S44 ' G E O G R 
ridian pafling tlirough Rhodes and Alexandria down to 
Syene and Meioe. And as the progrel's he thus made 
tended naturally to enlarge his ideas upon this I'cience, 
he attempted a (lill more arduous taflt, which was to de¬ 
termine the circumference of the globe by an aftiial 
meafurement of a fegment of one of its great circles, 
making his computation upon the whole by uniting 
certain accurate obfervations made in the heavens with 
a correfponding di(lance carefully furveyed and taken 
upon a meridian of the earth. 
The fegment of the meridian which he fixed upon for 
this purpofe, was that between Alexandria and Syene, 
the difiance of which v/as meafured, and found to be 
5000 fiadia, and the angle of the (hadow upon the 
icaphia or fun-dial which was obferved at Alexandria, 
,w'as equal to the jorh part of the circle; for at Syene 
there W'as no fiiadow from the gnomon- at the mid-day of 
the fummer folftice; and that this might be more accu- 
rately taken, they cltig a deep well, which being per¬ 
pendicular, was completely illuminated at the bottom 
W'hen the fun was vertical. Though this even wnisnot 
fully fu.dicient to give the exadt line of the tropic, be- 
caufe the fun was fotind to be vertical, or to call no 
fiiadow at all, for a circular fpace of 300 fiadia, becaufe 
the lull’s diameter, being equal to 32 minutes, would 
therefore appear perpendicular at the fame infiant of 
time to an extent of ground correfponding to that num¬ 
ber of minutes ; and therefore Ricciolus concludes, that 
this vertical obfervalion miifi have extended 150 fiadia 
on each fide of Syene. 
The fubfiance of this account is taken from Cleo- 
medes, who feciiis to have extradled it literally from 
Kratofihenes’s original work, iiititled MsTpijtrsi;, and it 
is piihliflied as Inch at the end of the Oxford edition of 
A-ratus, in 1672, though under the title of Merfov 
Trspiipefua;. By this account Eratofihenes made the cir¬ 
cumference of tr.e earth amount only to 250,000 fiadia, 
whereas a cloud of original authors have uniformly given 
the munbers to be 252,000. And to reconcile thefe two 
calculations, IDr. Murdoch has i.igenioufiy ftippofed, 
that inftead of 7° 12', the diflerence of latitude was 7® 
S'-i, which was the f of the circumference, which 
would bring the calcuiation to 250,000 fiadia, and tltat 
Cleoinedes negleiled the finall fractional part of the 
denominator; but that the principal mifiake was in 
ineafuring the difiance, and finding it to be 5000 fiadia. 
Indeed nothing is more common than to find a conftifion 
of numbers in the difiances given us by ancient aiuhors; 
for though thefe 5000 fiadia are mentioned as the difiance 
betwixt Alexandria and Syene by fo many authors, yet 
live know that Marimis and Ptolemy did nol allow above 
3600 fiadia to that difiance, as the 7° 12' amounted ex¬ 
actly to that number upon the proportion of 500, fiadia 
to a degree, which Ptolemy tells us, was agreeable to 
nienluratioiis that were allowed and acknowledged. 
The fame niuhber of 5000 fiadia is faid to have been the 
difiance lupjiofed by Pofidonius betwixt Rhddes and 
Alexandria, where he had concluded that the fegment 
of the meridian was the 4Sth part of a great circle from 
an obfervation of the ftar Canopus ; whereas Strabo tells 
us, that the feamcn only allowed it to be a difiance of 
4000 fiadia, and that Eratofihenes, by his gnomonical 
obfervations, concluded it to be only 3750. In like 
manner Pliny tells us, that it was 5000 fiadia betwixt 
Syene and Meroe; but m another pallage, after men¬ 
tioning the various meafures of Eratofihenes, Artemido- 
rus, aVid Sebofu.,-vlio dill'ered from each other, he adds, 
that the difputes upon that head had been lately deter, 
mined by furveyors lent thither by Nero, who found it 
to meafure 4(62 miles; tiiough from the intermediate 
difiance there fpecified, it appears that the colledted 
numbers give 874, and by multiplying each of thefe by 
8 to increafe them to fiadia, will give in the firlt 6896 
fiadia, and in the fecond 6992, both of which difier very 
jaaterially from 5000 fiadia. 
A P II Y. 
The inveftigation of this problem of the circumference 
of the earth, was efientially neceiTary for determining 
the radical principles of all maps, and therefore the moft 
eminent of the ancient afironomical geographers made 
repeated endeavourkto obtain an accuracy in this cal¬ 
culation. Eraiofiher.es, by making the circumference 
to be 252,000 fiadia, allowed therefore 700 fiadia to a 
degree; which by the redudtion of 8 fiadia to a Roman 
mile of 5000 feet, amounted to 87-^ Roman miles to each 
degree. Eipparchus added 25,000 fiadia to this meafure 
of Eratofihenes, according to Pliny, lib. ii. cap. 108, 
though the obfc rvation is not mentioned from which 
this concltifion is drawn, which it were to be wifiied 
Pliny liad done, as it increafes the error of Eratofihenes. 
This addition however makes the circumference to con- 
fifi of 277,000 fiadia, wliich was an allowance of 769 
fiadia, or 96 Roman miles, to each degree. 
Pofiidonius, in like manner, coinpqted the circum¬ 
ference ot the eartli to be 240,000 fiadia, by multiplying 
5000 {tl'.e fuppofed difiance betwixt Rhodes and Alex¬ 
andria) by 48, the fegment of the meridian according to 
his obfervation betwixt thefe two places; but Cleoinenes, 
when he mentions this conclufion, adds, “ if the dif- 
tance is 5000, but if not, in proportion to tlie difiance.” 
And as Eratofihenes had made tlte diftance to be only 
3750 fiadia, and Pofiidonius refiing Ills conclufion upon 
the deduction from his afironomical obfervation of the 
arch of the meridian, it was natural for him to infer, 
that if he admitted the difiance given by Eratofihenes 
to be true, being taken upwards of 170 years before his 
time, ujion tliat hypothefis, the circumference of the 
earth would be only 180,000 fiadia, as 3750 multiplied 
by 48 will produce that number; and indeed Strabo tells 
us, that this very calculation was approved of by Pofii¬ 
donius. It is for this reafon that he is quoted as having- 
had two opinions upon tiie quantity of the circumference 
of the earth, and that he was therefore the firlt geogra¬ 
pher who advanced the opi.fion of allowing only 500 
fiadia to a degree, wliich was afterwards adopted by 
Marinus and Ptolemy. By the fiifi hypothefis, there¬ 
fore, there would be 666 Itadia, or 83 Roman miles to a 
degree ; and by the fecond, 500 (Rdia, or 62-1- Roman 
miles. 
The difeoveries and improvements of Eratofihenes, 
as a geograplier, naturally led to an explanation of thefe 
various menfurations and calculations of the circumfe¬ 
rence of the earth, which indeed materially affedted the 
dimenfions of all the ancient maps. It is proper, how¬ 
ever, to obferve, that his map appears to have contained 
little more than the fiates of Greece, and the dominions 
of the fuccelfors of Alexander, digefied from thofe fur- 
veys already mencioiied. He had J'een indeed, and has 
quoted, the voyages of Pytiieas into the great Atlantic 
Ocean, wliicn gave him fome faint idea of tiie wefiern 
parts of Europe; but withal fo . imperfetil, that they 
could not be realifed into the outline of a chart. Strabo 
tells us, that he was extremely ignorant of Spain, Gaul, 
Germany, and Britain, as weli as of the Geti and Baf- 
tarni ; he was equally iguoraut of Italy, the coafts of 
the Adriatic, of Pontus, and of all the countries to¬ 
wards tne north. And he mentions in another paifage, 
that Eratofiiienes had made tlie difiance from Epidam- 
nus, or Dyrrachium, on the Adriatic, to the bay of 
Thernuc on the YEgean Sea, quite acrofs Epirus, to 
be only 900 fiadia, when it was really above 2000 fiadia; 
and in another inliance he had enlarged the difiance from 
Carthage to Ale.xandria to be 15.,000 fiadia, whereas it 
amounted to no more than 9000 fiadja. Major Rennell, 
in his “ Geographical Syfiein of Herodotus,” afeertains 
the Grecian fi.idium, or that employed by Herodotus 
in his fiatemeats of uifiance, to have been the part 
of a geographical aegfee ; that of Xenophon, y|o ; that 
of Eratofihenes, yjy ; that of Strabo, the fame ; chat of 
Pliny, yfy ; that oi Neacchus, yiy and ^ ; the mean lla- 
dium 01 the anciepts, y^-g-, or 505^ Englilh feet. The 
' pacC) 
