1000000030

nudow of

Ihal

perpendiclIl~r

edge is :\ "ertical cirele paf.

fing Ihrough the fun, in Ihe flereographic projeélioo.

The moolhs

b~ing

duly marked 00 Ihis dial, Ihe filO 's

declin~lioo, ~od

Ihe lenglh of the day al any lime, are

had by iofpeélioo(as alfo his ahirude, by means of afcale

of laogeols.)

HUI

ils chief propeny is, Ihal it may be

placed Irue, wheoever theCun !hioes, withoul Ihe help of

anyolher innrumeot.

Let

(Plate LXXI. fig.

l.)

be Ihe fuo's place io Ihe

nereographic projeélion,

Ihe parallel of the fuo's

declioalion.

l d

vertical cirele through Ihe fuo 's center,

Ihe hour.cirele; and il is evideot,

th~t

Ihe diamelcr

of this projeélioo beiog placed duly oorth aod foulh,

Ihefe Ihree cireles will pals through Ihe point

And

Iherefore, 10 give the dial ils due pofilion, we have ooly

lo IUro ils gnomon loward Ihe fuo, on a horizonlal plaoe,

until Ihe hour 00 Ihe eommon goomonic projeélioo coio·

cides wilh Ihat marked by Ihe hour·cirele

Pd,

which paf–

Ces

through Ihe inlerfeaion 01' the !hadow

l d

wilh Ihe

(jrele of Ihe fun's prefeot deelioalioo.

The

Babylenian

aod

[1.lian

dials reckon the hours,

not from the meridian, as witb us, but from the Clln's ri–

fiog aod fmiog. Thus, io

lla/y,

ao hour before fuo·fet

is reckooed the 23d hour; I\VO hours before fuoJet, the

22d hour ; aod fo of the rd!. And the !hadow that

marks Ihem 00 Ihe hour·lioes, is

Ihat

of Ihe point of a

flile. This occafions aperpetual variatioo belweeo their

dials aod elocks, which Ihey mun correa from lime to

time, tefore it arifes to aoy fenfible quantilY, by feuiog

Iheir elocks fo much fafler or Oower. Aod in

ltaly,

they begio their day, and regulale Iheir docks, 001 from

fun·fel, but from about mid.twilight, when Ihe

Ave Ma–

ria

is faid; which correas the difl'erence Ihat would o–

Iherwife be betwcen Ihe dock and thedial.

. The improvemenls which have beeo made inall forlsof

IOnruments aod machioes for meafuring time, bave reo·

dered fueh dials of li((le acconnt. Yel, as Ihe theory

of Ihem is ingeoious, and Ihey are really, in fome re·

Cpeas, the befl conlrived

aoy for vulgar uCe, a geoe·

ral idea of Iheir defcriplioo may OOt be uoacceptable.

Lel fig.

reprefenl ao ereél foulh wall, 00 which a

Bah,loni.n di.1

is tO be drawo, nlewiog Ihe hours from

fuo.rifing ; the lalÍrude of Ihe place, whofe horizon is

parallel 'o Ihe wall, beiog equaltothe angle

KCR.

Make,

as for a commoo dial,

KG=KR

(lVhich is perpeodicular

CR)

theradius of tbe equinoaial

JE~,

aod dmv

perpeodicular

for Ihe !lile of Ihe dial; Ihe nla

?OW of whofc poin,

is to mark the hOllrs, whcn

Sil

fel upright 00 Ihe plane of the dial.

Theo it is evident, Ihal, io Ihe coolingent lioe

JF.~,

Ihe fpaces

&c.

beiog I,keo equal 10 Ihe

taogeol' of the hour·din.oees frol1l Ihe

meridi.ln,

Ihe

raclius

KG,

ooe, IIVO, three,

eco

hoors afler fun.rifi ng,

on Ihe equiooaial

c1'y;

Ihe nl:ldow of the point

\Vi II

he fouod, al thefe times, refpetlirely io Ihepoinls, 1,2,

&c.

. Draw, for Ihe like hours after

ftlo.rifin~,

wheo Ihe fuo

IS," the tropic of Caprieorn

the lik< •

" 11111101\

lines

el),

CF., CF,

&c.

aod al thde !t"urs the flladolV uf lhe

point

will be fOllnn io lhofe hoes rdpellov..Jy. Fin.1

the lun's

alti lud~s

above lhe plane of the dial at Iltefe

VOL. II. No. H '

hours, aod lVilh their cO'laogeots

ti,

&c.

radius S

defcribeares ioterfeéliog the hour·iiues in Iht

poiOls

&c.

fo nlall the right lioes t

3[,

&c.

be Ihe lioes of 1, 11, 111,

&c.

hours after fuo·

riling.

The eoonruélioo is Ihe fame io evcry other cafe, dile

reglrd beiog liad lO Ihe difl'moee of loogilude of the

place al which the dial 1V0uld

horizootal, ,nd tite place

for which it is to feroe : Aml likewife, laking care to

draw 00 lioes bUI what are oeceff.,y; which m, ybe dooe

partly by the rules alreadygiveo for d<terminingthe liole

thal the fuo !hioes 00 aoy plaoe; aod partly from Ihi9,

Ihat 00 the tropical days, Ihe hyperbola defcribed by the

!hadolV of Ihe poiol

limits Ihe eXlent of alJ Ihe hour.

lioes.

o¡

Ihe right placing oj di./¡, and Aaving a IrU1 meridi.

an lillejor Ihe "gulalillg

clock! nnd 'Walchu.

The plaoe 00which the dial is 10 ren being duly pre,

pared, aod every thiog oeceffary for fixiog il, yOIl may

fiod the hour IQlerably exatl by a large eqiUnoaial ringo

dial, aod fet yom watch tO il. Aod theo the dial may

be fixed by Ihe walch at yOllr leiCure.

If yOIl would be more e.xaa , take Ihe fuo 's allilyde

by a good quadraol, ootiog the precife time of obffrva.

tioo by a dock or warch. Then eomrme Ihe time for

Ihe altitude obferved, aod fet Ibe walch lO ag,ee "'ilh

thal lime, accordiog tOthe fuo. A

Hadley's

quadrant

is very convenieot for this purpofe; for, by it yOIl mer

take the angle between Ihe fun aod his image ,eReacE!

from a balon of water; the half of which

~ngle,

fub–

traéliog Ihe refraaioo, is the altitude req uired. This is

ben done in fUOlmer, aod Ihe nearer the fuo is to !'he

prime venical (the ean or wen azimulh) when the obfer.

val ion is made, fo much the beller.

Or, iDfummer, lake tlVO equal alliludes of Ihefuo io

Ihe Came day; ooe aoy lime between 7 aod 10 io Ihe

moroiog, Ihe other bellVeen 2

~nd

in Ihe afternoon'

oOliog Ihe momenlSof thefe tlVOo¡'ferl'atians ¡'y a dock

or watch: aod if the \Vatch !hews the obferv.tioos to be

at equal diftaoces from 0000, it agrees exaaly with the

fuo : if OOt, Ihe walch mun be correattd by half the

diITerence of Ihe foreoooo aod aflemoon iotervals; and

theo thedial may be fel Irue by the wateh .

Tltus, for example, fuppofe you had lakeo Ihe fuo's

a1ritude wheo it was 20 mioutes pal!

VIII

io tht mol"

niog by the walch; and fnund, by obferviog io the afler.

nooo, Ihlt Ihe fuo had Ihe fame altilude 10 minutes be.

fore

IIIl ;

theo il is plaio, Ihat Ihe w'teh was

mintllts

too fa" for Ihe fuo: for

mioules after

XII

is the mido

die time bctweeo

VIII

h. 20

01 .

iA tite Oloroing .•

~nrl

III

in the ,flcrnooo; 3n<llhere(ore, 10 'nt.lke

Ihe watch agree lVilh Ihe filO, it muf! bc fct b,lck fiye

fllinllll's.

A good

,,/tridian lim,

for reglll:lliog docks or watch.

es,

01.1'

be had by the followior, ulLlh"d.

Mak~

a rOllod hnle, almol! a qllartcr of an inch di.l•

meler, in a Ihin pJ.ttc

01'

me,al ; and

fix

lite pl'l e in

Ih~

IOp 01'

a (¡lllI h wiodow, ill Il1eh a OIallller. Ih.11 it Olay

rtdinr f,onl Ihe ZCllilh " 'o :Ingle equallo Ihe ciJ.I,ti.

IlIde o( your pLlce, as nca ,ly as )'OUcao ellefs : foc li,en

lite