of the Fishery Board for Scotland. 357 
OxcAR Lighthouse — Mean Tidal Effect on Temperature. 
Temperature. 
Temperature. 
Differ- 
Month. 
ence. 
S3 - Sg. 
High Water, 
Low Water, 
S3 - S9. 
Low Water, 
High Water, 
9 A.M. 
3 P.M. 
9 A.M. 
3 P.M. 
S3. 
S3. 
1890, 
January, . 
6*0 
67 
07 
6 "2 
6-4 
0'2 
February, . 
4"6 
4 '8 
0'2 
4*9 
6*6 
07 
March., 
6"3 
6 '8 
5 "4 
5*5 
O'l 
April, . 
7-2 
7-7 
J 
7-0 
7-0 
O'O 
May, . . 
10-4 
11-3 
O'Q 
9-6 
9-3 
— O'^ 
June, . . 
11-1 
11-7 
0-6 
11-2 
10-8 
-0-4 
July, . . 
10-7 
12*3 
1-6 
10*8 
11 '4 
o'6 
August, 
9-7 
10-5 
0-8 
10-8 
10-9 
0"I 
September, 
October, . 
8-b 
9'b 
I'O 
8-b 
8-6 
November, 
7-4 
7-6 
0'2 
6-5 
7-2 
07 
December, 
1891. 
January, . 
3-4 
3-9 
0-5 
3-8 
5-2 
I '4 
February, . 
5-4 
5-4 
00 
4-9 
51 
0-2 
March, . . 
4-7 
5-4 
07 
4-0 
4-6 
0-6 
Mean, . . 
7-3 
7-9 
0-6 
7-2 
7-5 
0-3 
This shows that the average rise of temperature of 0°*4 is by no means 
uniformly produced. When high- water occurred in the forenoon and low- 
water in the afternoon, the average amount of heating was 0°"6 ; when 
low-water came in the forenoon and high-water in the afternoon, the 
amount of heating on the average was only 0°'3. In other words, there 
is on the whole a tendency for high-tide to produce a fall of temperature 
This is easily understood if the river and shallow-water is, on the average, 
warmer than the deeper sea-water. At low tide the warmer river-water 
comes down, raising the surface temperature ; at high tide the cooler sea- 
water streams up, lowering the surface temperature. In the foregoing table 
the result of the cooling effect of the seaward-water was shown to have 
neutralised the natural heating effect of the sun in May, and to have 
caused a fall of temperature in the afternoon of 0°'3 and 0°*4 in June 
and July. In winter, however, we know that the sea is warmer than 
rivers ; but the Oxcar results do not show, except in the month of Feb- 
ruary 1891, any marked rise of temperature with morning high-water. 
The specific gravity observations are of much value in throwing light 
on the physical changes taking place in the water. The ten-day 
means at Oxcar are given in Table XII., the figure 10 being in each 
case supplied. Thus 25*4 stands for 1025 "4, pure water having the 
specific gravity lOOO'O. The salinity is calculated as the percentage 
by weight of total dissolved salts. The monthly means are plotted 
for each station with regard to specific gravity and salinity in Diagram 
III. Plate XVII. 
At Oxcar, the average specific gravity for 1890 was 1024*6 at 10 a.m., 
and 1024'7 at 4 p.m., and for the first three months of 1891 it was 
1024-4 at 10 A.M., and 1024 6 at 4 p.m. Unfortunately the observa- 
tions at the North Carr Lightships are not available for comparison. 
The maximum observed at Oxcar was 1025*9 on August 1, 1890, 
