1887.] Dr H. R. Mill on Salinity and Temperature of Firths. 257 
run in this firth, and considerable variations were observed in the 
salinity of the water at high and at low tide. Temperature was 
found to fall uniformly towards the sea, the average being 55 0, 5 
at Alness, and 5 4° *5 at the Sutors, on the surface ; while on the 
bottom it was 54°*5 and 53° - 8 respectively. 
Table V , — Average Density of Water in Cromarty Firtli. 
Position. Alness. Invergordon. Bet. Sut’rs. 2 m.out.Sut’rs. 
Surface density at 15° *56 C., 1 ‘02308 1 ‘02331 1 ‘02465 1 ‘02515 
Bottom „ ,, 1-02405 1 ‘02445 1 ‘02530 1 ‘02546 
Number of cases, 12 7 5 4 
This shows a variation of density almost exactly the same for 
the 15 miles seaward from Alness as for the 15 miles seaward from 
Queensferry in the Firth of Forth. The identity extends to 
bottom as well as to surface water; but it must be remembered 
that the data compared are not really comparable, since they are 
on one side, the salinity of the Cromarty Firth in the middle of 
August 1886, and on the other the mean salinity of the Firth of 
Forth determined by numerous observations in 1884, 1885, and 
1886. Also, it must be pointed out that the salinity two miles off 
the Sutors of Cromarty is about equal to that at the Isle of May ; 
while five miles out in the Moray Firth, a salinity is found which, 
according to the German charts, is not to be met with nearer 
than 30 or 40 miles east of the Isle of May. 
The serial tidal observations at Alness are of considerable in- 
terest. There were two sets of these. The first on 5th August, 
for six hours, during the last three hours of flood-tide and the first 
three of ebb, brought out the exact equivalence of the curves of 
temperature and salinity, so that, substituting “ diminution of 
salinity ” for “ increase of temperature,” any statement with regard 
to tidal influence on temperature would be true of salinity also. 
This series was taken rather near the mouth of the Alness river, 
and sudden rushes of warm fresh water produced variations on the 
surface which were not found at any depth beneath it. The 
second series was taken from ll b 0 to 20 h 0 on August 12th, 
and as low water was at 16 h 0 it comprised five hours of ebb and 
four of flood. The resulting figures are given in Table VI., and 
represented graphically in PI. VIII. fig. 3. The densities of the 
