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Part III. — Fifteenth Annual Report 



wind to produce great changes. When a strong down-loch wind had 

 been blowing for some time, water of high salinity was always found at 

 the head of the loch, proving upwelling from below ; and at stations 

 nearer the outlet, the surface salinity was often lowered. These effects 

 were much more clearly brought out by the study of temperature changes. 

 During the years 1886-88, twenty-three series of observations were made 

 along Loch Fyne, which allowed of as many sections being drawn, show- 

 ing the distribution by means of isotherms on a vertical plane. The 

 direction of these isotherms showed conclusively that a continuous strong 

 wind up or down the loch produced a definite tilt in the layers of water, 

 and the farther study of individual temperature curves made it apparent 

 that the circulation so set up was of a somewhat complicated kind. It 

 appeared, however, that circulation by wind was a phenomenon which 

 occurred irregularly, sometimes giving rise to great disturbances in a short 

 time, at other times leaving the water practically unaffected for long 

 periods. As the observations were rarely repeated at a short interval, and 

 as it was impossible to make them at given phases of the tide, the result 

 was incomplete and unsatisfactory, requiring further observations for 

 confirmation. 



One result was, however, quite clear. In the North Channel and on 

 the Plateau at the entrance of the Clyde Sea Area the tidal currents 

 mixed the water so completely that the temperature was always uniform 

 from surface to bottom at any given time, or, in a word, homothermic ; 

 while, in the deep water of the Arran Basin beyond, the thermal changes 

 were of a different type, indicative of a much restricted vertical circulation. 

 The bar at Otter was found to act in the same way as the Plateau at the 

 entrance to the Area ; and in the deep Upper Basin the temperature at the 

 bottom was found to change very slowly, the lowest temperature for the 

 year being found in summer and the highest in winter, the seasons being 

 opposite to those at the surface. I summed up the temperature observa- 

 tions as follows : — 



' The difference between the thermal changes in the Arran Basin and 

 1 Loch Fyne is mainly due to the restricted entrance and the much steeper 

 4 slope within the sill, preventing the free mixture of the water from out- 

 ' side, and also to the low salinity of the surface water in the upper 

 ' reaches. The resemblance of the Channel and Plateau to the Gortans 

 1 Basin is very strong. The upwelling at Otter keeps the Gortans Basin 

 1 supplied with nearly homothermic water of much greater salinity than that 

 * found at the same depth in the Upper Basin ; but on account of the steep 

 ' slope beyond Furnace this water appears to spread over the cold layers of 

 ' Loch Fyne instead of following the ground and gradually mixing with the 

 1 mass down to the bottom. There is, indeed, a certain amount of mixture, 

 ' as is proved by the variations in the salinity of the deep water. How far 

 1 this is due to tidal action we cannot say. Wind is certainly a more power- 

 { ful agent for setting up vertical circulation in the water than tide is, but 

 1 a steady wind in one direction rarely lasts long enough to produce its full 

 4 effect. The tides, on the other hand, act continuously, and I have been 

 ' led to modify the opinion stated in Part II., p. 706, that tidal influence 

 ' was insignificant as leading to the formation of deep currents in the 

 ' Upper Basin. I do not find, in the temperature observations, enough 

 4 data to found an exact theory upon, and the precise share of steady 

 ' tidal action and spasmodic wind-action in stirring the depths of the 

 ' Upper Basin must remain for the present undetermined.' 



