of Edinburgh, Session 1885 - 86 . 
469 
The data already given represent the actual manner in which the 
salt or slightly freshened sea water fights its way up the bottom of 
the river from the sea, while the fresh river water runs down above. 
The greatest distance up the stream to which salt water penetrated 
on the bottom was not satisfactorily determined, but it was 
certainly less than half a mile. At high tide a very thin layer of 
salt water was found at the bottom in a deep hole close to the 
west bank of the river about a quarter of a mile from the mouth. 
On the east side the shallow backwater (fig. 1) was found on a 
calm still day to be filled with perceptibly brackish water at two 
hours’ ebb, the fresh upper water having evidently run off most 
quickly. On another occasion, when there was a gale blow- 
ing the fresh surface water into the backwater, the salt layer 
flowed out from the bottom during the first half-hour of ebb 
(Plate XIII. fig. 4, and Plate XIV. fig. 4). In this case the 
pressure of the accumulated water at the upper end of the cul-de- 
sac evidently set up a reaction current outwards, thus reversing 
the ordinary mode of emptying. 
In the open river the sea water appears always to flow and ebb 
under the fresh water ; but some shipmasters, who know the Spey 
well, told us that at the ebb the river often “ran salt” for a 
considerable time, the fresh upper water flowing away first : no 
such phenomenon was presented, according to them, on the flood 
tide, We looked specially for this effect, but did not observe it. 
On one occasion, August 11, two hours before high water, the 
density from the bottom up to 2 feet from the surface was that of 
sea water (over P024), and the river was brackish even at 6 inches 
from the surface. A strong wind was driving the river water out 
to sea on this occasion, and so stripping off the usual layer of fresh 
water. On another day, August 8, when the ebb was unusually 
rapid, there was a depth of 6 feet of perfectly fresh water lialf-an- 
hour after high tide. 
The salinity at any depth during flood or ebb tide is influenced 
by a number of conditions, such as the density of the sea water 
coming in, the rate of the out-going current, the height of the 
tide, the wind, and other variable factors. Supposing, to take 
the normal conditions, that just outside Spey bar, where the depth 
is about 1 fathom, the lower f fathom has a density of 1*0250 at 
