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XXII. On the extensive Atmosphere of Mars. By Sir James South, F.R.S. 
Communicated by His Royal Highness the President. 
Read June 16, 1831. 
That several of the planets as well as that which we inhabit are surrounded 
by atmosphere, astronomical observations have long since established ; the 
extent, however, to which in particular planets such atmospheres are diffused, 
is as yet not satisfactorily determined. The former rests principally upon phe- 
nomena observed on the planets’ discs, whilst the latter derives its support 
chiefly from those detected at or near their respective limbs. Every night, nay 
almost every hour, may give us indication of the one, whilst years are some- 
times necessary, as in the case of planets unattended by satellites, to help us 
to the other ; thus the hypothesis of the extensive atmosphere of Mars derives 
its origin from the observations of Cassini and Roemer, and has stood more 
than a century and a half without refutation or support. 
The observations to which I allude formed part of a series undertaken for 
the determination of the parallax of Mars, and are recorded in the Memoires 
de l’Academie des Sciences. Cassini’s were made at Briare and at La Charit6 
sur Loire ; whilst Roemer’s was obtained at the Royal Observatory of Paris. 
Of the Briare observation Cassini says, “Le premier Octobre 1672 & 2 h 45 m 
du matin a Briare, Mars vu par une lunette de trois pieds, sembloit toucher par 
son bord septentrional, la ligne droite tiree par la premiere et par la seconde 
de l’eau d’ Aquarius marquee \p, d’oh il n’6toit plus eloign6 que de six minutes. 
Cette etoile paroissoit si diminuee et si affoiblie de lumikre, qu’on ne la pouvoit 
plus distinguer ni & la vue simple, ni par une lunette un peu plus foible.” — 
Mem. de l’Acad. tome vii. p. 357. 
The La Charite and Paris observations entitled “ Eclipse de la Moyenne 
dans l’eau d’ Aquarius ” are thus narrated: “ Quoique le ciel fut alors assez beau 
de part et d’autre, et que l’on vit Mars pendant un assez long espace de temps, 
3 h 2 
