1875.] Selenography: its Past, Present, and Future. 215 
the slopes of the walls of the chief classes of lunar forma- 
tions, as well as the relative levels of the principal mares or 
great plains, both important features, with regard to which 
no data exists; and the determination of the dimensions 
under different illuminations of some of the peculiar white 
spots occupying at full the position of various lunar forma- 
tions. Means should also be taken for a re-determination of 
the apparent brightness of the different portions of the lunar 
surface, and of their colour; for good reasons appear to 
exist for believing distinct changes have occurred since Beer 
and Madler’s time, and it should be remembered that such 
changes were suspected by Madler himself. Measures of 
the height of many of the walled peaks, and of the depths 
ef the chief formations, are especially valuable, after the 
apparent disappearance of several of Madler’s deep craters, 
although the many unpublished measures of Schmidt will 
be, when accessible, a great addition to our present data on 
this subject. 
Measures of the lunar diameter, and observations of 
occultations of stars present themselves as highly important 
for the determination of the question of the existence of a 
lunar atmosphere, such as is at present indicated by the 
known apparent disagreement between the telescopic and 
occultation semi-diameter. Measures of the shadow length 
of different ring-plains at different periods might reasonably 
be expected to afford some indications of the presence of a 
denser local atmosphere, due to local causes. By deter- 
mining the distance between some conspicuous lunar 
point and a star before occultation, evidence of the strongest 
kind might be obtained with regard to whether a lunar atmo- 
sphere exists, though at present no suitable lunar point 
near the limb has had its position sufficiently well deter- 
mined to be available for this purpose. 
Much, likewise, remains to be done in the observation of 
the details of the surface, and the careful construction from 
these of a trustworthy map of them. Based on sketches 
made at intervals during a period of thirty-five years, and 
unaided by the results of systematic measures of their 
positions, Schmidt’s map, when issued, which is unfortu- 
tunately not likely to be for a considerable period, will, 
though of the highest value, add materially to the lunar 
work requiring to be done. It is certain to require much 
revision from its very nature, and must give rise to many 
uncertainties requiring settlement, whilst, as it differs in 
many points from Beer and Madler’s, whose general accu- 
racy is of the highest, these discrepancies will need most 
VOL. V. (N.S.) 2E 
