CLYDE SEA AREA. 701 



uniformity of salinity is maintained, and the difference between surface and bottom water 

 kept so low as it is. 



In an average year the rainfall over the Clyde drainage area amounts (Table VIII.) 

 to 2'646 cubic sea-miles, of which it is estimated that nearly 1 cubic sea mile is removed 

 by evaporation from the surface of land or water, leaving 1*646 cubic miles to be with- 

 drawn seaward. The data given above appear to countenance the hypothesis that this 

 fresh water is got rid of, not by the flowing away of a very fresh superficial layer after 

 each shower, but by mixing thoroughly with the mass of water, and slightly diluting 

 the whole retiring tidal stream. It is remarkable that the whole mass of river water 

 accumulated in the Clyde Sea Area is considerably less than the amount which passes 

 through it yearly, and less than one half of the total annual rainfall. There are about 

 670 tidal rises and falls in a year, and this represents a total introduction and withdrawal 

 of 890 cubic miles of Channel water — enough to change all the water of the Area thirty- 

 five times, or once in ten and a half days — and to that we must add the withdrawal of the 

 1*646 cubic mile of fresh water brought in during the year, or 0'18 per cent, of volume. 

 The incoming water in a normal region, having an average salinity of 97'5 per cent, pure 

 sea water, the outgoing water would contain on an average only 97 '3 per cent. The 

 densities corresponding to these quantities are 1 '02535 and 1*02530 respectively, a 

 difference just too small to be certainly detected by the methods employed. 



If the rainfall over the entire Area were to cease for a year, and evaporation proceeded 

 at the ordinary rate assumed above — 25 inches — the result would be the removal of 

 0'3172 cubic mile of fresh water. This would increase the normal average salinity from 

 9 5 '5 to 97 '0 per cent, of pure sea water, and if the state of matters continued for a second 

 year the average salinity would increase to 98*4, and in a third year of complete drought 

 the area would be occupied by pure sea water. When the equalising effect of tidal inter- 

 change is taken into account, however, it is evident that the total cessation of rainfall for 

 one year would convert the whole contents into pure sea water. It is difficult to assign 

 the time that would be required to entirely freshen the Area, supposing evaporation 

 suspended, as the tidal intrusion of salt water would continue, and probably assume a 

 limiting value, beyond which any fresh water added would flow away on the surface and 

 leave a layer of comparatively salt water in the depths. 



A comparison of the actual salinity of each of the basins on the occasion of maximum 

 salinity in August 1886, and of minimum salinity in February 1887, presents many 

 features of interest. The data are given in Table XXXI. 



[Table 



