of the Fisherif Board for Scotland. 



403 



year, being 0"*4 less at 9 a.m. and 0°*3 at 3 p.m., while the bottom tcm- 

 perature was 0°*2 lower in the afternoon only, the morning mean coinciding 

 with that of 1890. 



The highest temperatures were noted at the surface on July 18 at 

 3 p.m. (14°*6), and at the bottom on July 10 at 9 a.m. (18°*0) ; the 

 lowest readings of temperature were obtained on March 30 at 9 a.m., 

 when the surface water was 4° '3 and the bottom water 4°T. The 

 extreme range of temperature of surface water is thus 10° '3, and of 

 bottom water 13°*9. 



The water was warmest in July, and coldest in March. The mean 

 surface temperature varied from 13°*0 on July afternoons to 4°"9 on March 

 mornings ; that of the bottom from 13°*7 in July to 4°*7 in March. The 

 range of temperature was 8°T on surface, and 9°*0 at bottom, less than at 

 Abertay as regards the surface water, but more than at the Bell Jiock. 

 (PI. XVIII. fig. 2.) 



In the first ten days of July the temperature of the surface water aver- 

 aged 12°*0, whereas that of the bottom was as high as 16°*0 at 9 a.m. In 

 the afternoon the surface average was 12° "7, and that of bottom 14° '7. 



The temperatures of the air, of the surface, and of the bottom water 

 (24 fathoms), both for morning and afternoon, have been plotted, and the 

 curves are here reproduced (PI. XVIIL fig. 2) to show the connection 

 between them. It will be seen that while the water follows the air in 

 annual variation, the changes are slower and the range is less. The surface 

 water heats and cools more slowly than the air, and the bottom water still 

 less rapidly than the layers above it. 



During the spring and early summer months the bottom water is 

 usually colder than the surface water, which is colder than the air ; in 

 autumn and winter the surface water is warmer than the air, and the 

 bottom water warmer than the surface water. 



There are salinity observations for nine months only. Such deter- 

 minations are excessively difficult to make on board a lightship. During 

 these nine months the mean density at surface was 1025*1 (3*20 per 

 cent, of salts) at 9 a.m. and 1025*2 (3*21 per cent, of salts) at 3 p.m., while 

 at the bottom it was 1025*2 at 9 a.m. and 1025*4 (3*24 per cent, of salts) 

 at 3 p.m. The 9 a.m. surface water was densest in July (1025*4) • the 3 

 p.m. in July and September (1025*5); the bottom water was densest in 

 July and September (1025*6 in morning, 1025*7 in afternoon). The 

 minimum for the nine months was in November in the morning and in 

 December in the afternoon. (PI. XVIIL fig. 3.) 



Minimum Salinities at North Carr Rock. 





9 a.m. 



3 P.M. 



Surface .... 

 Bottom .... 



1024*6 in April.* 

 1024*7 in November. 



1024*6 in April.* 

 1024*9 in December. 



The percentage of salts in the water in which the lightship floats is 

 fairly constant, and evidently not greatly affected by the river water, 

 although it is not so invariable as that of the sea around the Bell Rock. 



The densest surface water was found to have a specific gravity of 

 1026*5 (Oct. 27, 9 a.m.), and the least dense 1023*5 (Nov. 4, 18, and 



* April 21-30 only. At 9 a.m., minimum of 1026*7 occurred in November, and 

 at 3 P.M., minimum of 1024*8 in December. 



