HOROLOGY. 
of the place, (or fitted on a frame, fo as to be applied oc- 
cafionally to a window,) will, throughout the year, give 
the mean folar time of noon far more accurately than any 
fun-dial of the ufual conftruction. On this account it is 
made public: becaufe it Ihould feem it might be conve¬ 
nient in molt country-places. For it requires no fkill in 
the obferver, but merely to fee that his clock or watch 
point to 12, or to fet it to that hour, 'when tlite centre of 
the fun’s image is upon the curve line belonging to that 
month. 
To defcribe three , and even four, Dials, on as many different 
planes, on which the hours may be known by the fhadow of only 
one axis .—Provide two rectangular planes, ABCD and 
CBEF, (Plate III. fig. 11 and 12,) equal in fize, and join 
them together by the line CB; fo as to form with each 
other a right angle, the one being horizontal, and the 
other vertical. Then divide their common breath B C 
into two <pqual parts in I; and draw the perpendiculars 
I G, IH, as the meridians of the two planes. Aflume the 
point G at pleafure, as the centre of the horizontal dial ; 
and if G I be made the bafe of a right-angled triangle 
GIH, in which the angle G is equal to the height of the 
pole, the point H will be the centre of a fouth vertical 
dial for that latitude. Defcribe thefe two dials, viz. a 
horizontal and a fouth vertical one, which will have the 
fame points of divifion in their common feftion B C ; and 
extend a piece of iron wire, as an axis, from the point H 
to the point G ; this wire will be the common axis-and 
ftyle of the two dials. Laftly, having with any radius at 
pleafure deferibed a circle, trace out on it an equinoctial 
dial, which muff be placed on the axis G H, in fuch a 
manner that it fhall pafs through its centre, and be per¬ 
pendicular to its plane, and that the line of 12 o’clock 
fhall be in the plane of the triangle GIH. 
■ If this triple dial be expofed to the fun, fo that the line 
GI fhall be horizontal and in the plane of the meridian, 
it is evident that the fhadow of the axis G H will fhow 
the hour on the three dials^at the fame time. 
If it be required to have a fourth dial, to fhow the hour 
by means of the fame ftyle; in the plane of the triangle 
GIH draw a line parallel to GH, and through that line 
a plane perpendicular to the plane of the meridian, which 
will cut the vertical plane in the line KL, and the hori¬ 
zontal plane in M N: the hour-lines of both dials will be 
cut by thefe two lines in points, every two correfponding 
ones of which muff be joined by tranfverfal lines ; for ex¬ 
ample, the point of feftion of 11 hours on the one, with 
the point of fection of 11 hours on the other, which will 
give on that plane parallel'hour-lines, fuch as ought to be 
on a polar dial that has no declination : thefe four dials 
will fhow the hour at the fame time, and by means of the 
fame ftyle or axis G H. 
Another Method— Provide a cube A B C E D, (fig. 13,) 
and having divided the fides AB, CE, and FD into two 
equal parts, in the points H, G, and I, draw the lines 
G H and G I: then affirming thefe lines as the meridians 
of the horizontal plane CD, and of the vertical one CA, 
and the point G as the centre, defcribe on the former a 
horizontal dial, and on the latter a vertical dial, each 
adapted to the latitude of the place. Affume the lines EM 
and EN, in fuch a manner, that the angle ENM fhall 
be equal to the latitude of the place ; make CO and CP 
equal to them ; and let a plane, pafs through M N and O P, 
fo as to cut off that angle of the cube: this plane will in- 
terfeft the hour-lines of the two dials already traced out 
in certain points, the correfponding ones of which will 
give the hour-lines of a third dial. 
Nothing then remains but to fix the ftyle, which will 
not be attended with any difficulty. For this purpofe, 
having drawn EQ perpendicular to MN, fix in a perpen¬ 
dicular pofition on the meridian KL, and in its plane, 
two,fupporters equal in height to EQ, bearing the ftyle 
R S, prolonged towards each end, and parallel to KL: the 
fhadow of this ftyle will fhow the hours on the three dials 
at the fame time. 
To cut a fone into feveral faces, oti which all .the regular 
dials can be deferibed. —Let the fquare ABCD (Plate IV. 
fig. 14) be the plane of the ftone, which is to be prepared 
fo as to receive all the regular dials. If we fuppofe the 
ftone to reprefent an imperfect cube, or any other irregu¬ 
lar folid, after all its faces have been fmoothed, it muft be 
fquared, and reduced to an uniform thicknefs. When 
this is done, proceed as follows : On the plane ABCD 
defcribe the circle HELF, with as large a radius as the 
ftone will admit; and dr,aw at right angles the two dia¬ 
meters FE and HL. Then make the angle FOI equal 
to 38^ degrees', which is the complement of the latitude 
of London, and draw the diameter IOM; make the an¬ 
gle EOG equal to the latitude 5if degrees, and draw 
the diameter G O K; then through the points I, G, M, K, 
draw tangents to the circle HELF, which will meet the 
other tangents palling through the points H,.E, L, F, and 
form part of the fides of the fquare ABCD, that repre- 
fents the plane of the ftone. Cut the ftone fquare, accord¬ 
ing to thefe tangeoits, in order to obtain planes or faces 
perpendicular to the plane of t^ie ftone ABCD, and the 
ftone will then be prepared for receiving on all its faces 
the dials which belong to them. 
On the face or plane which paffes through the line VX, 
defcribe a horizontal dial; on that palling through X N, 
an upper equinoftial dial, and on the oppofite plane, pac¬ 
ing through SR, an inferior equinoftial dial. An upper 
polar dial muft be deferibed on the plane paffing through 
TV, and an inferior one on the plane paffing through QP. 
On the plane paffing through T S make a fouth-vertical 
dial, and on the oppofite plane N P a north vertical one. 
Laftly, if an eaft vertical dial be deferibed on the fide of 
the ftone IM, and on the oppofite fide a weft vertical one, 
the whole will be complete. 
If it be required to have the ftone hollow, or rather cut 
through, nothing will be neceffinry but to draw lines pa¬ 
rallel to thefe tangents, and td cut the ftone fquare ac¬ 
cording to thefe lines, which will give, in the iniide of 
the ftone, furfaces parallel to thofe on the outfide. On 
thefe internal furfaces, dials fimilar to thofe on the oppo¬ 
fite external furfaces may then be deferibed. It is here 
to be obferved, that when the ftone is thus made hollow, 
neither an eaft nor a weft dial can be deferibed on it; but 
if it be placed on a pedeftal in the form of a regular off-a¬ 
gon, having one of its faces turned direftly towards the 
louth, different kinds of vertical dials may be deferibed 
on this pedeftal, viz. a fouth, a north, an eaft, and a weft, 
dial, together with four vertical declining dials ; fo that 
on this ftone and its pedeftal there may be twenty or 
twenty-five dials. 
If the fouth vertical dial be placed direftly fouth, and 
if the horizontal one be perfectly level, all thele dials to¬ 
gether will lhow the fame hour. 
To confnidl a Dial on the convex fufface of a Globe.— -This 
dial, which is.the limpleft and molt natural of all, is 
formed by dividing the equatorial circle into 24. parts. 
If a globe be placed on a pedeftal, ih Inch a manner that 
its axis fnall be in the plane of the'meridian, and ex-aftly 
elevated according to the height of the pole of the place, 
nothing then,will be necefTary to complete the dial, but 
to divide its equator into 24 equal parts. 
The globe, (fig. 15,) in this ftate, may be ufed without 
any farther apparatus; for, one half of it being enlightened 
by the fun, the boundary of the illumination will exactly 
follow, on the equator, the motion of the fun from eaft to 
weft. At noon, it will fall on thofe points of the equator 
turned direftly to the eaft and weft. At one o’clock, it 
will have advanced 15 0 ; and fo on. To render this, globe 
then fit for being employed as a dial, VI muft be inferibed 
at the divifion which correfponds with the meridian, VII 
at the following one, and fo of the reft; 1b that the 
twelfth will be exactly in the point turned towards the 
weft; then I, II, III, See. will be under the horizon. 
Nothing then will be neceffary, but to obferve what divi¬ 
fion correlpoqds with the boundary of the light and ffia- 
dow ; 
