GEOGRAPHY. 
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that arc still extant, also shew what care and 
j pains they had been at in making surveys in 
ail the different provinces of their empire, 
and Pliny has filled the 3d, 4lh, and 5th 
i books of his Natural History with the geo- 
graphical distances that were thus measured. 
Other maps are also still preserved, known 
bv the name of the Pentigerian Tables, pub- 
I lished by Welser and Bertius, which give a 
I good specimen of what Vegetius calls the 
I itinera picta, for the better direction of their 
I armies in their march. 
The Roman empire had been enlarged to 
I its greatest extent, and all its provinces well 
I known and surveyed, when Ptolemy, about 
J 150 years after Christ, composed his system 
of geography. The chief materials he em- 
I ployed in composing this work, were the pro- 
j portions of the gnomon to its shadow, taken 
by different astronomers at the times of the 
' equinoxes and solstices ; calculations founded 
on Urn length of the longest days; the mea- 
sured or computed distances of the principal 
j roads contained in their surveys and itinera- 
' ries ; and the various reports of travellers and 
navigators. All these were compared toge- 
, ther, and digested into one uniform body or 
; system ; and afterwards were translated by 
him into a new mathematical language, ex- 
! pressing the dilferent degrees ot latitude and 
longitude, after the invention of Hipparchus, 
j which had been neglected tor 250 years. 
Ptolemy’s system of geography, notwith- 
I standing it was still very imperfect, continued 
in vogue till the last three or four centuries, 
| within which time the great improvements in 
! astronomy, the many discoveries of new' 
countries by voyagers, and the progress of 
; war and arms, have contributed to bring it to 
a very considerable degree of perfection. 
Principles of geography. 
The fundamental principles of geography 
are, the spherical figure of the earth, its ro- 
tation on its axis, its revolution round the sun, 
and the position of the axis or line round 
j which it revolves with regard to the celestial 
luminaries. That the eartli and sea taken 
together constitute one vast sphere is demon- 
L strable by the following arguments : 1. To 
P people at sea the land disappears, though 
! near enough to be visible was it not for the in- 
1 tervening convexity of the water. 2. The 
I higher the eye is placed, the more extensive 
is the prospect; whence it is common for 
sailors to climb up to the tops of the masts to 
| discover land or ships at a distance. But this 
j would give them no advantage, was it not 
I for the convexity of the earth ; for upon an 
infinitely extended plane objects would be 
vi.-ible at the same distance whether the eye 
was high or low, nor would any of them va- 
nish till the angle under which they appeared 
I became too small to be perceived. 3. To 
I people on shore, the mast of a ship at sea 
appears before the hull; but was the eartli an 
; infinite plane, not the highest objects, but the 
largest, would be longest visible ; and the mast 
©fa ship would disappear, by the smallness of 
its angle, long before the hull did so. 4. The 
i convexity of any piece of still water of a mile 
or two inextent may be perceived by the eye. 
I A little boat, for instance, may be perceived 
j by a man who is any height above the water; 
j but if he stoops down or lays his eye near the 
f surface, he will find that the fluid appears to 
i rise and intercept the view of tue boat en- 
Vol. i. ' 
tirely. 5. The earth has been often sailed 
round, as by Magellan, Drake, Dampier, An- 
son, Cook, and many other navigators, which 
demonstrates that the surface of the ocean is 
spherical ; and that the land is very little dif- 
ferent may easily be proved from the small 
elevation of any part of it above the surface 
of the water. The mouths of rivers which 
run 1000 miles are not more than one mile 
below their sources, and the highest moun- 
tains are not quite four miles of perpendicular 
height ; so that, though some parts of the 
land are elevated into hills, and others de- 
pressed into valleys, the whole may still be 
accounted spherical, 6. An undeniable, and 
indeed ocular, demonstration of the spherical 
figure of the earth is taken from the round 
figure of its shadow which falls upon the 
moon in time of eclipses. As various sides 
of the earth are turned towards the sun dur- 
ing the time of different phenomena of this 
kind, and the shadow in all cases appears 
circular, it is impossible to suppose the figure 
of the earth to be any other than spherical. 
The inequalities of its surface have no effect 
upon the earth's shadow on the moon ; for as 
the diameter of the terraqueous globe is very 
little less than 8000 miles, and the height of 
the highest mountains on earth not quite four, 
we cannot account the latter any more than 
the 2000th part of the former, so that the 
mountains bear no more proportion to the 
bulk of the earth, than grains of dust bear to 
that of a common globe. 
A great many ot the terrestrial phenomena 
depend upon the globular figure of the earth, 
and the position of its axis with regard to the 
sun, particularly the rising and setting of the 
celestial luminaries, the length of the days 
and nights, &c. 
Though the sun rises and sets all over the 
world, the circumstances of his doing so are 
very different in different countries. The 
most remarkable of these circumstances is 
the duration of the light, not only of the sun 
himself, but of the twilight before he rises 
and after he sets. In the equatorial regions, 
for instance, darkness comes on very soon 
after sunset; because the convexity of the 
earth comes quickly in between the eye of the 
observer and the luminary, the motion of the 
eartli being much more rapid there than any 
where else. In our climate the twilight al- 
ways continues two hours, or thereabouts, 
and during the summer season it continues in 
a considerable degree during the whole night. 
In countries farther to the northward or 
southward, the twilight becomes brighter and 
brighter as we approach the poles, until at 
last the sun does not appear to touch the ho- 
rizon, but goes in a Circ le at some distance 
above it for many days successively. In like 
manner, during the winter, the same lumi- 
nary sinks lower and lower, until at last he 
does not appear at all ; and there is only a 
dim twinkling of twilight for an hour or two 
in the middle of the day. By reason of the 
refraction of the atmosphere, however, the 
time of darkness, even in the most inhospi- 
table climates, is always less than that of 
light; and so remarkable is the effect of this 
property, that in the year 1682, when some 
Dutch navigators wintered in Nova Zembla, 
the sun was visible to them 16 days before he 
could have been seen above the horizon, had 
there been no atmosphere. T he reason of aH 
this is, that in the northern and southern re- 
5,N 
gions only a small part of the convexity of 
the globe is interposed betwixt us and the sun 
for many clays, and in the high latitudes none 
at all. In the warmer climates the suu has 
often a beautiful appearance at rising and 
setting, from the retraction of his light through 
the vapours which are copiously raised in 
those 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 ones of the poles. In the high northern 
latitudes, thunder and lightning are unknown, 
or but seldom heard of; but the more terrible 
phenomena of earthquakes, volcanoes, &c. 
are by no means unfrequent. These, how- 
ever, •. eevn only to affect islands and the ma- 
ritime parts of the continent. 
Notwithstanding the seeming inequality in 
the distribution of light and darkne s, how- 
ever, it is certain, that throughout the whole 
world there is nearly an equal proportion ot 
light diffused on every part, abstracting from 
what is absorbed by clouds, vapours, and the 
atmosphere itself. The equatorial regions 
have indeed the most intense light during the 
day, but the nights are long and dark ; while, 
on the other hand, in the northerly and 
southerly parts, though the sun shines less 
powerfully, yet the length of time that he 
appears above the horizon, with the greater 
duration of the twilight, compensates tor the 
seeming deficiency. 
Was the earth a perfect plane, the sm* 
would appear to be vertical in ever) part of 
it; for, in comparison with the immense mag- 
nitude of that luminary, the diameter of lifts 
globe itself is but very small; and as the sun, 
was lie near to us, would do much more than 
cover the whole earth, so, though he w as re- 
moved to any distance, the whole diameter 
of the latter would make no difference in the 
apparent angle of altitude. By means of the 
globular figure of the earth also, along with 
the great disparity between the diameters of 
the two bodies, some advantage is given to 
the day over the night; for thus the sun, being 
immensely the larger of the two, shines upon 
more than one-half .of the earth; whence the 
unenlightened part has a shorter way to go 
before it again receives the benefit of his ravs. 
This difference is greater in the inferior pla- 
nets Venus and Mercury than in the earth. 
To the globular figure of the earth likewise 
is owing the long moonlight which the inha- 
bitants of the polar regions enjoy. The same 
thing likew ise occasions the appearance and 
disappearance of certain stars at some sea- 
sons of the year in some countries; for, was 
the earth flat, they would all be visible in 
every part of the world at the same time. 
Hence most probably has arisen the opinion 
of the influence of certain stars upon the wea- 
ther and other sublunary matters. Jn short, 
on the globular figure of the earth depends 
the whole present appearance of nature 
around us; and was the shape of the plain t 
we inhabit to be altered to any other, besides 
the real differences which would of conse- 
quence take place, the apparent ones would 
be so great, tiiat we cannot form any idea of 
the face w hich nature ■ on Id then present t o 
us. See Astro xc.w 
In geography the circles which the sun ap- 
parently describes in the heavens are suppos- 
ed to be extended as far as the earth, and 
