368 
Part III. — Ninth Annual Report 
the minimum was 1020 0 on March 19, 1891, a figure which was 
nearly reached on January 17, 1890, when 1020*5 was recorded. The 
main characteristic of this station is the uniformity of its salinity. 
Only four of the ten-day peiiods recorded in Table XII. show a 
lower specific gravity than 1024'0; these periods were January 21-31, 
and the whole month of November 1890. From February to October 
189(), the specific gravity hardly varied from a mean value of 1025-0. 
The efl'ect of tidal phase on salinity is naturally more marked tban 
that on temperature. All the observations were accordingly subjected 
to the same treatment as that described above with the following 
results : — 
OxcAR Lighthouse — Mean Tidal Effect on Specific Gravity. 
Specific Gravity. 
Specific Gravity. 
Month. 
High 
Low 
S9- S3 
Low 
High 
S3 - S9 
Water, 
Water, 
Water, 
Water, 
9 A.M. 
3 P.M. 
9 a.m. 
3 P.M. 
Sg 
S3 
S9 
S3 
1890. 
January, 
23-8 
22-3 
1-5 
24-0 
24-6 
0-6 
February, . 
24-9 
24-5 
0'4 
24-6 
24-9 
0-3 
March, . 
24-6 
24-4 
0*2 
24-5 
25-1 
o'6 
April, . . 
24-8 
251 
-0-3 
24-9 
25-0 
o*i 
May, . . . 
25-2 
24-7 
o'5 
25-1 
25-2 
June, . 
25-2 
247 
0-5 
25-0 
25-1 
July, . . . 
25-3 
24-9 
0-4 
24-9 
25-2 
0-3 
August, 
25-4 
25-5 
-O'l 
25-2 
25-6 
0-4 
September, 
October, 
24-5 
24 -3 
0'2 
25*1 
25-3 
0'2 
November, 
22-1 
227 
-06 
22-3 
23-3 
I 'O 
December, 
... 
1891. 
January, 
24-4 
24-3 
O'l 
24-4 
24-8 
0-4 
February, . 
23-9 
23-6 
24-9 
247 
-0-2 
March, . . 
24-4 
23 7 
07 
24-6 
24-9 
0-3 
Mean, 
24-5 
24-2 
0-3 
24-6 
24-9 
0-3 
Here we see, as might be expected, that the average range of salinity 
between high and low tide is the same, corresponding to a difference 
in specific gravity of 0*3. But we see also a somewhat unlooked for 
result, which is, that the salinity of the water is markedly greater 
in the afternoon than in the forenoon. This occurs at all times of 
the year, and cannot be assigned to the efi^ect of evaporation. Indeed, 
the magnitude of the difference — greater than that between high and 
low water — precludes the idea of evaporation being the chief agent 
producing it. , The explanation, probably, is that the spring tides 
at Oxcar occur about 4 p.m., and the neap tides about 10 a.m. Even 
granting this, there is the difficulty that the 9 a.m. low water due 
to spring tides appears to produce distinctly less freshening than 
the 3 P.M. low water due to neap tides. This matter demands further 
investigation, and may possibly be elucidated by considering the direction, 
speed, and duration of the tidal currents at different times of the 
month. 
The mean salinity at Oxcar may be put at 3*1 per cent, on the 
