338 On the Level of the Sea. 



portance to the inquiries whose object is to determine them witFi 

 accuracy. 



Almost all the basins of the sea communicate in various ways, 

 whether by wide canals, or by more or less contracted straits ; 

 and the waters in these different basins are subjected to the con- 

 ditions of equihbrium of communicating vessels. Only it must 

 be observed that the water of the sea is not a homogeneous liquid 

 in the whole extent of the mass: the temperature changes with the 

 latitude; and it also changes with the depth; the degree of saltness 

 changes in like manner ; and all these causes make the density to 

 vary in the different places, and from this there results a multitude 

 of motions, by which the equilibrium tends to be kept up. The 

 water of the Atlantic flows into the Mediterranean by the Straits 

 of Gibraltar, as is proved by the rapid current existing there ; 

 but it is not known whether, by an opposite current existing at 

 a greater depth, the water of the Mediterranean may not pass 

 into the ocean. If this second current exists, they are both with- 

 out doubt produced by the difference of density in the layers of 

 water. If it does not exist, it must be supposed that the Me- 

 diterranean loses by evaporation, or by other causes, more water 

 than it receives by the Nile, the Rhone, the Danube, and all the 

 rivers that empty themselves into it, and that the Atlantic Ocean 

 makes up the loss, in order to keep it at the height required 

 for equilibrium. 



The following are the results that have hitherto been obtained 

 respecting the level of the seas. During the French Expedition 

 to Egypt, a commission of engineers, under the direction of M. 

 Le Pere, determined the relative heights of the Red Sea, and 

 Mediterranean. This operation is worthy of great confidence ; 

 and, for result, it gives a very remarkable difference of level be- 

 tween these two seas, which are so near each other at the Isth- 

 mus of Suez, and which, besides, communicate with the Ocean. 

 At low water the Red Sea is 8 metres 12^' above the Mediterranean, 

 and at high water its excess of height rises to 9 metres 9". Thus is 

 confirmed the opinion of the ancients respecting the danger of 

 opening up a communication between the two seas. At the pre- 

 sent day, a great part of Egypt would be submersed by the 

 Red Sea ; and yet the bed of the Nile and the soil of Egypt are 

 constantly raised by the deposit of mud which every succeeding 

 inundation leaves. M. Girard has made very curious inquiries 



