439 / 868
N
A
v
1
G
12 milEs, AC egu.1
to
.o.~
aod ,he aogle BAC egu.1
10
33°
45', ,h; .dil!>n"< be, ,"eeo ,he
SEóS
aod
ESE
Ijnes. ' Vhcnce for the anLle
al
B.
il
will be,
As ,he fum
of
lh< fides AC aocl AB
3' 9
1.517'°
j~
10 lheir dill'ereoee
S·9 - 0 ·94930
fo io lhe
,~og.
of half 'he}
730,
07' _ 10 .5 1 606
fum of ,he .ngles B and C
to
taog.
of {lheir dilf.
41°, 43 '
T -
9·950 25
confequeotly ,he aogle B is
I
14°
5~ ',
aod fo Ihe leuiog
of ,he curreo' \ViII be
N
SI° 06'
E
or
EbE
2° 21 '
E.
Then for
Be
the
cu rrcol's
drifr in 3 hours,
it
wil!
be,
AsthelineofB
11 4°,51'
9·9' i OO
js 'o ,he dil!.oce run AC 20.9
1.3:02 5
foisthefioeofA
33°,45'
9·74474
t é
BC
12 .S
1.
10
7 19
lhe cllrrent's
drift
in
3 hours. and
co¡:¡fcquently
the
CUT·
rent fe\s
EóN
2° 2 l '
E 4.266
mi les an hour .
Sea. 8.
C'JI/('~,.nir.glh~
VAltlATION
of
l A!
COMPASS,
4nd
ho'UJ lo
jiJJd ;1
jro1Jl Ihe Iru,
4nd
objervd
A Ol PL l'
'TUDE8
Ir
A2IMVTHS O/th~ftln.
l.
T
H
E
variation of the compafs is how far the north
or
routh
poinr
of the needle Hands (rom the true
routh
or
north
paiOI
of the horizon towards tht:.
can
or
wen;
or
it
is 3n
arch
of lhe harizon intercepted
betw(!'eo the
m:·
ridian
of the place of obCcrvation éind the magnetic
me
ridian .
2. It is .abColutely necdTary to know (he variation of
the compaCs
al
f~a ,
in
order
[O
correél: {he {hip's courCe;
for fioee the !h¡p's eourfe i. direéted by the eompars, it is
evideot that
ir
the compars be
wrong
the
true
courfe wiU
differ from the o" fer",d, aod eoofe'luently the whole
reckoniog differ from {he truth.
3. The fun's true amplitude is an arch
oF
the horizon
comprehended becween che true eafl or wefi poinr thereof,
and lhe centre of the {un at ri{iog or Ceuine; or
it
is che
number
of
degrees,
&/,:.
that the centre
of
the Cuo is di·
fiant from
lhe
true ean or weH pOlDt
of the
horizon, to·
waFds the fouth or north .
4.
The
{un's
magnetic amplitude
i~
lhe
number
of
de·
grees
that tbe centre
of the
{uo is
from
(he
e;¡{l
or
~eH
POiOI
of the compaf., ,owards Ihe fou,h or oOrlh FOln, of the
fame at
rifing
or
feuing.
5.
Having thedeclinalionofthe fun,
together
with the
btitude of
the
place of
obrervation,
·\Ve
may from
thence
bod the fun's \Tue amplitude, by ,he following allronomic
proporition,
viz.
As the eo·line of tbe latitude
ís to
the
radius
S o is tbe
fine
of 1he
fun's
dceJination
tOlhe fine of lhe rlln'S lrue amp1itude
whieh
\ViII
be north or fouth aeeordi\lg as tbe fuo's declino·
tion is
north or romh .
EXAMPLE .
R equired lhe
fun's
true amplitude in lhe
lali,ude of 4to 50' oonh,
00
the 23d d.y of A pril 173 1.
Firll, I find (from Ihe lables of ,he fun's declioation)
tbatlhe fuo', declinalioo Ihe 23d of April is 15
0
54' north;
, hen for Ibe \Tue ampli,ude, it will be, by the former ana·
logy .
A, Ihe eo Iin, of Ihe !at, 410 50'
9 .8722
I
is
to
rad~us
10.COOOO
A
T
o
N .
fo i. ,he fine of Ihe der\. 15°, 5.1 ,
9 ' 437 69
tothefineoft.heampht. 21, 35 ,
9' S6S48
whieb
i.s
nonh, bee,lUfe lh e
decbnation
is nonh
at that
ti:lIt; anJ
confequently, in
the latitucie
of
41
o
50'
nonh.,
the fuo , ir.. on ,be 2
3d
of April 2'"
~5'
from the ..
11
pan of
the
horizon
tow;uds the
north, and
rets
fo much
from ,he \VeCl,he f, me \Vay .
6. The run's
true
3zimuth
jo;
the
areh
of the horiZOD
intercepted betwec n (he mcridian aod (be
vertical
circlc
.palTing through the centre
of
the fun
at
tbe time of
01.>.
fervation.
7. Tbe fuo's magoelie azjmUlh i. the areh of the he>–
riz.onintereepterl betwe.:n the
ma~netic
meridian aod th:
venieal,
pdJing
through the
(un .
S. Ha\'iog the la,i,ude of ,he place of obfer.alioo, 10-
gether
wirh
tbe
fur,'s
declinatioo
and
altitude
at
the time
of obrervat ioo, we m..
y
lind
his üue:lz. mUlh after the..
foll owing method,
t:iz.
I\
r.keil,
As
lhe
ungent
of halr
t~le
cOl1lplement of the htitude
¡J
to the
[angen{
of hCllf lhe
fUIn
of (he dinanee of the fUD
from the pol e aod eompl. meo, of ,he alli,ude
So is the (angent of h:.df th e diffc:renee bc:tween rhe
~i
Ilaoee of Ihe fun from the poie aod eomplemeot of tb,.
altitude
T o
the tangent
of a
fou
rth areh
whieh founh areh added 'o half the eomplement of Ihe
la,i,u:!e \ViII gi ve a 6f.h ",eh, and this 61,h areh lelfeoed
by
,he
eomplemeot of Ihe lati,ud" will give o
fix.,h
areb.
Then
make it
As
th e radius
is
to
the tangent of the alrilude
fo is the tangcnt
oC
(he
GXlh ..
arch
to.the
eo fine
of the (unJs
azimuth
which is to be
counted
from
the
fout h
or
nonh, to the:
eiR
or weet .
aeeording
as
the
fUD
is
fi,uated
with rcfpeél: to the
pl.ceof obfervatioo.
If Ihe latilude of ,he place aod decli .",ion of ,be fUTl
be both north or both romh , then [he
declioation
taken
fro01
90~
will give the fuo 's d:ll.oee from the pole; but
if the latitude aod dec1ioa.toion be on contrary lides of
the equator,
then
the
declinadon added
te
90°
",iJl
mvc
the fuo ', di!lanee from Ibe nearell pole to ,he pl..
e~
of
obrervation.
EXA""PLE.
In the Jatitllde of 51'-' 32 oorrh, rhe fun
having 19() 39'
north
declination,
his
altilude
was fOllod
by obfer. a,ion tO be 38° 'IS: Reguired Ihe azimulh.
By Ihe firll of Ihe foregoi ng aoologie., i, will
be
As the tangeot
of
i
the eom·
?
'
plemement of the Jatitude
5
19°. 14 9 '54
2
69
from lhe pole and eomple
61 ..
01 10.25
6 55
;, 'o the ' ageot of
t
the fum
~
of the diU . nee of ,he fuo
meot of (he ahitude -
fo i, the tangeol of
h.lftheir
differeoee
__
9 .. 19 7. 21 199
to ,he tango of a 41h areh
40 .. 20 9 ' 92S8,-
\Vhieh fou rth areh 40° 20', .dded 10 19° 14' half tbe
eomplemeol of ,he latitud., give o fiflh areh 59
0
34';
and this fif,h areh I.lfcoed by '3So
2S',
,he eomplemen,
of the lalilude, gives the Iixtb ",eh
H
o
06'; ,beñ for
the