340 On the Level of the Sea. 



the series of observations which he made with the same object 

 in 1812. All these measurements agree in placing the level of 

 the Caspian Sea much beneath the level of the Black Sea. Ac- 

 cording to the mean result, this difference may be estimated at 

 100 metres, or about 325 feet. Yet along the shores of the Cas- 

 pian Sea, and to a great distance from its present banks, there 

 are striking proofs to be seen of the abode of salt water. The 

 nature of the ground, its form and chemical composition, the 

 remains of shells, and the skeletons of fishes, with which it is fil- 

 led, seem to leave no doubt remaining, that the sea formerly 

 covered all these steppes to a distance of several hundred leagues. 

 How has the depression of level which is now observed been pro- 

 duced ? What is become of the mass of water which is wanting, 

 and which may be estimated as a volume of 30,000 square leagues 

 of surface by a metre of height ? These are problems, the solution 

 of which it will take a long time to effect ; for they are connected 

 with general geology, and perhaps with the great catastrophes 

 of which Caucasus has been the theatre. 



The mixture of the water of rivers with that of the sea, 

 also presents some hydrostatical phenomena, which it is curious 

 enough to observe. Fresh water being lighter, ought to keep 

 at the surface, while the salt water, from its weight, should form 

 the deepest strata. This, in fact, is what Mr Stevenson obser- 

 ved in 1818, in the harbour of Aberdeen, at the mouth of the 

 Dee, and also in the Thames near London and Woolwich. By 

 taking up water from different depths, with an instrument in- 

 vented for the purpose, Mr Stevenson found that, at a certain 

 distance from die mouth, the water is fresh in the whole depth, 

 even during the flow of the tide, but that a little nearer the sea, 

 fresh water is found at the surface, while the lower strata con- 

 sists of sea water. According to his observations, it is between 

 London and Woolwich, that the saltness of the bottom begins 

 to be perceptible. Thus, below Woolwich, the Thames, in place 

 of flowing upon a soUd bottom really flows upon the liquid bottom 

 formed by the water of the sea, with which it is no doubt more or 

 less mixed. Mr Stevenson, however, is of opinion, that, at the 

 flow of the tide, the fresh water is raised, as it were, in a single 

 mass, by the salt water which flows in, and which ascends the bed 

 of the river, while the fresh water continues to flow toward the sea. 



