of the Fisliery Board for Scotland. 367 
Abertay Light vessel — Mean Tidal Effect on Specific Gravity. 
Surface. 
Bottom. 
iVlUiM 1 Xl. 
High 
Low 
Low 
High 
High 
Low 
Low 
High 
Water 
9 a.m. 
Water 
3 P.M. 
Water 
9 A.M. 
Water 
3 P.M. 
So- S« 
Water 
9 A.M. 
Water 
3 P.M. 
Water 
9 A.M. 
Water 
3 P.M. 
■R R 
S9 
S3. 
S9. 
S3. 
B9. 
B3. 
B9. 
B3. 
looy. 
J line. 
2.5-9 
23-4 
2*5 
23-5 
25-6 
2-1 
26-1 
25-2 
0-9 
24-5 
26-0 
I "5 
0 11 IJ . 
26-4 
25-1 
1*3 
25-0 
26-1 
i-i 
26-5 
25-9 
06 
25-4 
26-4 
I-o 
Aug. 
26-3 
24-9 
I "4 
22-6 
25-9 
3"3 
26 3 
26-1 
0-2 
23-7 
26-3 
2-6 
oepi. 
25-8 
23-9 
1-9 
22-6 
25-9 
3"3 
26-1 
25-1 
i"o 
23-7 
26-1 
2-4 
\Jyy\i. 
25-7 
24-0 
17 
21 -6 
25-5 
3 "9 
25-8 
24-9 
0-9 
23-2 
25-7 
2-5 
"Vav 
ov. 
25-9 
22-9 
3'o 
20-9 
25-7 
4-8 
25-8 
25-5 
o"3 
23-4 
25-9 
2-5 
25-7 
22-1 
3-6 
20-6 
25-6 
5"o 
25-9 
24-5 
I '4 
23-2 
25-8 
2-6 
1 son 
J SiR. 
24-4 
15-9 
8'S 
18-9 
25-2 
6-3 
25-6 
24-4 
1-2 
22-1 
25-6 
3-5 
r CD. 
25-1 
22-0 
31 
18-5 
25-2 
67 
25-1 
23-0 
2"I 
21-5 
25-8 
4*3 
March. 
24-9 
19-9 
5*0 
18-9 
24-9 
6-0 
25-5 
24-8 
o'7 
22-6 
25-4 
2-8 
April. 
25-3 
22-8 
2-5 
21-9 
25-6 
37 
25-5 
24-4 
i-i 
23-5 
25-6 
2-1 
May. 
25-5 
22-7 
2-8 
22-2 
25-2 
3"o 
25-7 
24-1 
1-6 
23-2 
25-7 
2-5 
June. 
00.7 
27 
9.7.0 
iio 0 
3'o 
iii> 0 
0 
I "I 
OQ.fi 
0 
1-9 
July. 
25-7 
22-6 
3"! 
21-4 
25-4 
4-0 
25-9 
24-2 
17 
22-7 
25-6 
2-9 
Aug. 
2.5-7 
23-6 
2-1 
22-7 
25-8 
3'i 
25-8 
25-2 
0-6 
23-9 
25-9 
2-0 
Sept. 
25-6 
23-7 
1-9 
22-8 
25-8 
3*o 
25-8 
25-1 
07 
23-9 
25-8 
1-9 
Oct. 
24-8 
21-5 
3 '3 
20-4 
25-2 
4-8 
25-5 
24-5 
i-o 
22-4 
25-4 
3-0 
Nov. 
23-9 
20-4 
3'S 
15-2 
24-9 
97 
25-2 
24-1 
I'l 
18-7 
25-3 
6-6 
"Dec. 
24-4 
22-0 
2-4 
21-2 
25-0 
3-8 
25-0 
23 9 
IT 
22-7 
25-0 
2-3 
1891. 
Jan. 
25-2 
23-7 
1*5 
22-7 
25-1 
2-4 
25-2 
24-5 
07 
23-3 
25-2 
1-9 
Feb. 
24-9 
20-4 
4"5 
21-3 
25-2 
3 '9 
25-2 
24-3 
0-9 
22-6 
25-3 
2-7 
March. 
25-3 
21-8 
3-5 
21-1 
24-9 
3-8 
25-3 
23-9 
I "4 
22-6 
25-2 
2-6 
Mean. 
; 25-4 
22-4 
3"o 
21-3 
25-4 
4'i 
25-7 
24-7 
i-o 
22-6 
25-7 
1 
3-x 
( High Water, 9 A. M B9-S<)=0-3. ( Low Water, 9 a.m Bo-Sg=l-3. 
) LoW Water, 3 p.m B3- 83=2-3. (High AVater, 3 p.m B3- 83=0-3. 
This shows us that the range of salinity is much greater for tlie after- 
noon high water (spring tides) than for the morning high water (neap 
tides). On the surface the specific gravity for high water, at whichever 
hour it occurred, was 1025*4; but the afternoon low water (neap) only 
fell to 1022-4, giving a range of 3'0, v/hile the morning low water (spring) 
fell to 1021-3, giving a range of 4-1. The ebb tide is thus the more 
powerful agent in atfecting surface salinity. In other words, the river water 
brought down by the ebb is more powerful in changing surface conditions 
than the sea water brought up by the flood. Hence we would expect 
that the range of temperature, already discussed, is brought about by the 
lowering of the morning temperature in winter, and the raising of the 
morning temperature in summer, more than by any great disturbance of 
the afternoon temperatures. That the ebb tide in such an estuary as the 
Tay is stronger than the flood can be readily understood from the great 
volume of fresh river water, the downward current of which is dammed 
back at Perth by the flood tide and released when the ebb sets in. 
The bottom water also attains the same salinity at high tide^ whether 
the tide be flood or neap, its specific gravity being 10257. At low 
water springs (L.W. 9 a.m.) the range at the bottom between high 
and low water is S'l, at low water neaps (L.W. 3 p.m.) the range is 
only 1*0. This demonstrates the much greater influence of the tide in 
affecting the conditions of the upper than those of the lower layers of 
water. At high water, whatever hour it occurred, the average difference 
in specific gravity between surface and bottom was 0.3 ; but at low water, 
at 9 A.M. (springs), the ditferetice was 1*3, and at low water at 3 p.m. 
(neaps) the average difference was as great as 2-3. This resulted from 
