of Edinburgh, Session 1885-86. 
471 
of nearly equal depth, while the “ salt ” water below increases from 
a mere film on the bottom until it fills more than half of the river 
bed at the point of observation. The interface between the fresh 
and salt water is much sharper at some times than at others. We 
were anxious to find the direction of the plane of contact when 
it was clearly marked, and to see how it varied at different places, 
but to make an attempt of this kind a number of observers 
working simultaneously would be required, and these we could 
not command. 
The theory to which our observations have given rise regarding 
the mixture of salt and fresh water inside the bar is one which, 
although not proved, seems to be at least probably true. It is that 
the fresh water running down the inclined bed of the river meets 
the wedge of salt water pushing its way up and acting in virtue of 
its superior density as a sort of soft false bottom sloping in the 
the opposite direction to the inclination of the river bed. The 
lower layers of the fresh Water have now to run along a level sur- 
face or even to force their way up-hill, but in doing so the friction 
between the two tears off thin streams of salt water, and mixes it 
with the fresh to form the brackish layer. This is represented 
diagrammatically in Plate X V. fig. 1 . The part that diffusion plays 
in causing mixture is probably a small one, but it must be increased 
by the gouging action of the rapid rush of fresh water on the slow 
push of the salt wedge.* 
Observations on Salinity in Spey Bay. 
The work outside the river was seriously hampered in conse- 
quence of Spey bar being impassable during the least swell, and of 
the dependent fact that there are no fishing boats at Garmouth or 
Kingston. Once we went out in a little open boat, and on two 
occasions in a half-decked yawl from Port Gordon. The rest of 
the time devoted to outside work was spent on the shore taking 
samples of water in the surf, westward and eastward, from the 
* Since writing the above our attention has been directed to a paper by 
M. Adolphe Guerard, of Marseilles, read this summer (1885) and printed in 
vol. lxxxii. of the Minutes of Proceedings Inst. C.E . , pp. 805-836, in which ho 
employs (p. '334) almost identical expressions to describe the manner in 
which a tidal river is affected by the sea. His treatment of the subject is 
entirely from the engineer’s point of view. 
