1874.] The Lunar Atmosphere. 493 
about one-fiftieth of the density at the terrestrial surface. 
Notwithstanding this, however, as before stated, the ratio 
between the masses of the atmosphere and planet would 
be the same as onthe earth. All criteria directed, there- 
fore, against an atmosphere similar in density to our own, 
are quite inapplicable to the moon’s; and an examination of 
any of the works treating on the question will show how 
many are of this class. 
Further examination will also show that we have probably 
to deal with even a less density than some one-fiftieth of the 
earth’s. The moon’s mean density being only three-fifths of 
the earth’s, the moon’s surface is, in proportion to the lunar 
Mass, over six times greater than on the earth. Accordingly 
the atmosphere would be subjected to the action of a 
surface more than six times as great as has been on the 
earth. There can be little question but that at one time 
the terrestrial surface must have exerted a powerful action 
on our primitive atmosphere, and have absorbed and locked 
up much of what constituted our earliest atmosphere, for 
on no other conditions can be explained the known nature 
of the terrestrial surface. But as the moon’s primitive 
atmosphere must have been exposed to the aétion of more 
than six times the extent of surface, it would appear not 
improbable that her atmosphere must have lost much more 
in proportion than the earth. Even, therefore, if both 
planets originally possessed atmospheres whose masses, and 
those of the planets, were in similar ratio, the moon’s, thus 
exposed to a far greater absorbing surface, must in the end 
have lost the most ; and so the residue left possess a smaller 
ratio to the mass of the planet than on the earth. Wecan 
easily see, therefore, how—by the action of this proportion- 
ately six times greater surface—it is possible for the moon’s 
atmosphere to have been reduced to one-sixth the ratio to 
the mass of the planet that we find on our terrestrial surface. 
It is true that this would be a very difficult question to in 
any way prove, or even to maintain in its integrity, but this 
is immaterial for our present purpose; and it may well be 
admissible that, even had the planets started with the same 
proportional atmosphere, that of the moon may have been 
reduced to one-sixth. As the lunar atmosphere, under equal 
conditions, has been shown to be only one-fiftieth of the 
terrestrial density, reduced to one-sixth it would possess 
only one three-hundredth, or possess a density expressible 
on the earth as one-tenth of an inch, or some 2} millimetres. 
Now such an atmosphere is possible. 
But such an atmosphere is not small; the only thing 
VOL. IV. (N.S.) 3R 
