25 
i8 77.] Physical Changes upon the Moon's Surface. 
studied the phenomena presented by Plato, is that the 
darkening of the floor of Plato results from an adtual 
change due to heating adtion of the solar rays. Either from 
the volatilisation of some constituent of the substance com- 
posing the interior of Plato, or from some change in its 
constitution due to the heating effedt of the solar rays, the 
floor darkens in colour. Now, there are more than one sub- 
stance, known as a constituent of the crust of the earth, 
that thus darkens when submitted to heat, — some from loss 
of moisture, others from loss of other substances, and others 
again apparently from a molecular adtion. Selenographers 
cannot say which of these probably forms the floor of Plato, 
or rather to which of these does the substance probably 
forming the floor of Plato bear most resemblance ; but there 
is every reason to believe that to one of them the floor of 
Plato bears a strong analogy. 
It has been urged by Mr. Prodtor ( loc . cit., p. 504) that 
under these conditions the maximum change on Plato would 
not occur until long after the sun had attained its meridianal 
altitude on Plato, whereas observation shows that this takes 
place shortly after this period. It can be shown that this 
is not necessarily the case, because the time when the effedt 
of the solar rays reaches its maximum power depends very 
greatly on the adtion exerted by the lunar atmosphere. 
There is not merely a heating adtion only, but a loss of heat 
through various causes ; so that the problem is complex, 
and involves the balance of adtion between the opposite 
effedts. The question of the effedt of the lunar atmosphere 
in modifying the adtion of the solar rays upon the moon, 
though one of very great interest, is one involving very 
great difficulties, more especially as observation and theory 
appear to be at variance in one or two points. The influence 
of the lunar atmosphere is, however, sufficiently great to re- 
move the difficulty that has been urged on this point, just in 
the same manner that the adtion of the terrestrial atmo- 
sphere renders the hottest portion of the terrestrial day close 
after noon, rather than towards evening. The real power 
of this adtive agent, the lunar atmosphere, is rarely pro- 
perly appreciated, and, in fadt, the whole question of the 
atmospheres of the planets, as if it were forgotten that the 
whole subjedt is within certain limits as reducible to the 
control of the present powers of mathematical analysis as 
the motions of the planets themselves. In a future number 
this subjedt may be returned to, and the real power exerted 
by a lunar atmosphere on the phenomena presented by the 
moon, and on the condition of its surface, will be considered. 
