1000000031

tne a imulh, il will be, b7 lhe fecond of Ihe prccecling ana–

ics ,

As radius

10.00000

the lang. of lhe altilUde

38°, 18' 9.89149

fo is lhe tango of lhe fixlh areh 21 .. 06

Q. 58644

to lhe co·fine of the nitnUlb 72 .. 15 -

9 ' 48393

which, b. caufe the latilude is nonh and lhe fun fOUlh of

the

pla~e

of obfervalion, mull be counted from the fOUlh

towards lhe eaH or

wtll;

and confequelldy , if tbe alti ·

tud of the fun

~'as

taken in {he morning, the azimulh,

",ill be S 72° IS' E, or ESE 4"

Q5'

E ;. hUI if lhe al–

titude \,fas taken in the aftcrnoon, the

azimuln

will be

72° IS '

or WSW 4°

45'

\Vefter/y.

9. Having fouod lhe fUD 'S true ampli,ude or azimuth

by lh. preccding aDalogies. 3Dd

h lS

magnelic amplitude

or az.imUlh

obfervation .

il..il

evident,

they agne.

the-re

no y¡riation ; but

lhey

difagree, then

lhe

true . nd obferTed ampllmd" 3' lhe n Gng or felling of

,he fun be botb of lhe fame name,

i . • .

<ilher bO"1

north. or both

roulh,

their diff(rc!nce is the

Tariatíon ;

uut

rhey be of d¡ fferent names,

t .

one nort h anJ

the other

fOUlh ,

thcir fum is the variation . Again,

Ihe true and obferved a2.mulh be hoth of ,he fame name,

'•.

either bo,h ..H or hOlb well , ,heir difference is Ihe

variation ; bl1t

they be of different name" their fum

¡.

Ihe vuíation:

~nd

know

whclher the

variatlon ¡s

eaflerly, obrerve lhis gtneral rule,.

tJiz .

Let lhe obrerver', face be lu rned

the fun: then if

Ihe Irue amplitude or ;zimmh be lo Ihe righl

h.nd

,he ohferved, lhe varialion is ealler/y; bUI if i, be

Ihe

lcfl, \Vcll.r1y.

To explain which. lel NESVl (No. 30') reprefenl

compaC" and fuppofe the fun is really EbS al ,he lime

of obfe rv.\ion. hut lhe obferver fees him off lhe

e.rt

point of Ihe compaf., and fo ,he Hue amplilude or

ni–

mUlh of lhe fuo is

lloe righl of the magnelic or ob–

f.n·ed; here il is evidenl Iha, lhe E óS poio, of lhe

romp.:s oughl

lie where Ihe .eafl poin, is , and fo Ihe

Donh ",he.e Ihe NbW is ; confequeotly the nonh poiot

of (he compars is a poio::

lOO

far eall ,

i .

( .

the \"ariation

iD Ihis caf..is ealte r/y. Th

_me

\ViII hold \Vhen Ihe am–

plitude or azimulh is 'aken on ,lte well (ide of lhe me–

ridian.

Again, le, the Irue amplilude or azimulh be

Ihe

lefl hand of lhe obferved. Thus, fuppofe lhe fun is really

EbN

,h. ¡ime of obfe rvatioD, bu, tbe obferver fees

h,m off Ihe eall point of Ihe compafs, and fo the lrue

amplilude or azimuth tO the left of the obfo. ved: Here

il is evidenl th.. ,he EbN poinl of lhe compafs

o~ght

rllnd where the eall poinl is, and fo tite nonh where.lhe

NbE poin' is; coofequ<ntly Ihe Dorlh poio, of lh. com–

pafs lies a point

100

far weller/y, fo in this cafe ,he va–

rialion is \Vell. The fame wi/l hold ",hen Ihe fuo is ob–

(erved on ,he wefl lide of the mer idiaD.

EXAMPLE

Suppofe lhe fu n's 'rue amplitude al ri–

{j.~

is found tO be E

IJle>

20'

N, but by ,he compafs il

i, found to be E 26° 12': R equired the vari'lioD, aDd

which

w~y

it ¡s.

Sinee Ihey are bOlh ,he fame \Vay, ,herefor.

From lhe magDetie amplilude

E : 6

1: '

take

trU~

ampli lude

gRQ

•

and

Ihcre

remains the variation

JI . 52

which is cafierly, becaufe in lhis cafe the: true ampl.ludc

is Ih. right of ,he nbferved .

EXAMnt

11.

Suppofe lhe fun's Irue ampli,ude . tftt–

ting is

34° 26 '

andhis magnc,icamplilnde

\V,>"

S: Required the

,';\1

iJtion, and which

w~y

it ¡s.

Since ,hey lie bo, Ihe fame w'y, Ihererore

F rom ,he fun 's .'rue amplilude -

\V 34

n •

26'

lake his magnetic amplilude

23 , ' 1, S.

there rem;¡ins the variation

J' ,

v.¡.

which is wellerly. becaufe: the Une ampl itude:,

inthlsca.re

,he lefl hand of ,he obferved .

EXAMPLE

lll.

Suppofe the fun', \rue alti,ude at r¡Gng

is foundlobe 13° '4' N,andhis magneli'!: E ' 120 32' S:

Required the: \'ariation, and which way it lies.

S inee Ihe true and obferved amph,udes lie different

wa ys. thererore

'1'0

lhe Irue amplilude

add the magtn:,ic amplilude

E i3°,

24'

12 ,

Ihe fum is ,he varia,ion

2 S ' S6

\V .

wrnch

-ís

weHe:r1y,

bec~ufe:

the true amplilude is, io this

eafe,

th.

I.fl

of lhe obferoed.

E"ü,,"t

IV.

Suppofe the fun's "ue

altitude.at

fet–

ling is

fbll~d

8° 24' N, OUI his magoe,ic am–

p'l ilUd~

10° 13' S: Required Ihe voiri..ion.

to Ih. true ampli,ude

W 8°, . 24'

N .

add tbe magne,ic

VIlO,

13 S .

the funt is Ihe varialiod

)8 , 37 E .

\Vhich is eaHerly, becaufe ,he true amplitude is lO lbe

righ, of lhe obferoed .

EXA>t PLt

Suppofe lhe fu n's Irue azim u,h at ,he

time of obfervalion, i, found

be N 86

40' E, bUI by

lhe compafs il is N 73° 24' E: R equired ,be variatioD,

and which way i, lies.

From lhe fuo's true az.imulb,

take the magnetical,

0 ,

40'

N 73 '

24 E_

there remains the variation,

' 3 ,

16 E:

which is ealle r/y, becaufe lhe Irue azimulh is 'o lhe rigbt

of ,he obferved.

EXAM PLt VI. Suppofe the funs IrUe azimu,h is S

,o 24' E.

ana

,he magnelieal S 4° 36' W: R equired

the váriatioo, and which way it lies.

To the true azimuth

add lhe magnelical azimulb

the Cum is thevariatioo

8 ,

",hich is weller/y, becaufe Ibe "ue azimu\b is, in ,hi.

cafe,

lhe left of ,he obferoed.

JO. The va riation of the compaf, IVas 6rll obferoed

at London, in lhe ye.r

158o,

'o be 11° IS' eallerly,

and in the year 1622 it was 6°

E; alfo in the year

1634. i, was 40

05'

E , /lill decr<aling, aud ,he Deedle

approaching tbe true meridiao, till

coincided w,ith

¡t,

zod tbeo there \Vas

DO \'ariatioD;

after which, the viria-

ÚOIlI