MARS. 
231 
would appear, from a cursory view, as though the surface of 
Mars were pretty equally divided between land and water, the 
former, indeed, from Maedler’s map, having somewhat the pre- 
dominance, whilst on the Earth the ocean covers three times 
as much space as the land. 
Whilst observing the spots on Mars, we are somewhat 
astonished to see that, as they pass from the centre towards 
the margins of the planet, they become rapidly fainter, and 
that at the edges they are entirely obliterated. Even the 
darkest spots on Mars (those which are seen between twenty 
and forty degrees of south latitude) undergo this change as 
they approach the margins of the planet. Occasionally the 
whole circumference of the planet is surrounded, as it were, with 
a luminous ring, as in Plate XII., fig. 1, which represents the 
planet as seen by the writer in the 20-foot refractor at the 
Cambridge Observatory, in the opposition of 1856. This is 
generally held to be due to its atmosphere, which is considerably 
thicker at the edges than at the centre, just as during a fog we 
can perceive the blue sky and stars over-head, which are quite 
invisible at the horizon, where the atmosphere is considerably 
deeper and thicker. W e might thus conclude that Mars has an 
atmosphere of considerable density ; and this is confirmed by 
other circumstances. The older observers attempted to prove 
the existence of a dense atmosphere in Mars, by the disappear- 
ance of stars even at considerable distances from the planet, 
which they imagined had been eclipsed by its extensive gaseous 
envelope. This has been repeatedly tested in modern times, 
and with the best instruments, but without success. The 
elder Herschel followed very faint stars close up to the margin 
of the planet, and Sir J. South has repeated the experiment 
more recently without perceiving any change in the brilliancy 
or colour of the very small stars selected. Whilst Cassini 
could not perceive a star of the fifth magnitude within six 
minutes of the margin of the planet (on Oct. 1, 1672), Herschel 
could perceive stars of the twelfth and thirteenth magnitudes 
within three minutes of its disc. Herschel, however, fully believed 
in the existence of a dense atmosphere surrounding the planet, 
having often seen, besides the permanent spots on its surface, 
occasional changes of partial bright belts. The late Dr. Pearson 
