700 DR HUGH ROBERT MILL ON THE 



In the table the proportion of salts in the Estuary is an estimate founded on a small 

 number of observations, and does not claim to be so accurate as the others. In the 

 whole Sea Area, at low water, there were 5500 million tons of sea salt in the period 

 1886-87, and probably for a year of normal rainfall there would be 31 million tons less. 

 Each tide brings in and withdraws 1*3215 cubic miles of sea water, containing about 

 98 per cent, pure sea water, and about 290 million tons of sea salts. 



In ordinary circumstances it is evident that, if the average annual salinity of the Area 

 is maintained, as much fresh water must leave the system as enters in a year. The only 

 inlet of fresh water to be considered is rainfall, in its two forms of direct and occasional 

 precipitation on the surface and gradual and continual supply through streams. The 

 outlets are evaporation from the surface and outpouring into the open sea. It happens 

 that in this district the season when the evaporation is greatest is that in which rainfall 

 is nearly least, and when the rainfall is greatest evaporation is practically suspended. 

 This tends to intensify the annual extremes and leads to an accumulation of fresh 

 water in winter and early spring, when outflow is the only means of carrying it away, 

 and to a defect of fresh water in summer and early autumn, when evaporation comes 

 into the field as well. In winter the supply of fresh water is greatly increased, and the 

 means of removal diminished, while in summer the supply is greatly reduced and the 

 means of removal increased. 



The flood-tide is at many points of the coast less rapid than the ebb, and, although 

 the difference is too slight to be readily detected, the outflowing water must be somewhat 

 fresher than the inflowing. The action of the tide on the general salinity of the area is 

 to carry in salt water and carry out fresh water in the proportion required to maintain 

 the normal salinity at each season. It thus plays the part of a great sponge of salt water, 

 absorbing and removing a certain amount of fresh water at each application. The water 

 of the Channel, with 977 per cent, of pure sea water, is uniform in composition from 

 surface to bottom, in consequence of the overfalls and upwellings produced by the tidal 

 stream setting against the sloping seaward face of the shallow Barrier Plateau, on which 

 the incoming tide is mixed with fresher water to the average extent of 1 per cent. Here 

 the denser incoming water sinks to a lower level, and is covered by a layer of surface 

 water which is carried further inland by the flowing tide, becoming diluted as it proceeds. 

 The normal tidal effect in a wide open estuary like the Firth of Forth, is a sort of piston 

 action, the tidal current dams up and heads back the water of the estuary, so that any 

 given particle has only a short range to and fro ; but as the disparity of density between 

 surface and bottom water increases, the piston action changes to a wedge action, the 

 denser water ebbing and flowing underneath the fresher. 



The configuration of the Clyde Sea Area prevents the wedge action from setting in to 

 any extent, as the shallow bars separating the various basins act like the Barrier Plateau 

 itself, throwing up the bottom water and thoroughly mixing the whole section. It is to 

 this effect of configuration on a mass of moving water, and to the occasional and even 

 more effectual inversion produced by wind action in narrow inlets, that the remarkable 



