GEOGRAPHY. 
to geography to take notice of the difference betwixt 
the fame phenomena in different parts of the earth. 
Thus, though the fun rifes and fets all over the world, 
the circumftanccs of his doing fo are very different in 
different countries. The mod remarkable of thefe cir- 
cumftanccs is the duration of the light not only of the 
fun himfelf, but of the twilight before he rifes, and af¬ 
ter he fets. In the equatorial regions, for example, 
darknefs comes on very fbon after lunfet ; becaufe the 
convexity of the earth comes quickly in between the 
eye of the obferverand the luminary, the motion of the 
earth being much more rapid there than in any otiier 
part. In our climate, the twilight always continues two 
hours, or thereabouts ; and during the fummer-feafon 
it continues in a confiderable degree the whole night. 
In countries farther to the northward or fouthvvard, the 
twilight becomes brighter and ftronger as we approach 
the poles, until at laft the fun does not appear to touch 
the horizon, but goes in a circle at fome diftance above 
it for many days fuccelfively. In like manner, during 
the winter, the fame luminary finks lower and lower, 
until at laft he docs not appear at all ; and there is only 
a dim twinkling of twilight for an hour or two in the 
middle of what is called the day. By reafon of the re- 
fradlion of the atmofphere, ho'.vever, the time of dark¬ 
nefs, even in the moft inhofpitable regions, is alvvays 
lefs than that of light; and io remarkable is the eft'e6t 
of this property, that ia 1682, when fome Dutch navi¬ 
gators wintered in Nova Zembla, the lun was vifible to 
them fixteen days before he could have been feen above 
the horizon had there been no atmofphere, or had it 
not been endowed with any fuch power. The reafon of 
this is, that in the northern and fouthern Lititudes only 
a fmall part of the convexity of the globe is interpofed 
betwixt us and the fun for many days, and in the higher 
latitudes none at all.- In the warmer climates the fun 
has often a beautiful appearance at rifing and fetting, 
*>y reafon of the refradiion of his light through the va¬ 
pours which are copioufiy generated in thofe parts. In 
the colder regions, halos, parhelia, aurora borealis, and 
other meteors, are frequent; the two former owing to 
the great quantity of vapour continually flying from 
the warm regions of the equator to the colder parts of 
the poles. In the high northern latitudes, thunder and 
lightning are unknown, or but feldom heard of; but 
the more terrible phenomena of earthquakes, volcanos, 
See. are there not unfrequent. Thefe, however, feem 
chiefly to aftedl illands, and the maritime parts of the 
continent. See the articles Earthquake, and Vol¬ 
cano. 
Notwithftanding this feeming inequality in the gene¬ 
ral diftribution of light and darknefs, it is neverthelefs 
certain that throughout the whole world, there is nearly 
an equal proportion of light dift'ufed on every part, ab- 
Itradting from what isabforbed by clouds, vapours, and 
the atmofphere itfelf. The equatorial regions have in¬ 
deed the moftintenfc light during the day, but the nights 
are impenetrably dark ; while, on the other hand, in 
the northerly and foutherly parts, though the fun Ihines 
lefs powerfully, yet the length of time that he appears 
above the horizon, with the greater duration of the 
twilight, fully compenfates for every other deficiency. 
Were the earth a pcrfedf plane, the fun would appear 
to be vertical in every part of it : for in comparifon 
with the immenfe magnitude of that luminary, the dia¬ 
meter of this globe is comparatively fmall : and as the 
fun, were he near to us, would do much more than cover 
the whole earth ; fo, though he were removed to any 
diftance, the whole diameter of the latter would make 
no difi'efence in the apparent angle of his altitude. By 
means of the globular figure of the earth alfo, and the 
great difparity between the diameters of the two bodies, 
fome advantage is given to the day over the night: for 
thus the fun, being immenfely the larger of the two, 
fhines upon more than one half of the earth j whence 
'j 
the unenlightened part has a fhorter way to go before 
it again receives the benefit of his rays. In faff, on the' 
globular figure of the earth depends the whole pheno¬ 
mena of nature which furround us ; and were the fhape 
of the planet we inhabit to be altered to any other, be- 
fides the rea/ differences which would of confequence 
take place, the apparent ones would be fo incalculable, 
that we cannot form any idea of the face which nature 
would then put on. 
In geography, the circles and lines which the fun ap¬ 
parently deferibes in the heavens, are fuppofed to be 
extended to the earth, and marked on its furface ; and 
in like manner we may imagine as many circles and lines 
as we pleafe to be deferibed on the earth, and their 
planes to be extended to the celeftial fphere, till they 
mark concentric ones on the heavens. The connedlion 
of thefe circles and lines may be imagined to take place 
from the axis or poles of the earth, as well as by the 
interfections which they refpeflively make with each 
other at their angles of meeting, as reprefented by the 
armillary fphere, and the celeftial and terreftrial globes. 
It is from this connection of circles and lines that the 
fciences of aftronomy and geography are fo clofely in¬ 
terwoven ; to elucidate which we muft diftinClly illuf- 
trate them by name, and trace their utility and defigna- 
tion, both as they regard the heavens and the earth, and 
the tituation of places on the earth’s furface ; I'eferring, 
for correfpondent particulars, to the article Astrono¬ 
my, in our fecond volume. 
Principles and Definitions. —i. The axis of 
the earth is an imaginary line paffmg through its centre, 
about which its diurnal revolution is performed.—The 
poles of the earth are the extremities of its axis, north 
and fouth. 
2. The equator is the circumference of an imaginary 
great circle palling through the centre of the earth, per¬ 
pendicular to the axis, and at equal diftances from the 
poles.—If the axis of the earth be produced both ways, 
as far as the concave furface of the heavens, in which 
all the heavenly bodies appear to be placed, it is then 
called the axis of the heavens \ its extremities are called 
the poles of the heavens \ and the circumference produced 
by extending the plane of the equator to the fame con¬ 
cave lurtace, is called the equator in the heavens. —Circles 
drawn through the poles of the earth or heavens perpen¬ 
dicular to the plane of the equator, are called fecondarits 
of the equator. 
3. The horizon.^ properly fpeaking, is a double circle ; 
one of which is called the fenfible, the other the rational. 
■ —Hhe fenfble horizon is that imaginary circle, which, 
touching the lurface of the earth, feparates the vifible 
part of the heavensfrom the invifible. The rationalhorizoa 
is a circle parallel to the former, the plane of which 
pafles through the centre of the earth.—The poles of the 
horizon are two points, the one of which, over the head 
of the fpedlator, is called the zenith ; the other, which 
is under his feet, is called the nadir. —Circles drawn 
through the zenith and nadir of any place, cutting the 
horizon at right angles, are called vertical circles. 
4. A vertical circle paffing through the poles of the 
heavens, is a meridian, and is faid to be the meridian of 
any place through which it pafles.—The meridian of any 
place palling through the poles, and falling perpendicu¬ 
larly upon the horizon, cuts it in two oppolite cardinal 
points, called north and fouth. 
5. A meridian line, is the common interfeftion of the 
plane of the meridian and the plane of the horizon.' 
Hence any line which lies due north and fouth in an ho¬ 
rizontal plane, may be conlidered as part of the meridian 
line.—To draw a meridian line, perpendicular to an ho¬ 
rizontal plane, ereit a wire, or ftile, feven or eight inches 
long; and, as it is not eafy to determine precifely the ex¬ 
tremity of the fiiadow, it will be beft to make the ftile 
flat at top, and to drill a fmall hole through it, noting 
the lucid point projected "by it; mark, at feveral diffe¬ 
rent 
