CJ E- O G R A P H Y. 
rent times before noon, thefe lucid points, and through 
them draw concentric circles about the middle point of 
the wire’s ftation ; oblerve. in tlie afternoon when the 
lucid points again touch thefe circles; and find the mid¬ 
dle point of each arc between the points already taken: 
a li.ne, drawn through thefe middle points and the com¬ 
mon centre, will be the meridian line ; for, fince at equal 
didances from noon the fun is at the fame height, or in 
verticals equally didant from the meridian, the circle 
drawn through the zenith at equal didances from thefe 
verticals, is the meridian. This diould be done about 
the fuminer foldice, between the hours of 9 and 11 in 
the morning, and i and 3 in the afternoon. 
To obferve. the tranfit of any heavenly body over the 
plane of the meridian; place, in this plane, a telefcope, 
having two crofs hairs before its objedt-glafs, one ver¬ 
tical, the other horizontal, and obferve when the ver¬ 
tical hair paffcs through the centre of the heavenly bo¬ 
dy : or, hanging two plumb-lines exadbly over the me¬ 
ridiem line, place your eye clofe to one of the threads in 
fuch manner, as that it diall cover the other thread, and 
obferve when the body tranfits, or is behind the threads. 
—See the article Astronomy, vol. ii. p.456. 
6. The altitude or deprejjion of any heavenly body above 
or below the iiorizon, is the arc of a vertical circle in¬ 
tercepted between the body and the horizon, or the angle 
at the centre meafured by that arc.—The altitude of any 
heavenly body is found by the help of a quadrant, thus : 
bring the quadrant into fuch a fitiiation that the liar may 
be feen through the fights; then the angle, contained 
between the firing of the plummet and the fide of the 
quadrant on which the fights are not placed, is the alti¬ 
tude of the flar.—The prime vertical, is that which crofTes 
the meridian at right angles in the zenitii and nadir, cut¬ 
ting theihorizon in the cardinal points eafi and wejl. 
7. The azimuth, of a heavenly body, is the arc of the 
horizon intercepted between the meridian and the ver¬ 
tical circle palling through that body ; it is eaftern or 
weflern, as the body is eafl or well of the meridian. 
Z. 'Yh.t amplitude of a heavenly body at its riling, is 
the arc of the horizon intercepted between the point 
where the body riles, and the eall; its amplitude at fet- 
ting, is the arc of the horizon intercepted between the 
point where the body fets, and the weft : it is northern, 
or fouthern, as the body rifes, or fets, to the north or 
fouth of eaft or weft.—If a heavenly body riles, or fets, 
when the fun rifes, it is faid to rife or fet cofmically, if 
it rifes, or fets, vvlien the fun fets, it is faid to rife or let 
eichronically ; it is faid to fet or rife heliacally, when it ap¬ 
proaches I'o near the fun as to become invifible, or re¬ 
cedes-fo far from him as to become vifible. 
9. The latitude of a-place upon the furface of the 
earth, ns its diftance from the earth's equator- it is mea¬ 
fured by the arc of the geographical meridian of the 
place intercepted between the place and the equator ; 
latitude is either northern or fouthern.— Parallels of lati¬ 
tude, are circles on the furface of the earth drawn parallel 
to the equator; and a degree in the equator is to a de¬ 
gree in any parallel of latitude, as radius to the cofine 
of latitude. 
10. The longitude of a place, is the diftance between 
the meridian of that place, and the meridian of foine 
other place, taken at pleafure, and called the firf meri¬ 
dian ; it is meafured by the arc in the equator intercepted 
betw een thefe two meridians. Longitude is either eaft¬ 
ern or weliern, and is meafured 180 degrees each way. 
Or, as illuftrated by profeli'or Vince, let PAyiQJn the 
annexed diagram, reprefent the earth, PC/; its .ixis, P 
' the north pole, p the fouth pole ; and let j-sE Qjk. be a 
circle puffing through the centre C, perpendicular to the 
axis P/j, then tkat circle is called the equator. 1 his cir¬ 
cle divides the earth into two equal parts, APCLfalled 
the northern-, and A/iQ^c.illed the fouthern, hemifphere. 
Let K,.G, I, be the liuiations of three places upon the 
i'urface, and through them draw the great circles P K p, 
Vol. VIII. No. 508. 
34, 
PGp, Plpt caWeA meridians-, interfeiling the equator ia 
n, a, m, refpedlively. Now as every circle is fuppofed to 
be divided into 360 degrees, the diftance from the pole to 
the equator muft be 90 degrees. The latitude of a place, 
P 
is an arc of its meridian intercepted between the place and the 
equator, meafured in degrees. Hence, the latitude of K 
is meafured by the degrees of the arc ;i K ; and the 
latitudes of G and I are meafured by the degrees of the 
arcs a G, m I, refpeftively, and thefe are called north 
latitudes, the places lying in the northern h.emifphere; 
and the latitude of W is meafured by the degrees of tire 
arc aW, and is called fouth latitude, the place lying in 
the fouthern hemifphere. Let the fmall circle cGvde 
be parallel to the equator, then this circle is called n pa¬ 
rallel of latitude, becaufe every point of it has the fame 
latitude, all the arcs mv, a G, intercepted between it and 
the equator, being equal, on account of the circles being 
parallel. The longitude of a place is meafured upon the 
equator, and is the arc intercepted between the point 
from which you begin to reckon, and the point where 
the meridian of the place cuts, the equator, eftimated in 
degrees. Hence, all places in the fame meridian have 
the fame longitude ; the longitude of G is the fame as 
the longitude of W. Geographers of different coun¬ 
tries begin to reckon from difierent points, each begin¬ 
ning from that point where the meridian of its capital 
city cuts the equator; and if the city have a national 
oblervato’ry in or very near to it, that meridian is taken 
which paffes tlirough the obfervatory. This is called 
the firji meridian. We therefore define the longitude of a 
place to be an arc of the equator intercepted between the firf me¬ 
ridian and the meridian pqjjing through the place. In England, 
therefore, we begin from that meridian which paffes 
through the obfervatory at Greenwich; in France, they 
begir.-from that meridian which paffes through the ob¬ 
fervatory at Palis. Let therefore G reprefent the royal 
obfervatory at Greenwich, and a is the point of the 
equator from which we begin to reckon the longitude. 
Flence, tlie degrees of the arc a m is the longitude of the 
place I; and the longitude of the place K is meafured 
by the degrees of th.e arc an. Now vhe direction am 
from a is eaft, and the direition an is weft ; it is there¬ 
fore uf'ual to call am, eaft longitude, and an, weft longi¬ 
tude, each till you come to the point oppoiite to a, or 
till the longitude each way becomes 180 degrees. But 
fometimes the longitude is reckoned all the w-ay round 
in the fame direction; that is, the point w, wherever it 
may be, is called eaft longitude from a. 
If the latitude agd longitude of a place be given, 
the place itl’elf may be found; for if the longitude be 
known, fet off the arc am equal to it, if it be eaft longi¬ 
tude, and draw the meridian Vmp-, then if the latitude 
4 U be 
