DIA 
are genera!, for pricking down all dials With 
centres. See Dial. 
The other scales are particular, and give 
the several requisites for all upright de- 
clining dials by inspection. They are, 1. A 
line of chords. 2. A line for t!ie substile’s 
distance from the meridian. 3. A line for 
the stile’s height. 4. A line of the angle of 12 
and 6. 5. A line of inclination of meridians. 
DIALOGUE, in matters of literature, a 
conversation between two or more persons, 
either by writing or by word of mouth. 
DIALYSIS, in grammar, a mai'k or cha- 
racter, consisting of two points, ", placed 
over two vowels of a word, in order to se- 
parate them, because otherwise they would 
make a diphthong, as mosaic, &c. 
DIAMETER, in geometry, a right line 
passing through the centre of a circle, and 
terminated at each side by the circumfer- 
ence thereof. The chief properties of the 
diameter are, that it divides the circum- 
ference of a circle into two equal parts : 
hence we have a method of describing a se- 
mi-circle upon any line, assuming its middle 
point for the centre. The diameter is the 
greatest of all chords. For finding the 
ratio of the diameter to the circumference. 
See Circle. 
Diameter of a conic section, or trans- 
vase diameter, is a right line passing through 
the centre of the section, or the middle of 
the axis. The diameter bisects all ordi- 
nates, or lines drawn parallel to the tan- 
gent at its vertex. See Conic Sections. 
Diameter, conjugate, is a diameter, in 
conic sections, parallel to the ordinates of 
another diameter, called the transi'erse, or 
parallel to the tangent at the vertex of this 
other. 
Diameter, of any curve, is a right line 
which divides two other parallel right lines, 
in such manner that, in each of them, all 
the segments or ordinates on one side, be- 
tween the diameter and different points of 
the curve, are equal to all those on the 
other side. This is Newton’s sense of a 
diameter. 
But, according to some, a diameter is 
that line, whether right or curved, which 
bisects all tlie parallels drawn from one 
point to another of a curve. So that in this 
way every curve will have a diameter ; and 
hence the curves of the second order, have, 
all of them, either a right-lined diameter, or 
else the curves of some one of the conic 
sections for diameters. And many geome- 
trical curves of the higher orders, may also 
DIA 
have for diameters, curves of more inferior 
orders. 
Diameter of a sphere is the diameter of 
the semicircle, by whose rotation the sphere 
is generated ; in which sense it is thg same 
with axis. 
Diameter of gravity, in any surface or 
solid, is that line in which the center of gra- 
vity is placed. See Center. 
Diameter, in astronomy. The diameters 
of the planets are eitlier apparent or real ; 
the apparent diameters are such as tliey ap- 
pear to the eye ; and being measured by a 
micrometer, are found different in different 
cu cumstances and parts of their orbit. See 
Astronomy. 
DIAMOND. The diamond has always 
been regarded as the most valuable of the 
gems, and, consequently, as the most va- 
luable production of the mineral world, a 
superiority which it derives from its very 
high lustre, its transparency, and hardness. 
The first quality arises from its greater re- 
fractive power, which is such as to cause 
all the light to be reflected which falls on 
it at an angle of incidence greater than 
24J degrees ; and it is capable of being ren- 
dered still more brilliant by its surface be- 
ing cut into facets, which multiply the re- 
flections of light. From its hardness, too, 
its lustre remains uninjured : this hardness 
is such, that it can be cut, or rather worn 
down, only by rubbing one diamond against 
another, and is polished only by the finer 
diamond powder. 
This substance is found in India, in the 
districts of Visapore and Golconda, and 
likewise in Bengal, and in Brazil in South 
America. It is not found in its original si- 
tuation, but in tire beds of streams, or in 
a loose ferruginous sand beneath the soil. 
The Brazilian diamonds are inferior in trans- 
parency and pirrity to the Oriental. 
The diamond is found crystallized, being 
eitlier in perfect crystals, or in fragments 
often encrusted with a hard coating. The 
usual fonn is an octahedron, composed of 
tw’o four-sided pyramids joined by the base, 
tire faces being somewhat convex. Of this 
form there are some modifications : the an- 
gles being replaced by triangular faces, so 
as to give rise to a dodecahedron of twenty- 
four faces, likewise a little convex. These 
are the crystallizations of the Oriental dia- 
mond. The Brazilian is generally a dode- 
cahedron, with rhomboidal faces. These 
crystalline forms are often imperfect, pro- 
bably from tire attr ition which they have 
