470 



REPORT — 1891. 



The Abertay Liglit- vessel is anchored in about 8 fathoms of water, 

 the depth varying from 6 to 10 fathoms according to the tide, at the 

 mouth of the Firth of Tay. Its position differs from that of any of the 

 other stations, most nearly resembling Oxcar, but subject to much more 

 intense and variable conditions. The tides run very strongly, and the 

 shifting of the sandbanks on both sides of the entrance to the Firth must 

 produce corresponding changes in the direction of the tidal streams. 



The tidal effects at Abertay are remarkably distinct and striking. 

 They are brought out in the following table, which extends and confirms 

 the conclusions drawn from the Oxcar and North Carr observations. 



It appears from this table that the average change of temperature on 

 the surface between morning and afternoon is about a degree C, and the 

 daily range of temperature on the bottom something more than half a 

 degree C. The changes are sometimes in one direction, sometimes in 

 the other, and the mean of the whole series shows a slight warming in 

 the afternoon. 



The tidal relation of the water is remarkably simple, and affects 

 the bottom almost equally with the surface. It may be summed up 

 thus : — 



In the warm months, when high tide occurs in the morning, and low 

 tide in the afternoon, the water in the afternoon is nearly a degree C. 

 warmer than in the morning — the average of the month of June shows 

 even 2°25 C. of increased warmth. But in the warm months, when low 

 tide occurs in the morning, and high tide in the afternoon, the water is 



