( 492 ) (October, 
III. THE LUNAR ATMOSPHERE AND ITS 
INFLUENCE ON LUNAR QUESTIONS. 
By EpmMuND NEIsoNn, F.R.A.S., &c. 
ERHAPS no question in connection with the condition 
sof the moon’s surface has excited so much attention 
as the existence, or not, of a definite lunar atmo- 
sphere; and it must be conceded that it possesses the 
highest importance, when considered in its relation to the 
many problems of great interest to be decided in conne¢tion 
with the present and past history of our satellite. 
Curiously enough, although the most diligent and expert 
selenographers, with hardly, perhaps, but one exception, 
have long recognised both direét and indireét evidences of 
the probable existence of an atmosphere, yet among astro- 
nomers in general it has been usually considered as defi- 
nitely established that the moon is to all intents a perfectly 
airless globe. For even if it has been granted that it may 
possess, perhaps, the merest trifling trace of gaseous en- 
velope, it has been considered established, for a certainty, 
that this must be far less dense than the most perfect vacuum 
that has been artificially produced by an air-pump. 
There does not appear to exist, however, sufficient grounds 
for this widely entertained belief in the non-existence of a 
lunar atmosphere having been proved; for apart from the 
necessity, not observed in this case, of cautiously accepting 
positive conclusions based entirely on purely negative results, 
the subject has never been examined in a sufficiently com- 
plete and accurate manner. In consequence, many of the 
criteria that have been regarded as demonstrating the non- 
existence of a proper lunar atmosphere should never have 
been taken into consideration as applicable. Most of these 
negative results, which have been relied on as showing the 
absence of any gaseous envelope to our satellite, are only 
applicable to an atmosphere similar in density to our own. 
But such are the favourable conditions prevailing upon the 
terrestrial surface that if an atmosphere whose mass and 
that of the planets were in a constant ratio were to exist on 
every planet, the earth’s would be far the densest of any, 
excepting the giant of the family Jupiter. But on the moon 
the most unfavourable conditions prevail for the density of 
atmosphere; and under the conditions supposed, and these 
conditions the most probable imaginable, then the lunar 
atmosphere would possess at the surface a density only 
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