Astronomical and Nautical Collections. 191 
There is on this subject a singular confusion in the remark 
of Laplace, that Eratosthenes found the difference of latitude 
of Syene and Alexandria equal to 4, of the circumference: 
and this distance being estimated at 500[0] stadia, Eratosthenes 
inferred that the whole circumference was 250,000. ‘ But,” 
he continues, ‘* the uncertainty of the value of the stadium, 
employed by this astronomer, renders it impossible to appreciate 
the accuracy of this measurement.”—Exp. du Syst. du Monde, 
v. il. 
Now it is highly improbable that Eratosthenes should have 
committed any gross error in the measurement of the length of 
Egypt from north to south, and we may surely consider it as 
a sufficient determination of the stadium which he employed, 
that it was s,1;5 of the difference of latitude between Alexan- 
dria and Syene, or between 31° 13’ and 24° 5’, which is 7° 8’, 
or about 498 English miles; and 50 times this distance is 
24,900, giving 7,930 miles for the earth’s diameter : a result 
very fairly obtained from the operations of Eratosthenes, and 
as correct in reality as the distance of the sun in Plutarch has 
been rendered by accident only. Nor will the variation be 
material if we take the number 252,000, from Pliny, instead of 
250,000 stadia, as the exact extent assigned by Eratosthenes 
to the earth’s circumference. 
In referring to some of these numbers, we may observe one 
thing with respect to the numeration of the Romans, which has 
not been commonly noticed : that is, that though they had not 
invented a decimal arithmetic, they occasionally employed a 
centenary notation: thus in Pliny’s fifth book, chapter ix., we 
have xxvi.xxxix mill. passuum, for 2639 miles, and again 
xv.xLv, for 1545; the word centena being omitted, as was 
also usual in their computations of money: but they do not 
seem to have made any further step, either upwards or down- 
wards, in the decimal scale. 
Arr. XII. Corrections in Right Ascension of Thirty-six 
Principal Stars. By Jamus Soutn, F.R.S. 
{Coneluded from Vol. XIII., p. 393.] 
