or THE SURFACE OF THE MOON. 
335 
portions of the interior of craters, while other neighbouring portions have the full light. 
The effect is occasionally to produce half-tints on particular portions of terraces within 
the crater, as in the case of Theophilus, of which I present two drawings, one (Plate 
XVI.) showing this peculiarity in the morning light, the other (Plate XV.) not. The 
central mountains of that great crater are high enough to throw long shadows ; and these, 
as they catch upon other peaks or spread, softening with distance, over the surrounding 
plains, present far greater variety of shadow-tones than might be expected on a globe 
deficient, as the moon really appears to be, of both air and water. 
If we suppose the moon’s mass to have been derived from an outer ring of the earth- 
nebula, and, following the analogy of the planets in reference to the sun, admit its com- 
position to have been the same as that of the earth, we should have as the originally out- 
standing lunar atmosphere, only part of that which surrounds, or rather which 
at first surrounded, the earth. The surface of the moon being part of that of the 
earth, the barometric pressure o,f the atmosphere on the moon would be less than that 
on the earth, in the proportion of The height of the lunar atmosphere, taken in 
terms of the pressure at the surface of the moon, would be the same as that on the 
earth (5 miles +). The refractions would be less than on the earth, in the proportion 
of the function of density, but still very sensible, even in rude observations of occupa- 
tions of stars. 
In like manner the water due to the moon’s mass, would have its depth on the moon 
diminished to of that on the earth. If we assume the water on the earth spread 
over its whole surface to be 1 mile +, that on the moon would be 829 feet, a quantity 
not so great as to preclude the possibility of its being accumulated on the surface 
opposed to us, especially if we remember that the inequalities of internal earth-move- 
ment have in fact occasioned the greater part of our oceans to be collected on one half 
of the globe, while nearly all the land appears on the other half. In reference to this 
subject the opinion of Hansen, that the centre of gravity in the spherical moon is 
removed from the centre of figure 33^ miles in the direction from the earth, may be 
kept in mind. If it could be admitted, the oceans of water and air might be wholly 
collected on the remote half of the satellite. Moreover, it must be remembered that in 
the rocks near the surface of our planet both oxygen in oxides, carbonates, &c., and 
water in hydrates, are stored up in considerable quantities. We may suppose the 
process of volcanic incineration and aqueous absorption to have gone to greater extreme 
in the moon than on the earth, and so to have reduced the original atmosphere and the 
original oceans to very much smaller amounts. To judge by the absolute freedom from 
all change but that of degradation, which is observed on the visible disk of the moon, 
and by the perfect and perpetual clearness of its sharply defined mountain-bordered 
limb, cutting clear against the unbending rays of the stars, it would seem the most 
probable opinion that now at least there remains no visible trace of atmosphere or ocean. 
The different parts of the moon’s surface reflect light very unequally ; the dark parts 
have several degrees of darkness, the light parts several degrees of light. On the same 
mdccclxviii. 3 A 
