462 The Atmospheres of the Planets. [October, 
There are other points of some question in connection 
with the condition of the atmosphere of Jupiter, more espe- 
cially the shading noticed by Brett, Knobel, and Lassell on 
some of the minute white cloud-like spots. These appear- 
ances are, however, far from inconsistent with a moderate 
depth of atmosphere, such as it has been shown can well 
be supposed to exist on Jupiter. But in these, as in all 
other points, it is impracticable to found any trustworthy 
conclusion on so few observations as a score or so, and nu- 
merical data must be based on actual measures, for estima- 
tions on these delicate points can never be satisfactory. 
In considering the probable atmospheric conditions of the 
three smaller planets less uncertainty with regard to the 
two principal unknown quantities exist, for not only is there 
no reason for believing that the temperature of these three 
planets differs materially from that of the earth, but neither 
can the density or nature of their atmospheres. Their com- 
parative nearness also allows of telescopic investigation, 
affording a considerable amount of information with regard to 
their probable condition ; whilst their astronomical character 
indicate a great analogy in constitution of the earth. It will, 
therefore, not be necessary to regard them from the extreme 
points of view necessary in dealing with Jupiter; whilst 
their proximity to the earth permits an additional method of 
obtaining information with regard to the atmospheres to be 
made use of, namely, its horizontal refraction. 
Mars may be first considered as being the planet about 
whose physical constitution more information has been 
obtained, from telescopic and spectroscopic investigation, 
than any other. The appearance of Mars under favourable 
conditions is well known,—a distinét orange ground with 
faint shadings of brown, yellow, red, and grey, containing 
numerous permanent dull bluish grey markings, whilst the 
poles are marked by circular brilliant white spots. These 
last, from various considerations, have been, with justice, 
regarded as due to accumulations of snow and ice on the 
poles of Mars in the same manner as on the earth; and the 
dark bluish grey or, according to many observers, greenish 
spots appear to be true seas on Mars. ‘The existence of an 
atmosphere to Mars has been demonstrated by telescopic 
observation, which has shown many clouds, but apart from 
this is a necessary corollary to the existence of moisture ; 
and from the spectroscopic researches of Huggins and Jans- 
sen this atmosphere is very similar to our own, and accor- 
ding to the latter the presence of moisture is shown by its 
spectrum. Mars may therefore be considered to possess an 
