of Edinburgh, Session 1885 - 86 . 
483 
1 . Rivers luithout Firths , such as the Spey. — In such rivers the 
water inside the "bar is entirely fresh during the greater part of ebb- 
tide, since all the sea water which forces its way up the short 
estuary during flood- tide runs out again along the bottom, and does 
not at any time render the surface layers brackish. The fresh 
water, running out at all states of the tide, spreads over the surface 
of the sea, and persists in a thin layer or in patches to a consider- 
able distance, overlying water of uniformly high salinity. The con- 
ditions as to temperature are not fully known, "but except during 
certain periods in spring and autumn, the river water is either much 
warmer or much colder than that of the sea. 
2. Rivers with narroiv, shallow Firths , such as the Tay. — The bar 
lies out at sea, where the water is always salt. The tide exerts a 
great influence throughout the estuary, the sea water being only 
partly withdrawn by the ebb ; and the currents, setting up and 
down with great velocity, effect a tumultuous though transitory 
mixing of the water. During ebb-tide the water inside the bar is 
comparatively fresh at the surface, and the ridge it has to pass over 
in reaching the sea directs it out over the surface of the salt water 
in a btackish layer, like a river of the first class. The temperature 
relations are not well known, but appear to be similar to those of 
estuaries of the third class. The range is, however, greater, and 
variations more irregular. 
3. Rivers with wide and deep Firths , such as the Forth. — Here 
the sea water is never withdrawn from the estuary by the tide, and 
the tidal currents are less rapid than in rivers of the second class. 
The landward portion (the “river proper compartment” of engineers) 
is comparable to a river of the first class, but the conditions are 
much less distinctly defined. Further seaward comes a division 
(the “ tidal compartment ”) resembling an estuary of the second 
class, without a bar, but exhibiting all its other characteristics in a 
modified degree. Finally, nearest the sea, and comprising most of 
the firth, there is the region specially characteristic of this kind of 
river entrance. The difference of density between surface and 
bottom is slight, and it is scarcely affected by the tides ; the water 
throughout its whole mass is slightly freshened, and so meets the 
true sea water beyond as a great wall of only slightly reduced 
salinity. The curve representing the salinity of the surface water is 
