THE PLANET MAES IN JANUAEY 1867. 
29 
telescope may be used, and our own atmosphere (as tested by the 
performance of the telescope on double stars) may be in a state 
as favourably fitted for definition. The colour of the planet is 
also variable ; the redness (compared to a faint tinge of Indian 
red by some observers, and to a coppery tint by others), and the 
greenish-grey tint of the darker parts of the disc, being much 
more marked on some occasions than on others. Another phe- 
nomenon — the paleness of the disc round the edges— is also 
variable. 
The variations in the appearance of Mars are clearly expli- 
cable on the natural hypothesis of an atmospheric envelope, 
such as that surrounding our own earth, bearing clouds and 
mists over the surface of the planet. Judging by the analogy 
of our own earth, we may consider that the planet’s cloud- 
covering would vary in density not only from place to place 
upon the surface, but, considered as a whole, from season to 
season, and from year to year. It gives a high idea of the dif- 
ficulty of the problem attacked by astronomers, in examining 
Mars, to note that, for favourable research, we must have a fine 
night upon our earth, and a clear day on Mars, combined with 
favourable circumstances of distance, altitude, and presentation ; 
that we cannot watch the planet through* any single Martial 
year, but must be content to piece in the observations of diffe- 
rent seasons of different years ; and, finally, that Mars, when in 
opposition at the solstice of one of his hemispheres, is nearly 
twice as far from us as when in opposition at the corresponding 
solstice of the other hemisphere. 
Since, in January 1867, the north Martialists are enjoying 
early summer, while the southerners are entering on their 
long winter, the observer on earth may expect to find the 
northern snow-cap smaller than the southern ; though the 
extent of the latter will not, in all probability, approach the 
dimensions observed by Delarue in 1858, when the southern 
hemisphere had lately passed through the full severity of the 
Martial winter. The plate represents the appearances to be 
looked for when an inverting telescope is used, so that the north 
pole is below ; it is only necessary to invert the plate to obtain 
what we are in the habit of considering the natural presentation 
of such a globe. 
A little consideration of fig. 1 will show that, though Mars 
will gradually present more and more of his northern polar 
regions towards the sun as he moves on to the positions M,m, 
and M 2 , yet to observers on earth the north polar presentation 
will not change in the same way. In fact, while Mars is 
retrograding (or until the middle of February ; see map), his 
north pole will be retiring so far as observers on earth are con- 
cerned. Afterwards Mars will gradually present more and 
