584 
POPULAR SCIENCE REVIEW. 
is clearly due to tlie fact tnat at the surface of the earth a 
number of mixtures and combinations of elementary bodies 
exist in a state of permanent equilibrium^ and in the three 
states of earthy water and air, these states being themselves 
due to pecuhar distribution and circulation of heafc, light, 
electricity, gravitation, and chemical agency proper to the 
position of the earth in a state of permanent equilibrium, in 
that part of the great system of bodies occup}dng space, in 
which we find ourselves to be. It is impossible to deny this, 
and equally impossible not to see that the form of the bottom 
of this chief of all lake basins is, in the logical sense of the 
word, an accident. Certainly, the essential fact of water 
resting on land is not in any way the cause of the form of the 
ocean floor. 
But, besides the general depression which is, under all cir- 
cumstances, occupied by water, and which may fitly be called 
the open ocean, there are numerous smaller seas, more or less 
nearly enclosed, and many depressions of all conceivable forms 
and dimensions in that part of the solid mass which rises 
above the mean level of the sea. Many of these communicate 
by channels with the open ocean, and their level necessarily 
approximates closely to that of the sea. Of this kind the 
Mediterranean is a familiar example. The Black Sea, again, 
opens into the Sea of Marmora, and that into the Mediterra- 
nean, and is only dependent on the ocean through two inter- 
vening bodies of water. The Sea of Azof opens only into the 
Black Sea, and is thus dependent on the sea in a very 
indirect manner. The levels of these seas are different, 
and the state of saltness of their waters is very different from 
the ocean ; but still they are inland seas, and not lakes of the 
more distinct and well-marked kind. The Caspian Sea is 
another step removed from the ocean. It is a vast sheet of 
salt water; but the proportion of salt it contains is much 
smaller than in the open sea, whilst its level is more than 
eighty feet below that of the ocean. This great lake has 
always received the drainage of two great rivers (the Volga 
and the Ural), but is probably nothing more than a partially- 
dried and separated part of the ocean, which once came in 
from the north. The Dead Sea is a small but very remark- 
able sheet of water, whose surface is upwards of eighteen 
hundred feet below the sea. It is, however, apparently the 
remains of a larger sheet, and was probably at one time a 
part of the Red Sea. Owing to its peculiarities of position 
and separation from the Red Sea, and partly also to the 
enormously greater evaporation than supply from its surface, 
but partly to the influx of springs connected with volcanic 
emanations, it has attained its present character. I might 
