Astronomical and Nautical Collections. 411 
“¢ The celebrity of his successor Eratosthenes is principally 
due to his measurement of the earth, which indeed is. the first 
attempt of the kind that has been recorded in the history of 
astronomy. It is very probable that in much earlier times astro- 
nomers had not wholly omitted to make experiments of the 
same kind; but nothing has been left of these earlier operations, 
except some estimations of the circumference of the earth, which 
have been reduced, by means of comparisons more ingenious 
than demonstrative, to something like an agreement with more 
modern determinations. Eratosthenes having considered that 
at Syene, the sun, at the time of the summer solstice, shone 
into a well throughout its depth, and, comparing this obser- 
vation with that of the meridian altitude of the sun at the same 
solstice, as observed at Alexandria, found the celestial arc, 
comprehended between the zeniths of these two cities, equal to 
the fiftieth part of the whole circumference; and as their dis- 
tance was estimated at about five thousand stadia, he gave 
252,000 stadia as the whole length of the terrestrial meridian. 
It is, however, very improbable, that, for so important a pur- 
pose, this great astronomer should have been contented with the 
coarse observation of a well enlightened by the sun. This con- 
sideration, and the relation of Cleomedes, authorise us to con- 
clude that he observed the shadow of the gnomon at the sum- 
mer and winter solstices, both at Syene and at Alexandria: 
and in this manner he obtained the difference of latitude of 
these two cities very nearly such as it has been found by modern 
observations. But the greatest uncertainty, that this measure- 
ment has left us, relates to the length of the stadium employed 
by Eratosthenes, which it is difficult to determine among the 
multitude of different stadia that were employed by the Greeks.” 
P. 34, 35. 
iii, Elements of a Comet, communicated by Professor Scuvu- 
MACHER. 
v 
Tue comet now visible, 3 October, gives us still much labour. 
The totality of the observations of Dr. Olbers and my own, 
