HOROLOGY. 
290 
diredly towards the fouth, then make the angle ECK 
or thp arc EK (fig. 5), equal to the height of the pole; if 
C K V be then made a right angle, the point V will be the 
centre of the dial; and the angle CVK, which will then 
be equal to the complement of the latitude or of the ele- 
vation of the pole 3 will denote the angle which the il} le ? 
in the plane of the meridian, ought to form with the 
plane of the dial. 
Of the-North Vertical Dial. —If the vertical dial be north ; 
make, as before, the angle OCA (fig. 5), equal to the 
height of the pole, and the angle C A H a right angle : the 
point H will be the centre of the dial and the angle C HA 
will be that which the ltyle forms with the meridian. 
The ftyle, inftead of being inclined downwards, muft be 
turned in a contrary diredion, as may be readily con¬ 
ceived when we confider the pofition of the pole in legald 
to a vertical plane turned direftly towards the north. 
Of Polar Dials.— To make a polar dial, draw the meri¬ 
dian XII XII, (fig. 6,) and XZ perpendicular to it. From 
the point M, in this line, make on each fide the fame con- 
ftruftion as taught in the preceding figure.. If parallel 
lines be then drawn through the points of divifion, they 
will be the hour-lines. For it may be eafily feen, that, as 
the pole is in the continuation of this plane, they cannot 
meet but at an infinite diftance, or that the centre of the 
dial is at an infinite diftance; whence it follows that the 
lines muft; be parallel. The ftyle muft be placed in a per¬ 
pendicular direction in the point M ; and in height muft 
be equal to the diftance between 12 and 3 ; or if an iron 
fpike be placed at that diftance from the meridian XII 
XII, and parallel to that line, it will fltow the hour by 
its whole length. 
Of Vertical Eajl and Weft Dials.— Next to the dials already 
deferibed, the fimpleft are thole which direftly front the 
eaft or the weft. The method of conftrufting them is as 
follows : Draw the horizontal line HR, (fig. 7,) and af- 
f'ume in it any point P, for the bottom of the ftyle, the 
upper extremity of which is intended to fhow the hours. 
At the point P, make, towards the left for an eaft dial, 
and towards the right for a weft one, the angle HPE, 
equal to the complement of the latitude, or the elevation 
of the pole above the horizon ; and continue EP to N. 
The line EN will be the equinoftial. Then through the 
point P draw the line CA, in fuch a manner as to form 
with the line H R the angle A PH, equal to the elevation 
of the pole; then A C, which will interfeft the equinoc¬ 
tial EN at right angles, will be the hour-line of VI in the 
morning, and alfo the fubftylar line. When thefe lines 
have been traced out, the hour-lines may be drawn in the 
following manner. In the fubftylar line AC, a flume a 
point A, at any diftance from the point P, according to 
the intended fize of the dial ; and from A, as a centre, 
deferibe a femicircle of any radius at pleafure. Divide 
this femicircle into twelve equal parts, beginning at the 
point P, and then from the centre A draw dotted lines 
through each of the points of divifion in the femicircle, 
till they meet the equinoftial EN; if lines parallel to the 
fubftylar line be then drawn through the points where 
thefe dotted lines cut the equino&ial, they will be the 
hour-lines required, the fubftylar line being that of VI in 
the morning. The parallels above the fubftylar line, in 
the eaft dial, will correfpond to IV and V in the morning ; 
thofe below it to VII, VIII, &C. in the afternoon. The 
ftyle, the figure of which is feen in the plate, at S M is 
placed parallel to the line of VI, on two fupports raifed 
perpendicular to the plane of the dial, and at a diftance 
above it equal to that of VI hours from III or from IX. 
It is here evident that a weft is exaftly the fame as an 
eaft dial, only in a contrary fituation, (fee fig. 8 ;) but 
inftead of marking on it the morning hours, as IV, V, VI, 
See. you muft inferibe on it thole of the afternoon, as I, 
II, III, IV, See. It may be eafily feen that thefe dials 
cannot Ihow the hour of noon : for the fun does not be¬ 
gin to illuminate the latter till that hour, and the former 
ceal'es to be illuminated at the fame period. We fhall 
therefore introduce in this place a curious dial, which 
Ihows the time correft at noon only. 
Wollajlon’s Univerjal Meridian Dial. —It it well known 
that common dials give what the Englifh call apparent, 
and the foreign aftronomers true, time; which, on ac¬ 
count of the unequal motion of the fun, is unequal ; the 
natural day being fometimes longer,and fometimes fliorter, 
than the mean day, fliown by clocks and watches which 
go equably. In order, therefore, to find whether a clock 
or watch goes right, by a common fun-dial, or to let a 
clock or watch to mean time, when wrong, it is necefiary 
to add to, or fubtraft from, the time fliown by the dial, a 
certain number of minutes and feconds, ufually called 
“ the equation of time ;” in order to find the time which 
the clock or watch ought to fhow, or to which it fliould 
be fet. As the quantity of this equation is continually 
changing, and is, belides, fometimes additive, and fome¬ 
times fubtraftive, it is not furprifing that it fliould create 
difficulties among common people, and be the reafon, as 
Mr. Wollafton aflerts it is, why the time fhown by the 
clocks and watches in one pariffi differs fometimes nearly 
half an hour from the time fliown in an adjoining one ; 
for the equation of time, at one feafon of the year, 
amounts to fixteen minutes and a quarter. 
To remedy this inconvenience, Mr. Wollafton has con¬ 
trived a dial, which, notwithftanding it will not give the 
time at any other part of the day, will always fhow the 
inftant when it is 12 o’clock, without any regard to the 
equation of time ; as well as the inftant of apparent noon ; 
and, confequently, the equation of time alfo, which-is al¬ 
ways the difference between the two. 
The principle upon which this dial is formed, is as fol¬ 
lows :—As a ray of the fun, being let through a fmall 
hole into a dark room, gives his image on the floor; and, 
if a Jlraight line be drawn truly in the meridian from be¬ 
neath that hole, the centre of the image, when cut by 
that line, fhows apparent noon ; fo it was conlidered, that, 
if a curve were drawn at a proper diftance on each fide of 
that line, according to the equation of time and latitude 
of the place, the centre of the fun's image crofting that 
curve might fhow the mean time time of noon, and con¬ 
tinue true throughout the year. For fince what is called 
the equation of time arifes principally from the variation 
of the fun’s apparent motion in the ecliptic, together with 
the obliquity of the ecliptic to the equator; both of 
which depend upon his longitude or place in the eclip¬ 
tic ; and fince his declination depends upon the fame too ; 
whatever be the fun’s declination afeending or defeend- 
ing, the equation of time would very nearly correfpond 
with it; and this would hold good for half a century at 
leaft, without any fenfible difference. On this idea a me¬ 
ridian curve has been formed, and found to anfwer ; and 
from it the following dial was contrived, on a more ge¬ 
neral plan ; whofe conftruftion it is apprehended will eafi¬ 
ly be underftood. 
A Table, here fubjoined, was firft made of the equa¬ 
tion of time of each degree of the fun’s declination ac¬ 
cording to his longitude ; in which, the middle column 
contains the fun’s declination north or fouth ; the next 
adjoining on each fide, the fun’s longitude ; next to them 
are the refpeftive equations of time; and the outermoft 
columns contain thofe equations converted into degrees, 
and correfted by the co§ne of the fun’s declinatiop. 
TABLE 
