390 



PHYSICAL CONDITIONS OF 



Tides. The crest of the tidal wave is almost simultaneous over the Clyde 

 Sea-area, high water occurring at the heads of the remotest lochs within 

 17 minutes of the time of high water at Lam lash in Arran. In the river- 

 there is more retardation, about 1^ hours in the case of Glasgow. The tidal 

 currents in the broad channels and at the heads of the lochs are rarely more 

 rapid than 1 knot per hour ; but in the narrow channels, and especially where 

 the depth as well as the \vidth is reduced, there are tidal races which give 

 rise to a choppy sea, and mix the water thoroughly from surface to bottom. 

 This is especially the case in the North Channel off the Mull of Cantyre 

 and at the entrance to the Gareloch, at both of which the spring tides run at 

 over 5 knots, while at Otter Spit at the entrance to Upper Loch Fyne the 

 ordinary spring tide attains 3^ knots. These velocities are increased or 

 diminished in a notable degree according as the wind conspires with or 

 blows against the current. 



Salinity. The saltness of the great body of the water in the Clyde Sea- 

 Area is remarkable ; the deeper layers, even at the head of the remotest 

 lochs, are nearly pure sea-water, while the brackish water on the surface 

 only forms a thin skin. The average salinity of the water is subject to change 

 by evaporation in dry weather, and by dilution in wet weather. The follow- 

 ing table shows the average and the extreme range of the salinity of the 

 whole mass of water in each division as observed in 1886-88: 



TABLE II. SALINITY OF WATER IN THE CLYDE SEA-AREA. 



The " pure sea- water " of which the percentage present is given above is 

 assumed to be of 35'00 per mille salinity, or of specific gravity S^^ =1 '02600. 



Although the salinities are calculated from specific gravities, the great 

 diversity in the forms expressing the latter factor is a sufficient reason for 

 using salinities for purposes of comparison either in the form of total salts 

 or per cent, of pure sea-water. In order to show the difference between the 

 salinity at surface and bottom and the slight influence exercised by the 

 former on the salinity of the whole mass of water a few selected stations are 

 compared in Table III. 



