12 Memoir on the Diminution of the 
part of the circumference. Thus, according to this astro- _ 
nomer, the sun being at the zenith of Syena at the summer 
solstice, he found on the same day its distance’ from the . 
zenith of Alexandria 7° 12’. This distance was from the 
upper edge of the sun ; for the ancient astronomers did not 
correct the sun’s altitude as observed by the gnomon, to 
obtain that of the sun’s centre: which is the reason why 
their latitudes were too stall by the apparent diameter of 
the sun. This is evident for Alexandria, whose latitude is 
calculated by Ptolomy at 30° 58’; while, according to Nouet’s 
observations, it 1s 31° 1371 5"3 greater therefore by 15’ 5”, 
which is nearly the sun’s semi- ediameter. The apparent 
altitude of the sun at the s.mmer solstice at Alexandria, as 
observed by Eratosthenes, must then be corrected by the 
sun’s semi-diameter, the refraction and parallax 5 which 
gives 7° 27' 50” for the distance from the. sun’s centre 
fo the Pent of Alexandria at the said solstice. Sub- 
tracting it from the latitude of Alexandrja, as observed by: 
Nouvet, the difference 23° 45’ 7” will be the obliquity of the 
ecliptic in Eratosthenes’s time, or towards the year 250 be- 
fore our era. According to the formulas of Méc. Cél, it 
was at that epocha 23° 45’ 19”, which agrees remarkably 
well with Eratosiliencs’s observations. These observations, 
together with Pytheas”'and the preceding Chinese, all com- 
bine therefore to show that the ecli iptic’s obliquity, previous 
to our era, was very nearly the same as is given by the for- 
niulas of Méc. Cél. Let us now consider the observations 
made since the commencement of our era. 
OF ANCIENT OBSERVATIONS POSTERIOR TO OUR AIRA, 
Chinese Observations. 
The first of these observations dates from the year 173 of 
our era. It is recorded as follows, tn Father Gaubil’s MS. 
(Conn. des Tems for 1809, p 395.) 
<¢ Ou the 9th of October 173. at Loyang, meridian sha- 
dow ten feet. On the 7th of February 174, meridian shadow 
nine feet six inches. These shadows were observed care- 
fully.” | 
The gnomon was eight feet Jong. 
The altitude “it the sun’s centre resulting from the first 
shadow is 38° 22" 15", after being corrected by refraction, 
and parallax. The one resulting from the later shadow, 
also corrected} is 39°31’ 9,4. Let a be ba altitude of + he 
equator at Loyang. Jf we calcvlate from the new Tables 
of the Sun just pub lished by the Board of Longitude, the 
sun’s 
\ 
