17 
1 877.] Physical Changes upon the Moon's Surface . 
Madler, who were the first to notice the existence of definite 
variations in tint, were struck by the analogy they bore to 
the changes which would result from processes of vegetation 
on the lunar surface, though, as they pointed out, this 
seemed incompatible with the absence of masses of water 
upon the moon. It is remarkable that, despite this appa- 
rently fatal objection, other experienced selenographers have 
been led to the same conclusion, from evidence of widely 
diverse nature. Other instances of colour changes on the 
moon, also periodical in character, appear to indicate the 
effects of the solar rays continuously poured down on the 
lunar surface for nearly fifteen days at a time ; and to this 
class appears to belong the variation in the floor of Plato, 
which has been discussed by Mr. Pro (51 or in the “ Quarterly 
Journal of Science ” for January and Odtober, 1873. There 
are other instances which, unlike the last, are not period- 
ical in nature, and which indicate in a very forcible manner 
the effedt of the slow, but energetic and increasing, disinte- 
grative agents which have destroyed all the older lunar 
formations, and the principal of which appears to be the 
extensive and all-powerful lunar atmosphere. The capabili- 
ties and power of this last member of the forces moulding 
the surface of the moon are generally overlooked, on the 
supposition that its small density, when compared with the 
extremely dense atmosphere of the earth, must destroy its 
energy and capabilities, forgetting that what it may want in 
density it may make up in volume.* 
The variation in the tint of the floor of Plato is one of the 
* It is to be remembered that the influence of the atmosphere upon the 
surface of the moon depends almost entirely upon its mass, and not upon its 
surface density. It is not through its pressure that the lunar atmosphere 
exerts its influence upon the surface of the moon, but through its chemical 
adlion upon the constituents of the crust of the moon, or through modifying 
the great variations in temperature to which the lunar surface is exposed. In 
its chemical adtion the effedts of the atmosphere will depend almost entirely 
upon its mass, because, through diffusion and similarly adting causes, all por- 
tions of the atmosphere will successively come in contact: with the surface. 
Were the adtion of the atmosphere very rapid, so that the entire chemical 
power of any portion was liable to be exhausted before its place could be taken 
by another portion through the agency of diffusion, then it is true a decrease 
in the density of the atmosphere might produce a decreased adtion in a given 
time. But the adtion of the atmosphere is so excessively slow that any effedt 
of the kind stated is out of the question. The effedt of the atmosphere in 
modifying the variation in temperature to which the lunar surface is exposed 
likewise depends almost entirely upon the mass of the atmosphere, and not 
upon its surface density. Thus, in the retardation of the radiation of heat 
from the moon, and in the absorption of the solar heat, it is not the surface 
density of the atmosphere on which these effedts depend, but, as both Tyndall 
and Magnus have shown, upon its mass. Yet the strange idea that the powers 
of the lunar atmosphere lay entirely in its surface density seems to be enter- 
tained by a hostile reviewer of my work on the Moon. 
VOL. VII. (N.S.) 
C 
