, fbaight lines, to the .JJj points; o£,the compafs ; when
therefore you draw ihe.itraight- fine f ro n rA to -B , you
mil ft obfervc to, which.' of,the .3?.; line:;, it huparalbl, or to
which it is .-nearefi: ,fo, and* you,,thus:g.et the'.-rhumb, or
the point o f the compafs you muft continue to fail upon,
in order.t0i.go from A to(B . H For iuftance, if you
End the line ^Ja is.paralleljto the fqnth; weft fine,of the
Compafs,'ijlen i f you .continue to fail on the fputh-wefl
point, yoii .
245. * In all maps, the,fupper part is northern, the
lo\yér7pàrtftfputhern,-thé right hand,.fide. js jë a ft^ ;i and
thé left handifi'de is weftern. On the r]gh.t.diidleft.ljdq>î
the degrees o f latitude are marked ; and on the top, and
bÓ‘tforn,;,t'hè degrees marked., When the
maps are very large, the degrees may b e ju b divided into,
halves, quarters, & c r •,
V246. When the meridians and parallels o f latitude are_;
f ir night and parallel lines, the latitude o f a place as,found'
by flrêriihîmg a thread over the ;place,-fo that.-it mayciit,
ûid'fame degree of,latitude cur the1 right and, leftrfid eo f
thqmângand that degree is the latitude olMt'he-, place."
A n d to find'.the longitude, ftretch a thread over the place,
fo th a t it may cut thefame degree of-longitude, on. t^e
tpp'andbbttom,,ahd that tjegreeis the longitude o f the
place. For inftance, i f we tak.e the chart o f the Eaft
India illand's,.and ftretch a firing over.Sin»;, we-fliall.
find that it will cut each fide at 14° N. laf. and the top
andhottom. at 1 o®. 10' E . long. -Thefe . therefore are
the latitude and longitude; o f that place.
. 2.47. On the, contrary,v.,if the latitude and Iongi|ude:
o f ‘a place he given -to find , the place, ftretch one thread.
over the given .degree o f latitude- on each fide, and another
thread o v e r th e . given degree o f longitude at
the top and bottom;'and at the . interfe&ion o f the .
thrra&as th,e place required. . B y this means you may
ptit down in-a map, any place whofe latitude and lon g i-1
tude are known.
'‘ 248- Now let the meridians and.pàrâiïete o ^ p i tü d e ,
be curve’dinef. Then to find thé latitude o f 'ai.placera-
p"àialleftpf..datitude " mult be drawn through jit, .by the
dame .rules as the otheriuftalleh. are drawn,, and it cuts
the fides at the degree, o f latitude o f the placé; . A n d to .
find the longitude, "of thé place, dra w ai circle, oflpngitu.de
through it, by thêdàmè '■ liules^ash tfie i^hw^cifclès'arë
..drawn,, and it cuts the top/and bottom at. the degree d f
longitude o f the"'place, -v Bu t as -it is. troublefotne tb
draw thefé;cifclesfufne"follow]’ngmethodfmay generally
be fu & t t it ly g c ë ^ ^ tó d T o find theda&uc/r, find b y a
pair- ° f compaflès and a'fcale o f equal pajsyhbw far the
placets frbmthe tWo paralhls.between which it lies, and"
divide the1- ciiftance o f the parallels in' that proportion,
arid you get" very nearly thc -latitiicie. Suppofc, for in-
ftadee, ithe'-diftance between thei parallels ï d j ^ î f f a id
that qneSikï"îqàfaUet pf 45°, and the, o t h e r - b f - a n d
fuppofe the -place ’tçrjberwithin 3 parts o f the parallel o f
4 j° , and 7. parts’ o f tfiéo^afallêl o f 50? ; then 5° muft be
divided in to '10 parts, and 3 o f thofe. parts meft be
added; to 45°, and it gives the latitude, This; is done.
b y proportion,i.thus, '3 + (7,- dr to -:r$-:: 50: '3X 5 *_
" - " ■ 16 ~
LL zcx f-0 ; therefore the latitude is 46'i ° nearly. In the
10 " -i • r f T . f , , y
- very. fame’nShnher^tp.iaima^ fin d the loiighicde nearly.
■ 249.- "©h'tHe - contrary, if tKeiiatitude'addlongifude
o f a place-bei .giyen,;-to find tire place,-- ch aw • a circle o f
-latitude through the given latitude .orreach fide, and, a
circle ofdchigftude thrtHigh the given V1 1 1 at the
top a a ltd bottom,--'and their- interfedliou denotes the1
place. O r e s you kuowbetvvecu'what two parallels: o f
latitude and o f . longitude the place is, “.ybusknow b y
what.' four lines it is honnded ; and as you ‘‘knowtme.
-proportional diftance from e'aeh line, you may eafilyjyfiy
trial,' find the {ions'.. 5•
'••2jOC’Wht-n we'undertake a voyage;‘w e ought to he
acquainted with the illands, rocks, lauds1, ftraits, rivers,
&c, near which-we are to' iai»S*lfhe ,Tendings arid the,
runnings1 out o f the ihorcs,’ &c. we1 ftfould‘.alfd know the
figns o f being near laud; Which are'freqadntly'g'B ffithe
appearing o f birds ; *i h e ‘ floating o f weeds upon the feet
ther de'fjthd-'and’ *c&tatff'drf me water. S^S>recApr,xJwet
iheukbknow ithe-times'-wheii.'inhe vriridshfefyiiT, partied-1
larly the;jriade winds bjj e- ,fesd|ni^dwhe id
ftorms'and7hurricanes are to be ^p^?i^,antgtrie'fi^6?o f
“ their approach ’ ‘the 1 motions p>ft_current8^»plAlJ5 more*
efpecially'of th e tides. A l l thefrthmgs ard to b e leaingd?
by good fea-charts, and journals o f vo jages^ *
; O-ti'dbe Mariner{s Comfajs.
‘ ,25 r. -T h e 1.' earth ‘psHefiefit ajTcr^igindtKS-r fubftimee-
which hasfth^iprgperty o f aitrafting ii on and fteel only,
and. this fnbftance is called a natural, magnet, didpddjlonti.
The fame property m^y^afwt|eTpmm.anica£fid'teiiron.
and fteel, and thefe are ck\\e&Jartificial magnets.
252. j l f apiece o f wire; or. a .needle.be rendered magnetic,
and.be fufpended upphcal fine, point at its middle,'
fp.tha t.it &anifieely turn m'an hpr-izbntal plant, one end.
will always be dirccied towards t.lic northern part o f the
horizon, aud thc,.otlicr. towards tile Ifoathefn v T h e
farmer end is callccbt-ljo■ northpoh, and the ISfttfeSendTlie
foitth.pole. -Thefe tpoleslaje not. direfted to thejiorth
and foutb poles o f the earth, but-vary confiderabl
them, and" dScrOntly m j|jferent pjacts, and- this i fa llen.
9 j‘e:-vdrititwn oflhey, compafs-, and" even, in the fame:
place,, theyfare. fuhj.edt.to" a yery fmafi;griidug;l‘ ya'riStion..-
The fiire&gn in which the magnet lands,/is, called ther
magnetic meridian, g
253. T h e mariner’ si compafs, or, gas it is. called, the
compafs,- tilc-fieefirig^compafi, or the;needle,, confifts of, three- •
pai ts, the box, the cp d or. fly, and the needle. T h e
. card is a circle o f fiiif paper reprefentigg thehorizoti,,
. with the 32 point8bof the compafs marked upon i t ; the
, fixed .
the centre,of the needle, is’perforated,; and, a c.ap^^
conical agate, at its top is fixed in this perforation j„.thi?.
cap ia hung on ajfteej'piri,.which is fix.ed.to the botfoin,
o f riie box, fo that the card, hanging on the pin, turns
freely round its centre,; and jEhe needle lies in the direction
o f the N. and, Srpoints, o f ffie ;gard;, and .therefore
thef^rpciint^' wfil idwky SHbe'direp:ed' tri ifx^fridgnnic north
and fquth-.poirifs S f the. horizon, the needle-fixirng- itfe lf
in. fe e magnetic;.meridian..,.. The box which contains the
card ana needle, is .ajchgujaf b|afs b q S h a n g within another
box b y two .iCOncentrip, rings, .caUediftjimbals;fo
fixed by|prof^ centres, to, the two boxes, that the inner
01'u bft./ll 1 ;tain a horizontal fituation gr all .the motions
,of. the ihip. T he top o f the ittner .box, has % .cover o f
glafs, to prevent, tlic card from-, being ;difturib"ed' b y th e .
.wind.f ,
254. In order to" determine the true point o f the
cb'mpafs"on which alhip fails, it iVneceiTary to know, the
variation of the compafs..af , ,the place where you are, on
which" accbunt, every means have beentifed" to determine,
-by obfervat-ion, what the '• variation is ; -and thefe obfer-
vations have been put down in good fea-charts, for the
;uf"e o f ,navigators, Thefe.ho'wever can ferve but for a few
years",, 'em account of. their being variable at the fame
place nor has it been difeovered how much the variatio
n is.fubjedtto vary. (The following table from Mr.
C rv y L L o ’ s Treatife on Magnetifm, contains the variation
at the places and times therein inferted, and upon
many occafions may be found very ufefuL
Latitude N. - ; ÿ •LoWitude. W . t ; ' Variation E . •' I1 Years»
? I70”‘ u4îri"l6"S®. 24' i' 21' ■
69. m m M « ■ 64'. 11 u, ;;.0i ..\.J ' 1778 ••
{ 66. : 1 ! .16 7 . j W s I - *2§1 j
1 t â - . 43|r H T 34 ! ■ 2 7 . ,
; «3 - ‘ W ■ 1 • iC y. " 4 8 - . . ‘2Ö.
Y -J.9- . 39 • /■ -*49r j 8 54 1 '
58 . »4 ■ -•-'f 39' ■4 T9 f 24. Im M
" 'f ' 'Hr •• 12 i ; . - is s s • V;q" ,4 - ? 3- 29 v :':-
1 S 3- 37 > E " i 134. 53 ■ -io . 32 «
H H "8 "• ^ ‘ g 14-j & o , f 1 Vârîation W ,
. 48. 44 i B " « f 1 !■ ', i 2©. W - ' i 40- 41 t S i " i f i i . 1X0 ■ 'ft -22".; s "‘ t f f " ' 38 1
33- 45 4 -^*4-. i s o * . " 22‘. 37 • t
3 1 ! jo . f t 15. m m i |7 0
28, : ■ .,a° -h • f i j . f o ., . m i 43 ^
m 54 ;-i8. 20 .. S lfV, - ' l l
b ■ 4 1
■4 "':zc0ih 3P* - ; t fp g X .;3'"**- m B 35' •
45 ; [ I >J 39 . 11
•• ; 16. 37 -b.1 . 22. 1Mû » ' 33 ; .
:..®5 ;■ îf fl ft6 m m m m .
1 ■ . 32 R 23‘ 4 5 ' t 45- . : , 12. ; 21 t 2ft- 54 ■ ; : 48 •
11. 5 1 «4- . . b , s . . 19 ... ■ ’ "'y-'sf: 22. ■ 5° , - r.: m m
- . 1 4, ■ 1 20. . 5 - 44 f f i l i 1 ' 'è-l.;'- r-.T?-. i-,9- -a. '■ J
i 1 | H 45 22., "34 - - : -'27 .
- 2. S? , 7- 1 42" B ,
t ’ ||t‘ j - 26. r 2 . i f B i p a i , 35 -, , '
I .-'Ô? i * 7-f;î 4-
I wBKI ■ 7 27. ^ 11 4 - -,
Latitude.iSi; . : ■
i s . 5®, 1 , 3- 12 J
h 3 Latitude