, as this shrinking of the arches must be I 
attended with very ill consequences, by break- i 
ing the texture of the cement after it has 1 
been in some degree dried, and also by open- I 
ing the joints of the voussoirs at one end,. 1 
so a remedy is provided for this uiconve- 1 
nience, with regard to bridges, by the arch of I 
equilibration in Dr. Hutton’s book on bridges; I 
but as the ill effect is much greater in pow tier- I 
magazines, the same ingenious gentleman. I 
proposed to iind an arch of equilibration for I 
them also, and to construct it when the span 1 
is 20 feet, the pitch or height 10 (which are 
the same dimensions as the semicircle), the 
inclined exterior walls at top forming an 
angle of 1 13 degrees, and the height of their 
angular point above the top of the arch 
equal to seven feet. 
MAGI, or Magians, an antient religious 
sect in Persia, and other Eastern countries, 
who maintained, that there were two prin- 
ciples, the one the cause of all good, the 
other the cause of all evil ; and abominating.' 
the adoration of images, worshipped God. 
only by fire, which they looked upon as the 
brightest and most glorious symbol of Oron 
masdes, or the good God: as darkness is the- 
truest symbol of Arimaniits, or the evil God. 
This religion was reformed by Zoroaster*. 
The sect still subsists in Persia, under the 
denomination of gaurs, 
MAGIC Lantern. See Optics. 
Magic Square, in arithmetic, a square* 
figure made up of numbers in arithmetical 
proportion, so disposed in parallel and equal 
ranks, that the sums of each row, taken either 
perpendicularly, horizontally, or diagonally* 
are equal : thus. 
powder-magazines were made with Gothic 
arches; but M. Vaubaji, finding them too 
weak, constructed them in a semicircular form, 
whose dimensions are 60 feet long within, 25 
broad; the foundations are eight or nine feet 
thick, and eight feet high from the founda- 
tion to the spring of the arch.; the floor is two 
feet from the ground, which keeps it from 
dampness. 
One of our engineers of great experienc 
some time since, had observed, that after 
the centres of semicircular arches are struck, 
they settle at the crown, and rise up at the 
Natural square. Magic square. 
Magic squares seem to have been so 
called, from their being used in the construc- 
tion of talismans. 
MAGNA CHARTA, the great charter 
of the liberties of England, and the basis of 
our laws and privileges. 
This charter may be said to derive its ori- 
gin from king Edward the Confessor, who 
granted several privileges to the church and 
state, by charter; these liberties and privi- 
leges were also granted and confirmed by 
king Henry I., by. a' celebrated gregt charter 
now lost; but which was confirmed or re- 
enacted b\ king Henry II. and kihg John, 
llenry III., the successor of this last prince, 
after having caused twelve men to make 
enquiry into tiie liberties of England in t he 
reign of Henry I., granted a new charter 
which was the same as the present Magn: 
Charta ; this he several times confirmed, aw 
as often broke; till in the thirty-seventh yea: 
of his reign, he went to Westminster-iiall 
and there, in the presence of the nobilitv am 
bishops, who held lighted candles in'tliei 
I hands, Magna Charta was read, the king ul| 
1 1 
