30 LEVEL OF THE SEA. 



water is deep and clear, lying on a dark-coloured, 

 rocky bottom; as is the case on many parts of the west 

 coast of Ireland. Where it is green the bottom con- 

 sists of yellow sand, whose colour, mixing with the es- 

 sentially blue tint of the water, has the same effect 

 as the blending of gamboge with Prussian blue on a 

 painter's palette. The white, red, or brown colours of 

 the water on our channel coasts are due to chalk, to 

 red sand and to mud. The variable redness of the Red 

 Sea, and the brown of the Antarctic Ocean, in the 

 neighbourhood of ice, are caused by the presence of 

 minute vegetables; and, to one or other of these causes, 

 all marine colours, other than green or blue, may be 



Speaking in a large sense, the level of the sea is the 

 same in all latitudes, and has been made the common 

 basis for determining the comparative heights of points 

 of land, which may be said to project, like minute in- 

 equalities, from the surface of the globe, whose figure 

 is determined by the uniform curve, assumed by the 

 waters under the influence of gravitation. Minute 

 differences of level, however, have long been recog- 

 nised in certain enclosed seas of limited extent. Thus 

 the Baltic, into which several great rivers flow, and 

 which is connected with the Northern Sea by a narrow 

 channel, is about a foot higher than that part of the 

 ocean ; and, except on rare occasions, after long-con- 

 tinued westerly or north-westerly winds, the current 

 sets strongly outwards. The Black Sea, also fed by 

 great rivers, and having a still narrower outlet, is con- 

 siderably higher than the Mediterranean. The latter, 



