of the Fishery Board for Scotland. 



269 



Fishery Board direct-reading hydrometers. The readings were reduced 

 to a constant temperature 17°'5 C, by a correction obtained by direct 

 observation, so as to be useful for comparison, inter se. These densities, 

 although published, are not supposed to have any absolute value, and 

 should not be compared with densities taken at other times or in other 

 places. They are quite trustworthy, however, as showing the changes 

 of salinity due to tidal phase. An attempt was made to measure the 

 velocity of the current at each observation by timing a chip floating 

 alongside between measured marks on the gunwale. This was successful 

 only while it was calm ; and as a strong breeze was blowing most of the 

 time, the vessel did not swing true to the tide, but lay nearly across it, 

 thus making it impossible to carry out the experiments. The strength 

 of the wind also deprived the observations of much of their value as 

 indications of the tidal currents. On September 8th observations were 

 begun shortly before half-flood, and continued to the following low-water. 

 They thus covered half a flood and the whole ebb. Before high-water 

 it was calm, but the ebb was accelerated by a fresh easterly breeze. The 

 strength of the current round the point of Otter Spit was tremendous, 

 two men being unable to row a light boat against it. On the 10th 

 observations were commenced at low- water, and continued until close on 

 high- water, thus covering the whole flood-tide; but the tide was retarded 

 by an easterly wind rising to a gale, which made it difficult to read the 

 instruments accurately on account of the rolling of the vessel as she lay 

 between wind and sea. 



On the 9th, at half-tide, the surface temperature was 55°*5, while that 

 at the bottom was 52°"3. As flood-tide proceeded, the surface layers grew 

 cooler, and the deeper grew warmer, indicating mixture of the water, 

 until at high-tide the surface and bottom temperatures were 54 0, 7 and 

 53° *4 respectively. As ebb-tide set in, the water became more and more 

 nearly homothermic, until at low-tide the range was only frcm 54° - 3 to 

 53° "6. The study of the flood-tide next morning showed the first rush of 

 homothermic water inwards past the ship, but this gradually gave place 

 to water warmer above and colder below, until at half-flood the surface 

 temperature was 54° "6, and that at the bottom 52° *3 ; then mixture began 

 to occur, and the last two hours of flood, like the first two, consisted of 

 nearly homothermic water. Remembering that a temperature of 52°*3 

 was only found outside Otter bar, at a depth of 30 fathoms, it is demon- 

 strated that the effect of flood tide is, first, to drive back the mixed and 

 therefore homothermic water which had passed out of Gortans Basin on 

 the ebb, and then to draw up the deep, cool layers, from the Arran 

 Basin, which, at half-tide, are being forced into Gortans Basin ; but soon 

 the upwelling water becomes mixed, and the last two hours of flood 

 carry in freshly-mixed water coming from all depths. The commence- 

 ment of the indraught of deep water from outside seems to correspond 

 with the covering of Otter Spit about half flood, thereby relieving the 

 rush through the Narrows ; the subsequent slacking of the surface 

 current before high-water might be considered to relax the upward pull 

 on the lower layers, and so admit only the homothermic surface water. 

 The ebb appears to consist entirely of almost homothermic water, as is 

 necessarily the case when we consider the homothermic character of the 

 Gortans Basin. 



The range of salinity being much less than that of temperature, the 

 density observations are much less satisfactory as indexes of circulation ; 

 but their indications, as seen by the table below, point in the same 

 direction. 



