500 The Lunar Atmosphere [Otober, 
less density. Consequently, the amount of atmosphere 
acting on the moon’s surface is not much smaller than on 
the earth, and, as far as a single square mile of surface is 
concerned must be estimated by millions of tons. Moreover, 
from these considerations as to the horizontal refraction 
exerted by a lunar atmosphere, it is evident that, were its 
density a little less than one-thousandth of the terrestrial 
density, the retardation at the limb during occultations would 
no longer be detectable from its being smaller than the 
probable errors of the method, and yet it cannot be questioned 
but that this atmosphere would still exert a most marked 
influence on the physical condition of the lunar surface. 
There are many selenographical problems dependent on 
the question of the existence or not of a lunar atmosphere, 
and prominent amongst them stands the absorbing subject, 
so often debated, oflunarchanges. Though the configuration 
of the surface of the earth is undergoing a never-ceasing 
change from the action not only of volcanic forces, but also 
of the various atmospheric and aqueous causes, were it to 
be regarded from the moon, it is doubtful whether during 
the last century, with all our telescopic aids, any unmistak- 
able alteration in the formations constituting it, or in its 
appearance even, apart from changes of tint, could be detected. 
Similarly on the moon there may be even more energetic 
action in constant work, without its making such vast 
changes in the appearance of our satellite as to direct our 
attention unmistakably to the circumstance of lunar, 
volcanic, and other energies being still in active progression. 
For such is our imperfect knowledge of the details of .the 
configuration of the surface of the moon, that, were at 
intervals of a few years new volcanic cones, craterlets, and 
hills to come into existence, it is improbable in the extreme 
that they would be recognised as new formations instead of 
new objects detected. 
Suppose the new manifestation of lunar volcanic energy 
took the form of some of those volcanic cones, as occur on 
both the earth and moon—a steepish hill, some hundred 
yards in diameter and hundred feet high, with a deep mouth 
of no great dimensions; or else, perhaps, gave rise to a new 
ridge of mountains, such as are so common on the moon, 
some miles long and a mile broad, covered, perhaps, with 
small crater cones not exceeding some hundred yards in 
dimensions—there is not one ten-thousandth of the moon’s 
surface where these new formations would possess the 
slightest probability of being taken for the result of the action 
of present lunar volcanic energy. Or were, from some vast 
