ASTRONOMY. 187 



c. Astronomers, who would compare their observa- 

 tions of these eclipses, should be furnished with 

 telescopes very much alike in distinctness and mag- 

 nifying power. The greater perfection of the te- 

 lescope will give the immersions later, and the e- 

 mersions earlier. On the whole, it is best to em- 

 ploy, as DE LAMBRE advises, telescopes of mode- 

 rate power. Abrege d'Astron. legon 22. 1 4-. 



d. The apparent instant of immersion or emersion, will 

 also be affected by the proximity of Jupiter to the 

 Earth, and by the altitude above the horizon. M. 

 BAILLY has given rules for correcting the effects 

 arising from all these circumstances. See TJieorie 

 des Satellites, par M. BAILLY, Also Mem. Acad. 

 Roy ale des Sciences, 1771 ; and Phil. Trans, vol. 

 LXIII. LA LANDE, 304-7. VINCE, vol. i. $ 460. 



191- By observing the time required by the 

 satellites to enter into the shadow, the angles 

 which their diameters subtend at the centre of 

 Jupiter may be computed. 



According to WHISTON, the first satellite employs 

 1m 10s; the second, 2m 20*; the third, 3m 4Qs; 

 and the fourth, 5m 30s, to enter completely into the 

 shadow, when they enter it perpendicularly. LA 

 LANDE, 3038. From more accurate observations, 

 M. BAILLY computed their diameters, as seen 

 from Jupiter, the first, 60' 20" ; the second, 



29' 42"; 



