104 



Part III. — Mleventh Annual tteport 



20) ; and the densest liottom water was of specific gravity 1026-6 (3 p.m., 

 July 10 and Sept. 28), and the bottom water of least density 1023*8 

 (9 a.m., November 27). 



Dunbar Harbour. 



Observations were commenced at Dunbar in July ; and will be of 

 especial use in connection with the work of the zoological station at 

 Dunbar. 



The air temperature and that of the water are read twice each day at 9 

 A.M. and at 4 p.m. 



. The highest mean monthly temperature was in July — 13°*4 in the 

 mornings, and the lowest during the six months 6° '4 in December 

 mornings, the maximum reading being 13°*8 (July 21 and 24, Aug. 13 

 and Sept. 12), and the minimum 5°*8 on December 22 and 23. 



Oxear Lighthouse. 



This station is situated on a small isolated rock towards the northern 

 shores of the Firth of Forth, opposite the mouth of the Almond. 



The mean annual temperature of the surface water was 9 o, 0 at 9 a.m. and 

 9°'6 at 3 p.m., a degree higher than in 1890. 



During the month of July the surface water was hottest (14°'3 at 9 a.m. 

 and 15°1 at 3 p.m.), while in January it was coldest (3° -9 at 9 a.m. and 

 4° "3 at 3 p.m.), a range of 10° "4 in the mornings and 10° *8 in afternoons. 

 This is considerably greater than that of Abertay, where the difference 

 between the hottest and coldest monthly mean was only 9 o, 0, whereas here 

 it was 1 1°*2. 



The highest temperature was read on July 17 at 3 p.m. (17°*2), and the 

 minimum on January 10 at 9 a.m. (2 c, 4), giving a range between extreme 

 temperatures of 14°-8. These results again show that Oxcar is an excep- 

 tional station, for, whereas its mean annual temperature for 1890 was 

 nearly a degree lower than that of any other station, this year it is nearly 

 a degree higher. The mean annual temperatures of the other East Coast 

 stations were all lower in 1891 than 1890, whereas at Oxcar it is a degree 

 higher. A glance at the diagram (PI. XVII I. rig. 1) shows the change of 

 temperature from month to month at various stations. The general trend 

 of the 1891 curve, however, is much the same as that of the 1890 one, 

 although the maximum occurs in July (1891) instead of in June (1890). 



The mean density of the surface water at Oxcar varied from 1025-7 

 (9 a.m.) and 1025*6 (3 p.m.) in August to 1023*2 (9 a.m.) in November and 



1023- 4 (3 p.m.) in December (PI. XVIII. tig. 3). The mean for 1891 was 



1024- 5 (3-08 per cent, of salts) at 9 a.m. and 1024*6 (3*10 per cent, of salts) 

 at 3 p.m. Thus the water of the Firth of Forth 5 miles below the Forth 

 Bridge is salter than that at the mouth of the Firth of Tay. But the 

 extremes are not so great, Abertay water being often about as dense as that 

 at Oxcar, but also often very much lighter, so that the influence of the 

 volume of fresh water brought down by the Tay is more powerful where 

 estuary and sea join, than that carried by the Forth where the estuary 

 merges into the Firth. The difference between the two cases is indeed (as 

 has been previously pointed out by one of us) that between an estuary 

 properly so called, such as that of the Tay, and a firth in the stricter sense, 

 J ike that of the Forth. The extremes at Oxcar are from a maximum 

 specific gravity of 1025*9 (July 17, and August 4 and 17) to a minimum 

 of 1021*1 (Nov. 24). 



