WITH THE SALT WATER OF THE SEA. 511 



Although the Frith of Tay is very ill calculated for experi- 

 ments of this kind, from the circumstances already taken no- 

 tice of, still the premises which we have stated seem to war- 

 rant the conclusion, that when the wave of the tide obstructs 

 the motion of a river, and causes it either to become station- 

 ary, or to move backwards, the effect is produced by the salt 

 water presenting to the current of the river an inclined plane, 

 the apex of which separates the layer of fresh water from 

 the bed of the channel, and suspends it buoyant on the sur- 

 face *. 



It may here be observed, that this inferior current of salt- 

 water, will never reach that point of the bed of the river, which 

 is intersected by a line drawn perpendicular to the altitude of 

 the wave of the tide, in the ocean, at the mouth of the river. 

 This point is undoubtedly the place at which the salt-water 

 would arrive, at every flood, were there no fresh-water current, 

 as has been demonstrated with regard to the waters of the 

 Tay, by the accurate observations of Mr James Jardine. 

 But as the motion of the current of salt-water is retarded by 

 the opposite current of the fresh-water, and the apex of the 

 wedge which it forms, also washed away by the same agent, 

 the point which the salt-water reaches will be considerably 

 lower than the summit of the tide-wave with which it is con- 

 nected. 



The surface of the higher part of the river, whose elevations 

 and depressions are influenced by the movements of the tide, 

 will necessarily attain a higher level than the summit of the 

 tide-wave, in consequence of the lower specific gravity of the 

 river-water, when compared with the denser column of sea- 



wateiv 



* I understand that my friend Mr Robert Stevenson has made similar obser- 

 vations at the mouth of the Dee, near Aberdeen, and also on the Thames, and that 

 his conclusions and my own nearly coincide. 



