DECLINATION, 
Decutnation, Circle of, is a great circle of the fphere 
pafling through the poles of the world; and on which the 
declination of a flar is meafured. See Decuinarion, in 
haus 
ECLINATION, Parallax of, is an arc of the circle of de- 
clinton, whereby the Saar ve of the altitude increafes or di- 
_ declination of af 
ECLINATION, Refradio aa the, an arc of the circle . 
declination, whereby the Beige of a ftar is increated © 
Giminifhed by means of the refraGtion. 
eee in a gener ral 
fixe nt. The 
mariner’s 
o 
ct 
= 
o 
< 
° 
s 
ity 
= 
mn 
“Oo 
fon] 
aa 
orizonta peel car de cepte bal bee 
the 
twenty degrees weit, the meaning is, thatif a ftraight line be 
drawn through the centre, or point of fufpenfion, of th 
through the true north point of the Boron: 
then the north half of the needle lies on the weft of that Line, 
and makes an angle of 20 eine with it. If the ssauas 
tion is faid to be 12° ea eaning is, that the north 
half of the magnetic needle ae on the eaft fide of fis Tine 
It is evident 
that when the north half of the needle lies on the eaft of the 
above-mentioned line, which, in fact, is the meridian of the 
place ; vi other half of the feels mult lie on the oppofite, 
{ weit ma of hat meridian line, making the fame 
makin 
ae it. ng t ear declination, the north 
rut ion, however, of t magnetic 
bee as rete a in ent aoe bane g very r rere nd unfit 
for navigation, it is moft probable that the compafs began 
the pte t! of pon until the time of Columbus's firft 
voyage to America, which took place in the year 1492 ; for 
it is exprefsly mentioned in his life, that the declination of 
the m c needle fr true meridian was ifco- 
at this 
Some writers have attributed the 
declination to. Sebatftian abe 
that the quantity of it varied in the fame place, fo as to be - 
ps gaan at different times. The difcovery of this variation 
€ magnetical declination is generally attributed to Mr. 
Gelibrand a ans r in- can college, who firft obferv- 
ed it aay ee ; yet Mr. Bond, in his « Longi- 
tude Found,” are 5 and 6 on that the declination was 
found to vary and to decreafe firft by Mr. John Mair, fe» 
condly by Mr. Gunter, thirdly by Mr. Gellibrand, and then 
a that time it has been 
ot only varies after, ac 
no theor rule ha 
ifh the means of foretelling. 
mathematicians, 
i accuracy the declination of Ge magnetic needle, for any. 
future period, at any particular place. 
That the declination is not owing to any imperfeGion in 
the conftruGion of the needles or compafles, or to the varie 
ous ftrength of the magnets, : proved by obferving = all 
os aagnetic needles of compaffes that are fituated n 
ne fpot, fhew exadily = fame eur hese provi ided d they 
are c freely fufpended, and out of the influence of each other, 
well as of any other (jeunes ‘tubes 
” The eee of the declination at tates times, 
and in different places. being one of the greateit impediments 
to the perfection of navigation; philofopkers, mathema- 
ticians, navigators, and mec cs, have endeavoured, with 
admirable affiduity, fir ; to aera inftrum ents o 
<< 
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i] 
= 
ae 
Q 
G& 
jot 
rx) 
™ 
ees 
a 
oO 
m 
w 
am 
@ 
ns a manner 
ifomicies 4 are ‘bet ie to h cecinaton of 
the magnetic needle, will be found defcribed under the article 
Co S. f is ‘to 
not be aed Ge the declination, pari a ia taaraet ae 
fea, fhould be very accurate. t ruments, 
ith the 
and the molt expert obfervers, the Beclaadon aa found _ 
may at wane - ad piace upon within eight or ten mie 
nutes; but with the common lefs per 
t hecrlinaryelal ofnautical ober the refult of the oa: 
ations hardly ever differs from the truth lefs than half a de- 
gree 5 an h 
ferving the exaG@ magnetieal azimuth of the fun, and from 
the difficulty of i ite ae true and the magnetical azimuths 
at the very fame moment.. Another fource of error arifes 
from the influence of iron upon the compafs, which at fea is 
7, ‘impoflible to be avoided. Ca ptain eae who 
articular, 
the 
ae 5 (Phil. Trant. for 1805, ; “6 Whilft furveying aig 
fouth coal of New Hollandin 1801 and 1802, I obferved a 
confiderable difference in the direGion — the magnetic 
ne when there was no other apparent caufe for it than, pot 
ofthe fhip’s head, being in a different dire€tion. This o 
fioned much perplexity in laying down the bearings, and in . 
ng : 
3 
e 
ng a exertions of the gre eat 
nor : 
ar the: 
ry 
