294 HORO 
xxxviii.S. The ancient Bramins feem alfo to have been 
acquainted with this inftrument, and to have applied it to 
aftronomical purpofes. The Chinefe and Egyptians were 
likewife acquainted with it at a very early period. Learn¬ 
ed men have generally afcribed the invention of it to the 
Babylonians, from whom the Jews derived it before the 
time of Aha?,. Herodotus traces the knowledge of the 
art of dialling among the Greeks to the fame fource. 
The diverfity of fun-dials arifes from the different fitu- 
ation of the planes, and the different figure of the fur- 
faces, whereon they are defcribed ; whence they become 
denominated equinoctial, horizontal, vertical, polar, di¬ 
rect, ereft, declining, inclining, reclining, cylindrical, &c. 
Dials are l'ometimes diltinguiflied into primary and fe- 
condary. Primary Dials are fuch as are drawn either on 
the plane of the horizon, and thence called horizontal 
dials : or perpendicular to it, and called vortical dials ; or 
elfe drawn on the polar and equinoctial planes, though 
neither horizontal nor vertical. Secondary Dials are all 
thofe that are drawn on the planes of other circles betides 
thole lafl mentioned; or thole which either decline, in¬ 
cline, recline, or deincline. Each of thefe again is di¬ 
vided into feveral others, as follow: 
EquinoElial Dial , is that which is defcribed on an equi¬ 
noctial plane, or one parallel to it. 
Horizontal Dial, is defcribed on an horizontal plane, ora 
plane parallel to the horizon. This dial fhows the hours 
from fun-rife to fun-fet. 
South Dial, or ErcEl direEl South Dial, is that defcribed 
on the l'urface of the prime vertical circle looking towards 
the fouth. This dial fhows the time only from 6 in the 
morning till 6 at night. 
North Dial, or ErcEl direEl North Dial, is that which 
is defcribed on the l'urface of the prime vertical looking 
northward. This dial only fhows the hours before 6 in 
the morning, and after 6 in the evening. 
Eajl Dial, or ErcEl direEl Eajl Dial, is that drawn on the 
plane of the meridian, looking to the eaft. This can only 
Ihow Lhe hours till iz o'clock. 
Wejl Dial, or ErcEl direEl Wcjl Dial, is that defcribed on 
the weftern fide of the meridian. This can only fhow the 
hours after noon. Confequently this, and the laft pre¬ 
ceding one, will lhow all the hours of the day between 
them. 
Polar Dial, is that which is defcribed on a plane pafiing 
through the poles of the world, and the eaft and weft 
points of the horizon. It is of two kinds; the firft look¬ 
ing up towards the zenith, and called the upper-, the lat¬ 
ter, down towards the nadir, called the lower. 
Declining Dials, are erect or vertical dials which decline 
from any of the cardinal points ; or they are fuch as cut 
either the plane of the prime vertical, or of the horizon, 
at oblique angles. Declining dials are of very frequent 
ufe; as the walls of houfes, on which dials are moftly 
drawn, commonly deviate from the cardinal points. Of 
declining dials there are feveral kinds, which are deno¬ 
minated from the cardinal points which they are neareft 
to: as decliners from the fouth, and from the north, and 
even from the zenith. 
Inclined Dials, are fuch as are drawn on planes not eredt, 
but inclining, or leaning forward towards the fouth, or 
iouthern fide of the horizon, in an angle, either greater 
or lefs than the equinoctial plane. Reclining Dials, are 
thofe drawn on the planes not eredt, but reclined, or lean¬ 
ing backwards to the zenith towards the north, in an an¬ 
gle greater or lefs than the polar plane. Dcinclined Dials, 
are fuch as both decline and incline, or recline. Thele 
l ift three forts of dials ard very rare. 
Dials wthout Centres, are thofe whole hour-lines con¬ 
verge fo (lowly, that the centre, or point of their con- 
courfe, cannot be expreffed on the given plane. 
Cylindrical Dial, is one drawn on the curve furface of a 
cylinder. This may firft be drawn on a paper plane, and 
then paited round a cylinder of wood, &c. It will fhow 
LOGY. 
the time of the day, the fun’s place in the ecliptic, and 
his altitude, at any time of obfervation. 
There are alfo Portable Dials, or on a Card, and Univcrfal 
Dials on a Plain Crojs, &c. 
RcfraElcd Dials, are fuch as fhow the hour by means of 
fome refracting tranfparent fluid. 
■ijiing Dial, is a (mail portable dial, confifting of a brafs 
ring or rim, about two inches in diameter, and one-third 
of an inch in breadth. In a point of this rim there is a hole, 
through which the fun-beams pafs, and form a bright 
fpeck in the concavity of the oppolite l'emi-circle, which 
gives the hour of the day in the divifions marked with¬ 
in it. 
General Principle of Sun-Dials. —Conceive a fphere, with 
its twelve horary circles or meridians, which divide the 
equator, and confequently all its parallels, into twenty- 
four equal parts. Let this fphere be placed in a fituation 
fuited to the pofition of the dial; that is, let its axis be 
directed to the pole of the place for which the dial is con- 
ftruCtad, or elevated at an angle equal to the latitude. 
Now if we fuppofe a horizontal plane cutting the fphere 
through its centre, the axis of the fphere will reprefent 
the ftyle, and the different interfeClions of the horary cir¬ 
cles with that plane will be the hour-lines; for it is evi¬ 
dent, that, if the planes of thefe circles were infinitely 
produced, they would form in the celeftial fphere the ho¬ 
rary circles, which divide the folar revolution into twenty- 
four equal parts. When the fun therefore has arrived at 
one of thele circles, that of three in the afternoon for ex¬ 
ample, he will be in the plane of the fimilar circle of the 
fphere above-mentioned; and the fhadow of the axis or 
ftyle will fall upon the line of interfeclion, which that 
circle forms with the horizontal plane ; this line then will 
be the line of 3 o’clock ; and fo of the reft. 
All this is illuftrated in Plate I. fig. 2, which reprefents 
a part of the fphere, with fix of the horary' circles. P p 
is the axis, in which all thefe circles inferfedt each other; 
AHB/i tlie horizontal plane, or horizon of the fphere, 
indefinitely continued ; A B the meridian ; D E the dia¬ 
meter of the equator, which is in the meridian ; and 
DHEi the circumference of the equator, of which DHF, 
is a half, and DH a quarter. This quarter of the equa¬ 
tor is divided into fix equal parts, D 1, 1 2, 2 3, 3 4, 4. 5, 
5 6, and through thefe pafs the horary circles, the planes 
of which evidently cut the horizon in the lines C 1, C 2, 
C 3, C 4, C 5, C6 : thefe are the hour-lines; and if we 
fuppofe them continued to AF, which is perpendicular 
to the meridian CA, they will give the hour-lines Cl, 
CII, CIII, CIV,CV, CVI. The ftyle will be a por¬ 
tion C S of the axis of the fphere ; which confequently 
ought to form with the meridian, and in its plane, an 
angle, SC A, equal to the height of the pole, or PC A. 
Should this reafoning appear too tedious, another me¬ 
thod may be employed to acquire a clear idea of the prin¬ 
ciples of dialling. Conftrudl a folid fphere, divided by 
its twelve horary circles, and cut it in fuch a manner 
that one of its poles fhall form with the plane of the lec¬ 
tion an angle equal to the height of the pole of the given 
place. If the fphere, cut in tnis manner, be then made 
to reft on a horizontal plane, with its pole directed to¬ 
wards the pole of the world, the points where the horary 
circles interfedt the horizontal plane will be readily feen; 
and the common fedlion of all the circles, which is the 
axis, will fhow the pofition of the ftyle. 
For the fake of illuftration, we have here iuppofed the 
fedlion of the fphere to be formed by a horizontal plane; 
but, if the plane be vertical, the cafe would be fimilar, and 
the lines of interledfion would be the hour-lines of a ver¬ 
tical dial. If the plane be declining or inclining, we 
fhall have a declining or inclining dial; it may even be 
eafily feen that this holds good in regard to every fur- 
face, whatever be its form, convex, concave, or irregular, 
and whatever may be its pofition. 
The Jlyle is an iron rod, generally placed in an inclined 
3 direction, 
