1875.] The Atmospheres of ihe Planets. 463 
atmosphere bearing a considerable resemblance to our own, 
though, from the small number of clouds and the general 
transparency, perhaps slightly inferior in density. The 
temperature at the surface is also probably very similar to 
our own, for the following reasons :—Were the temperature 
of Mars, in any marked degree, warmer than our own, from 
the presence of considerable bodies of water, an envelope of 
moisture of considerable dimensions would be formed, the 
polar ice zones would be much smaller; numerous clouds 
must at times occur, and the column of aqueous vapour 
traversed by the sunlight reflected from the surface of Mars 
would be so considerable, that the spectroscopic evidence of 
its existence would be of the most marked character. 
Were, however, the temperature in any degree much colder 
than our own, the seas of Mars would be permanéntly frozen 
and snow covered; the general brightness of Mars being thus 
greater than the polar zones even, whilst the amount of 
aqueous vapour, though very variable; would be far too small 
to be revealed by the spectroscope, and clouds would be en- 
tirely absent. Only, therefore, if the temperature of Mars 
resembles that of the earth can its telescopic and spectro- 
scopic characters be as they are. The conditions, therefore, 
of the atmosphere of Mars, in so far as its density and 
temperature are concerned, may be regarded as very similar 
to those of the earth; that is to say that its density is 
probably not over five times greater or five times less than 
our own, and its mean temperature at the surtace within 40° 
C. of that of the earth. 
Under these conditions the law of decrease of tempera- 
ture of the atmosphere of Mars ceases to possess the all- 
important charaCter it has when the variations in tempera- 
ture are more marked, for the decrease must necessarily be, 
whether slow or quick, so small as to exert comparatively 
slight effect. The conditions are, however, sufficiently 
similar to those of the earth to render it probable that the 
law that best represents the rate of decrease of tempera- 
ture on the earth will also best represent the same on 
Mars. In the equations (12), (13), and (15), putting r= 
unity, and f=4, and they then reduce to the well known 
equations employed by Ivory, in his celebrated theory of the 
Astronomical Refraction, regarded by Plana and other emi- 
nent astronomers as best representing the condition of the 
earth’s atmosphére. Substituting, then, the constant for 
Mars, and the height corresponding to any given density of 
the atmosphere of Mars will be found. From this it appears 
that, at a height of 8°59 miles, the density would be reduced 
VOL. V. (N.S.) 3 .N 
