244 OBSERVATIONS on TWO ECLIPSES. 
At firft it was thought ufelefs to endeavour to obferve 
the magnitude of the eclipfe with the quadrant, becaufe of 
the flow movement of the two planets vertically, yet upon 
reflection he thought that the paflage of the horns and 
limb of the fun and moon to the vertical thread only, 
would be fufficient to give the differencies of the altitude 
and azimuth of the centers of the fun and moon, and of 
courfe their difference of latitude and longitude. He 
then obferved them towards the end of the eclipfe, and at 
the fame time obferved the time of their arrival at the ho- 
rizontal thread, but without any expectation of their be- 
ing of great fervice to calculate the diftance of the cen- 
ters from. 
In the firft obfervation the paflage of the upper horn te 
the vertical thread, is a little doubtful; at the time it was 
thought proper to give the preference to the paflages of 
the limb and lower horn of the moon to the fame vertical 
thread, in making the calculations from this ob- 
fervation; by this means we may find the lines 
NB and CO, the firft of which combined with the 
two diameters LN and ST will give LE, the difference 
of azimuths and of the centers of the two planets; the 
apparent difference of the altitude from the fame centers 
may be eafily obtained, by calculating SF and CM, fides 
of the right-angled triangles CSF and CLM, in which 
are known the other two fides. 
Plate Ill. 
Figure 18. 
In 
culant SF et CM cotes des triangles rectangles CSF et CLM dans les quels on connoit 
deja les deux autres cotés. 
Dans la feconde obfervation on a obmis par diftraction, ’inftant du paffage du bord de la 
lune au fil vertical, on fe fervira donc pour la calculer des paffages des cornes au méme fil les 
quels donneront immediatement C A et CO d’ou fouftrayant ST, on aura CD et C F con- 
noiffant ces deux lignes et le demi-diametre du foleil on calculera les angles CSD, CSF 
fouftrayant leur fomme de 180°, on aura l’angle CSL partageant cet angle par la moitié on 
aura l’angle CSL du triangle CLS, on connoitra donc aifement SI, coté de cetriangle et du 
triangle SLE calculant enfin ce dernier triangle SL E dans le quel on connoit deux angles 
et un cété on obtiendra L E; difference d’azumuth et SE; difference apparente de hauteur des 
centres des deux aftres, 
La derniere obfervation etant plus complette on pourra la calculer indifferement de l’une ou 
V’autre maniere. On pourra méme faire vfage du paflage au fil horifontal pour conclure la dif+ 
ference de hauteur attendu que le movement des deux aftres dans le fens vertical commencoit a 
devenir moins lent lorfque cette obfervation a eté faite. 
‘ Si 
