WITH THE SALT WATER OF THE SEA, 5it 
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 *. < 7 . 
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 Jarprne. 
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- 
water, 
* T understand that my friend Mr Rosert Srevenson 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. 
