1887 .] Dr H. R. Mill on Salinity and Temperature of Firths. 259 
appears there, and does not influence the surface for a considerable 
time. 
Dornoch Firth . — This firth is shallower than that of Inverness, 
and in addition to its being shut off, like the Firth of Tay, by a 
bar at its mouth, the channels inside are narrow, tortuous, ex- 
tremely shallow, and constantly changing on account of the sand- 
banks. The “ Garland ” navigated this firth under the charge of a 
pilot, and only two days were spent in it. It is impossible to say 
much regarding the variation of salinity with position, but this 
appeared to be more rapid than in the other inlets examined. At 
the Dune of Creich the density of the surface water was 1*01750, 
and that of bottom w T ater 1*01919 at high tide; off Dornoch, 
inside the bar, the surface had a density of 1*02395, and the 
bottom 1*02517 at \\ hours ebb, the distance from the Dune 
being 9 miles. A few miles beyond the bar a density of 
1*02588 reigns from surface to bottom. Temperature was high 
in the Dornoch Firth (over 57°), but rapidly fell as the sea was 
approached. 
A very complete set of observations was made off Ardjachie 
Point during the last 4J hours of ebb tide and the whole suc- 
ceeding flood, hourly readings being made for twelve consecutive 
hours. The data are given in Table VII., and the corresponding 
curves in PI. VIII. fig. 1. 
This is the most interesting record we obtained of the tidal move- 
ments of salt and brackish water past a given point. From 4J to 
1J hours before low tide both surface and bottom water grew 
gradually fresher, while maintaining nearly the same difference in 
salinity, i.e., the whole mass of water was moving seawards as a 
uniform current. At \\ hours before low water the rate of 
decrease of salinity in the bottom water diminished, that in the 
surface water increased, and the difference between the two grew 
greater. The second observation after low water showed a marked 
increase in the bottom salinity, while the surface was at its mini- 
mum ; this shows that the current on the bottom was slowed and 
reversed before the outward surface current was affected. During 
the next hour the surface water grew salter more rapidly, and then 
for two hours gained on the bottom water ; so that 4|- hours after 
low tide the water in the channel was nearly homogeneous, as far 
