620 
POPULAE SCIENCE EEVIEW. 
are hid exactly the time they should be. If there was an 
atmosphere round the moon, the time during which the star is 
out of sight would be shorter, as the ray of light proceeding- 
from the star would be bent from behind the moon^s disc, and 
we should see the star attached to the moon^s edge after it has 
really disappeared, as we see the sun and other objects above 
the horizon when they have set. If the distance between two 
stars at a short distance from each other, and which were 
about to be occulted, were repeatedly observed shortly before 
the time of occultation, it would be found, as Arago suggests, 
that if a lunar atmosphere did exist, the distance of the stars 
would go on gradually increasing. We do not know v^hether 
this last method, by which any error which may exist in the 
measurement of the mooAs diameter is got rid of, has ever 
been tried, but the &st has been repeatedly calculated, and it 
has been found that the density of the lunar atmosphere is less 
than that given by the best au*-pumps. Sir J. Herschel, how- 
ever, thinks that from the figure of the moon being elongated 
towards the earth, the opposite and larger hemisphere may be 
provided with a lake of water and a stationary volume of air. 
In respect to changes on the mooAs surface, it must be 
admitted that our knowledge is still in a very imperfect state. 
Considering that an object may appear to change its form or 
become visible or invisible, according to the manner in which 
the sun^s light falls upon it, it is certain that even with the 
same instrument and observer, mistakes may occur on this 
point. Though Schroeter had very keen sight and powerful 
telescopes, yet Olbers was able to detect two small craters 
which had escaped his attention, and which he was not able 
to see until he had repeatedly tried. Even in the magnificent 
map of Beer and Maedler, which took so many years of con- 
tinuous and persevering labour to complete (Jan. 1830 to 
Oct. 1836), there are numerous omissions ; as for instance, the 
numerous rills which are seen in the interior of the mountain 
Gassendi. We are, however, now promised a chart which will 
be the most magnificent yet projected, and it is to be hoped 
that the design will be carried out in all its integrity. We 
allude to the map spoken of at the present meeting of the 
British Association, where the survey of a small part of its 
surface will be apportioned to different observers who shall 
direct all their care and energy to the most minute particulars 
of its surface ; and it is to be hoped that the project may be 
more successful than a similar one started nearly twelve years 
since by the same learned society. The writer of this article 
then made several sketches and measurements of Plato and 
the lunar /|7.9, with the twenty-foot refractor at Cambridge ; 
but it was found to interfere too much with the ordinary work of 
