418 SIR JAMES SOUTH ON THE EXTENSIVE ATMOSPHERE OF MARS. 
oil ne vit point l’etoile moyenne qui devoit etre cachee par son disque. Le 
diametre de Mars etoit alors de 25 secondes. — Mem. de 1’Acad. tom. vii. 
p. 358. 
“ Les nuages qui survinrent. ne permirent pas d’en voir la sortie ; et l’on ne 
sgait pas ineme si on l’auroit pu voir immediatement, car trois quarts d’heure 
aprhs le ciel s’etant decouvert a Paris, M. Roe me r la chercha attentivement 
autour de Mars, et il ne la trouva qu’apres l’attention de deux minutes, quand 
elle ytoit deja eloignee bu bord oriental de Mars de deux tiers de son diametre. 
C’etoit alors ll h 15 m , et le parallele de l’etoile coupoit le diametre de Mars 
en raison de 2 a 3. II commenga de la voir sans difficulty, quand elle etoit 
eloignee de Mars de trois quarts de son diametre.” — Mem. de l’Acad. tom. vii. 
p. 359. 
Hence we learn that a star of the fifth magnitude at the distance of six 
minutes from the planet Mars became invisible to Cassini ; and that after 
occultation by the planet, the same star could not be detected by Roemer, till 
the planet’s limb had receded from it, almost seventeen seconds of a degree. 
Experience, however, has long shown us that stars of the same magnitude are 
visible even when in actual contact with the moon’s enlightened limb ; to 
what cause then is the invisibility of the star when in the vicinity of Mars 
referable ? Cassini attributed it to the atmosphere of Mars ; and although it 
seems difficult to imagine one of such enormous extent as the Briare observa- 
tion would require, still, as any other hypothesis would involve us in greater 
difficulty, I shall adopt it, and shall present it, as also the comments which 
precede it, in Cassini’s own words : “ Cette difficulty de voir cette etoile de la 
cinquieme grandeur trks proche de Mars est considerable, d’autant qu’il n’y a 
point de difficulty a voir des etoiles de la meme grandeur jusqu’au bord de la 
lune. Ce qui pourroit fairjuger que Mars est environne de quelque atmo- 
sphere.” — Mem. de l’Acad. tom. vii. p. 359. 
Admitting, then, that an extensively diffused atmosphere of Mars is indicated 
by the observations above quoted, let us see if modern observations can con- 
firm it. 
On the 27th October 1783, Sir William Herschel, with a new 20-feet 
Reflector of 18.7 inches aperture, saw a star of the 13th or 14tli magnitude at a 
distance of two minutes and fifty-six seconds from the planet, “ not otherwise 
