614 
POPULAE SCIENCE EEVIEW. 
full^ covering as it does one-fourth part of its surface. Other 
great seas will be seen in the eastern and northern portions 
of the moon ; the parts most free from them being the 
southern and south-eastern^ which is thickly covered with 
annular mountains. But it is at once seen that these level 
plains are very different in respect to their darkness, and 
even in their colour. The Mare Grisiimij situate near the 
western boundary of the moon, is very dark, and the Mare 
Tranquillitatis is almost equally so, whilst the Mare Nuhiumj 
lying between Tycho and GoiDernicus, is of a much lighter 
shade. In some of those seas the surfaces are very smooth, 
and in some parts almost perfectly polished; but others are very 
irregular, and covered wdth great isolated rocks, craters, and 
even mountain- chains. They are sometimes traversed by long 
veins or very extensive and level ridges, which ascend to the 
higher qjlcdeau by a very gradual slope. Perhaps the long flat 
plain which Mr. Key lately discovered at the circular contour 
of the moon may have been one of the unmapped seas which 
come into sight at certain periods. A similar appearance, on 
a much less extensive scale, was seen by Mr. Lassel, in May, 
1864, but seems to have been flat land between two project- 
ing mountains ; the latter being very common at this part of 
the moon, which comprises the highest mountains, or those of 
Doerfel and Leibnitz. Both observations are very remarkable, 
and prove that the contour of the moon is far from being 
circular, and that great irregularities exist at certain times, 
according to the libration of our satellite. 
Colour has been detected in those seas, but is very difficult 
to be perceived, and some eyes can only detect the ordinary 
grey or neutral tint of different intensities. The Mare 
Sumorum” is that in which this green tinge is considered 
strongest, although all the other seas partake of it more or less. 
It has been debated whether it is due to the presence 
of the vegetable or mineral kingdom in the moon ; if 
in the former, it must, of course, be taken as a proof of the 
existence of an atmosphere. The seas are by no means 
the darkest parts of the moon. The floor of the mountains 
Chbrnaldi and Riccioli, at the east part of the moon, are con- 
siderably darker, as likewise the walled plain Plato. The 
latter, on account of its dark colour, was formerly denomi- 
nated the Lacus Niger, although it is a perfect example of the 
type of annular mountains. Sir J. Herschel considers those 
vast plains or seas as of a decidedly alluvial character. 
The seas are the only parts of the moon which can be 
seen by the naked eye, and that very imperfectly. With the 
telescope there comes into sight a much more remarkable 
view of its surface, and amidst the first objects which catch 
