1000000030

find the time of the day by this dia!' Having fet

it on a level place in fun·nline. and adjuHed it by the

leveling fcrews

and /, until lhe plumb.line hangs over

the black line upon lhe edge of lhe,quadrant, and paral.

lel to the fa id edge

move the femicirele in Jhe qua·

drant, umil the line of

'V'

and:!!: (where the cirele

touches) comes tO the latitude of your place in tile qua·

drant: then tUín the whole meridional plane

BD,

lVith

itscirele

uponthe horizontal plane

umil the edge

of the Ihadow of the cirele falls precifely on the day of

the month in the femicirele

and then, the meridional

plane will be due nonb and fouth, the axis

will be

parallel to the axis of the world, and will can a Ihadow

upon tbe true time of tbe day, amoog tbe bours on the

cirde.

found exaél eoo\1gh for feme Gther requifites in dialiag,

IVe n,all take il iDoccafionally.

The

~onnruélion

of fun·dials on all planes wharever,

may be rncluded io one general

ru~:

inrclligible, if that

of a horizonlal dial for any given lalillldc be well under.

nood. For there is

plane, however obliquely fitua.

led with refpeél to any given place, but what is parallel

to the horizoo of fome dther place

and therefore, if

IVe can fi nd tha!

othe~

place by a p;oblern on lhe lerre.

nrial globe, or by a lrigonomerrical calcularion,

and

conaruél a horizonlal dial fe r il; lhal dial, applied

!he plane where il is te ferve, IVill be a trile di. lfor lha!

place.--Thus, anereél direél foutn dial in 51{ degrecs

nOrlh latilllde,IVould be ahorilOfttaldial OA the fame meri.

dian, 90 degrees fouthwa-rd

degrees nonh latitude;

which fa lls in IVith 38{ degrees

fouth lo¡itude. Bu!

if the uprighl plane declines froro facing thefou!h at the

given place, ir would ílill be a

horizOot~1 pl~ne

90 de–

grees from that place, bUl f{)f a dilferen! longirude,

which would alter the reckoning of lhe houls accord·

ingly.

A S E 1.

As, when the inarument is thus reélified, the

quadrant and fCRlicirele arein tbe plane of tbe meridian,

fo lhe cirele is lhen ilt the plane of the equinoélia!.

Therefore, as the fun is above the equinoélial in fum·

mer (in nonhern latitudes) and bclow it in winler

the axis of lbe femicirele lVill can a lhadolV on tbe hour

of lhe da

on the upper furface of the cirele, from lhe

twentielh of March to lhe lwenty·tbird of September :

and from the tIVenty·third of Seplember

lhe tlVen·

Ln us,fuppofe, rnQt an up,ight piane at LonJon

tielh of March, the hour of lhe day IVill be determined declines 36 degrecs IVellward fram faóog the fOUlh

and

by the lhadolV of the femicirele, upon the lower furo that it is required

find a place on the globe,

lV~ofe

face of the cirele. In lhe former cafe, the Ihadow of horizoo the faid plane is parallel

and alla t-he dilfucoce

the drde falls upoo lhe day of the month, on lhe10IVer of longitude between London and that place.

pan of lhe diameler of lhe femicircle

and io lhe lauer

Reélify the globe to the lalitude of London, and bring

cafe, on the upper pan .

London 10 lhezenith under rhe brafs meridian, lhenthu

The method of laying down themonlhs and figns upon point of the globe IVhich lies in lbe horizon at the given

thefemicircle is as follolVs. DraIV the right line

ACB,

e· degree of ioclinalion (coumed lVaflward from lhe fouth

qual to thediameler of the femicircle

ADB,

and crofs it point of the horizoA) is the place at IVhich the abovc·

in lhemiddle at right angles lVith the line

ECD,

equal io mentioned plane IVould be hOrJZonta!.--NolV, to find

length 10

ADB j

then

lVill be the radius of the circle the lalilude and loogitude of that

pIQc~,

keep )'our eye

FCe,

which is the fame as lhat of lhefemicircle. Upon

upon the place, aod tUCA the globe eanward, until it

as acentre,defcribe thecirele

FCe,

onwhichfeloff lhearcs comes under the graauated edge of lhe braf6 níeridian

and

Ci,.

each

eq~al

to 23';' degrees, and divide them then, tlle degrees ,ef ehe brafs meridia-o

lh~l

llands di–

~ccordingJy

ioto that Dumber, for the fuo's declination. reélly over the place, is its IHilade

and the number

Theo, laying the edge of a ruler over the cenlre

and . of degrees in lhe equator, which are intercepted be–

~Ifo

over the fuo'sdeelinatioD for every fifth day of each t\Veen lbe meridian of London a'!ld the brafs meridian,

momh (as in lhecard dial ) mark the poinls on lhe dia· is lhe place's dilferenee of longitude.

meter

of lhe femici rcle from

which are CUl

T hus, as lhe lalilude of.Londbn is 5ti degrees nOrlh,

bythe ruler

and

thm

place the days of the rnonlhs and lhe deelination of rhe plaee

36 Eleg,ees \Ven

ele·

accordingly, anfwering 10 the fun's declination. This vale the north pole

p {

d~g-rees

aoo.e fhe

heri~on,

aod

?one, fettingQne fOOI of lhe compaffes in

and eXlend· turn lhe globe unlil LondoA aomes te the zenith, or un–

rng.lhe orner 10

defcribe the femicirele

aDei efg

der the graduated edge of lile meridian

lhen COUO! 36

whrch divide ioto fix equal pans, aod lhrough lhe points degrees on lhe horizon lVenward (rom the fOUlh poinr,

divifiondraw righl lines, parallello

CD:

for tbe be· and

m~ke

a mark on lhal place of the globe ol'er which

grnning of the fines (of which one half are on one fide of lhe reckoning enas, and bringing me mark lHlder lhegrao

the femicircle, and lhe olher half on lhe other) and fet dualed edge of lhe brafs meridian, it will be found 10 be

Ihe charaélers 'of the figns te lheir proper lines, as in the under 30¡ degrees in fOUlh latillrde: keeping il lhere,

figure

counl in the equaror the number of degrees belween the

Having Ihewn holV to make fun·dials by the aflinance meridian of London and the brafen meridian (which

of a good globe, or of a dialing.fcale, IVe Ihall nolV pro· nolV becomes the meridiao of tbe required

pl~ce)

and

ceed 10 lhe melhod of connruéllOg dials arithmelically

you IViII find it to be

42{-.

Therefore an upright plane

which wilJ be more agreeable tO lhofe \Vho have learned at London, declining 36 degrees wenward froOl lhe

the elemenu of rrigllnome!ry, becal,fe globes and fcales fOUlh, would be a horizonlal plane al lhat place, whofe

can nel'er. be fo atcurate as the logari!hms in finding lhe.

1~litude

is 30± degrees footho.f

~he

equalor, and longitude

angular d,nances of lhe hours. Yet, as a globt may be

42t

degrees wea of lhe mendran

London.

VOL. Il. No. 43.

Which