464 
Proceedings of the Royal Society 
sounding-line was used, by fixing a weight to the neck of the bottle 
and paying out line enough to let it lie flat. 
It was impracticable, on account of the time required, to use the 
delicate hydrometer for determining the density of the hundreds of 
samples taken in tracing the mixture of sea and river water ; and 
we were obliged to be content with occasionally reserving a bottom, 
intermediate, and surface sample for careful determination, while a 
small directly-graduated hydrometer was made use of in the boat. 
This hydrometer was marked to read “specific gravities” from 
1*000 to 1*030 at 60° F., but since only an indication of the salinity 
was required the temperature, which averaged about 58° and which 
often varied 1° or even 2° between surface and bottom, was 
neglected. 
Throughout this paper the density of water when given to the 
third or fourth decimal place ( e.g ., 1*026 or 1*0255) is only roughly 
approximate, and is not corrected for temperature ; the observations 
were made, as a rule, while the water had a temperature of from 
12° to 16° C. When it is given to five places (e.g., 1*02345) it is 
accurate to within 0*00005, and is calculated to 15° *5 6 C. whatever 
the original temperature of the sample was. The observed tem- 
perature of water and air is given in degrees Fahrenheit, in con- 
sistence with the usage of meteorologists. Instrumental errors are 
not allowed for, as the same thermometers were always used for air, 
surface-water, and deep-water temperatures respectively; the absolute 
uncertainty, which is a constant for each thermometer, does not 
exceed 0°*2 in any case. Time is reckoned from 0 hours, midnight, 
to 24 hours. 
Observations on Tidal Variations of Salinity. 
Numerous observations were made at a point about 100 yards 
inside the bar when tide was flowing, and some in the same position 
while it was ebbing. The mode of working was to anchor the coble 
in mid-stream, and, beginning at the surface, to take samples at 
every foot of depth, to observe the density with the small hydro- 
meter, and where there was a sudden rise in density between any 
two specimens, to examine samples from every 3 inches in that 
foot. By this means the vertical distribution of salinity was 
traced. After a few minutes the process was repeated, and so on 
