834 
GEOGRAPHY. 
marked on Us surface; and in like manner 
we may imagine as many circles as we please 
to be described on the earth, and their planes 
to be extended to the celestial sphere, till 
they mark concentric ones on the heavens. 
The most remarkable of those supposed by- 
geographers to be described in this manner 
are the following. 
1. The horizon. This is properly a double 
circle, one of the horizons being called the 
sensible, and the other the rational. The 
former comprehends only that space which 
we can see around us upon any part of the 
earth, and which is very different according 
to the difference of our situation. The other, 
called the rational, is a circle parallel to the 
former, and passing through the centre of the 
earth, supposed to be continued as far as the 
celestial sphere itself. To the eyes of spec- 
tators there is always a vast difference be- 
tween the sensible and rational horizons; 
but from the immense disparity betwixt 
the size of the earth and celestial sphere, 
planes of both circles may be considered as 
coincident. Hence in geography, when the 
horizon, or plane of the horizon, is spoken 
of, the rational is always understood when 
nothing is said to the corttrary. In conse- 
quence of the round figure of the earth, every 
part has a different horizon. The poles of the 
horizon, that is, the points directly above 
the head, and opposite to the feet o't'the ob- 
server, are called the zenith and nadir. 
2. A great circle described upon the 
sphere of the heaven, arid passing through 
the two vertical points, is called a vertical 
circle, or an azimuth; and of these we may 
suppose as many as we please all round the 
horizon. In geography every circle obtains 
the epithet of great whose plane passes 
through the centre of the earth; in other 
cases they are called lesser circles.. The al- 
titudes of the heavenly bodies are measured 
by an arch of the azimuth or vertical circle 
intercepted between the horizon and the body 
itself. The most accurate method of taking 
them, with regard to the sun and moon, is 
for two persons to make their observations 
at the same time ; one of them to observe 
the altitude of the upper limb, the other of 
the lower limb of the luminary; the mean 
betwixt these two giving the true height of 
the centre. The same thing may also be 
done accurately by one observer, having the 
apparent diameter of the luminary given. 
For, having found the height of the upper 
edge of the limb by the quadrant, take from 
it half his diameter, the remainder is the 
height of his centre; or having found the alti- 
tude of his lower edge, add to it half the dia- 
meter, and the sum is the height of the centre 
as before. When the observations are made 
with a large instrument, it will be convenient 
to use a sextant, or sixth part of a circle, ra- 
ther than a quadrant, as being less unwieldy. 
3. Ahnucantars are circles supposed to be 
drawn upon the sphere parallel to the hori- 
zon, and grow less and less as they approach 
the vertical points, where they entirely va- 
nish. The apparent distances betwixt any two 
celestial bodies are measured by supposing 
arches of great circles drawn through them, 
and then finding how many degrees, minutes, 
Ac. of these circles are intercepted between 
them. 
4. Sometimes the visible horizon is cqiisi- 
d- red only with regard to the objects which 
are upon the earth itself, in which case we 
may define it to be a lesser circle on the sur- 
face of the earth, comprehending all such ob- 
jects as are at once visible to us; and the 
higher the eye, the more is the visible hori- 
zon extended. It is most accurately observ- 
ed, however, on the sea, on account of the 
absence of those inequalities which at land 
render the circle irregular; and for this rea- 
son it is called sometimes the horizon of the 
sea, and may be observed by looking through 
the sights of a quadrant at the most distant 
part of the sea then visible. 
5. The equator is a great circle upon the 
earth, every part of which is equally distant 
from the poles or extremities of the imagi- 
nary line on which the earth revolves. In 
the sea-language it is usually called the line, 
and when people sail over it they are said to 
cross the line. 
b. The meridian of any place is a great 
circle on the earth drawn through that place 
and both poles of the earth. It cuts tiie ho- 
rizon at right angles, marking upon it the true 
north and south point; dividing also the 
globe into two hemispheres, called the eastern 
and western from their relative situation to 
that place and to one another. The poles 
divide the meiidians into two semicircles, one 
of which is drawn through the place to which 
the meridian belongs, the other through that 
point of the earth which is opposite to the 
place. By the meridian of a place, geogra- 
phers and astronomers often mean that semi- 
circle which passes through the place, and 
which may therefore be called the geogra- 
phical meridian. All places lying under 
this semicircle are said to have the same me- 
ridian; the semicircle opposite to this is called 
the opposite meridian. The meridians are 
thus immoveably fixed to the earth as much 
as the places themselves on its surface, and 
are carried along with it in its diurnal rota- 
tion. When the geographical meridian of 
any place is, by the rotation of the earth, 
brought to point at the sun, it is noon or 
mid-day at that place; in which case, was 
the plane of the circle extended, it would pass 
through the middle of the luminary’s disk. 
Supposing the plane of the meridians to be 
extended to the sphere of the fixed stars, in 
that case, when by the rotation of the earth 
the meridian comes to any point in the hea- 
vens, then, from the apparent motion of the 
heavens, that point is said to come to the 
meridian. The rotation of the earth is from 
west to east; whence the celestial bodies ap- 
pear to move the contrary way. East and 
west, however, are terms merely relative, 
since a place may be west from one part of 
the earth, and east from another; but the 
true east and west points from any place are 
those where its horizon cuts the equator. 
7. All places lying under the same meri- 
dian are said to have the same longitude, and 
those which lie under different meridians to 
have different longitudes ; the difference of 
longitude being reckoned eastward or west- 
ward on the equator. Thus, if the meridian 
of any place cuts the equator in a point 15 
degrees distant from one another, we say 
there is a difference of 15° longitude betwixt 
these two places. Geographers usually fix 
upon the meridian of some remarkable place 
for the first meridian, and reckon the longi- 
tude of all others by the distance of their me. 
ridians from that which they have determined 
upon as the first; measuring sometimes east- 
ward on the equator all round the globe, or 
sometimes only one-half east and the other 
west; according to which last measurement 
no place can have more than 180° longitude 
either east or west. By the antient Greek 
geographers the first meridian was placed fn 
Hera or Junonia, one of the Fortunate islands, 
as thyy were then called, which is supposed 
to be the present island of Tenerifi’e, one of 
the Canaries. These islands, being the most 
westerly part of the earth then known, were , 
on that account made the seat of the first me- 
ridian, the longitude of all other places bSing 
counted eastward from them. Among mo- 
dern geographers indeed, it is now become 
customary for each to-makejthe first meridian, 
pass through the capital of his -own country; 
a practice, however, whicli is certainly im- 
proper, as it is thus impossible for the geo- ■] 
graphers of one nation to understand the , 
maps of another without a troublesome calcu- 
lation, which answers no purpose. By the 
British geographers the royal observatory at 
Greenwich is accounted the place of the "first 
meridian. 
8. If we suppose 17 great circles, one of 
which is the meridian to a given place, to in- 
tersect each other at the poles of the earth, 
and divide the equator into 24 equal parts, 
these are the hour-circles of that place. These 
are by the poles divided into 24 semicircles, 
corresponding to the 24 hours of the day 
and night. The distance betwixt each two i 
of these semicircles is 15°, being the 24th part 
of 3ti0; and by the rotation of the earth each 
succeeding semicircle points at the sun one ] 
hour after the preceding: so that in 24 hours ] 
all the semicircles point successively at the < 
sun. Hence it appears, that such as have ■ 
their meridian 15° east .from any other have 
likewise noon one hour sooner, and the con- 
trary; and in like manner every other hour of ; 
the natural day is an hour sooner at the one i 
place than at the other. Hence, from any 
instantaneous appearance in the heavens ob- 
served at two distant places, the difference of 
longitude may be found, if the hour of the • 
day is known at each place. Thus the be- 
ginning of an eclipse of the moon, when thenj 
luminary first touches the shadow of the 
earth, is an instantaneous appearance, as also 
the end of an eclipse of this kind, when the 
moon leaves the shadow of the earth visible 
to all the inhabitants on that side of the globe. 
If therefore we find, that at any place an 
eclipse of the moon begins an hour sooner 
than at another, we conclude that there is a 
difference of 15° of longitude between the two 
places. Hence also was a man to travel or sail 
round the earth from west to east, he would 
reckon one day move to have passed than 
they do who stay at the place whence he set 
out; so that their Monday would be his dues- ; 
day, & c. On the other hand, if he sails west- 
ward, he will reckon a day less, or be one : 
day in the week later, than those lie leaves 
behind. 
9. The equator divides the earth into two 
hemispheres, called the northern and south- 
ern ; all places lying under the equator are 
safd to have no latitude; and all others to 
have north or south latitude according to 
their, situation with respect to the equator. 
The latitude itself is the distance from the 
equator measured upon the meridian, in de- ] 
grees, minutes, and seconds. The eomple- 
