of Edinburgh, Session 1885 - 86 . 
791 
that of the sea or firth; its length and position in any system 
depend on many conditions, and may vary slightly with rainfall, 
tidal range, wind, and other variable factors. In an estuary, salinity 
increases rapidly from river to firth or sea. A firth is an inlet of 
the sea connected with a river through an estuary ; * in it the water 
gradually increases in salinity from estuary to sea, and tidal effects 
are not strongly marked. The term river-system has been used here 
in a restricted sense to describe the connected system made up of 
four parts — river, estuary, firth, and adjacent sea ; but this does not 
affect its more general geographical meaning, which includes 
tributaries and feeding lakes. 
Applying these definitions to the Forth system, the river terminates 
in ordinary weather between Stirling and Cambus, the estuary 
stretches from the end of the river to about a mile east or west of 
Queensferry, and the firth then begins, extending to beyond the Isle 
of May. 
Examination of the estuary was conducted in the manner adopted 
for the Spey, only the temperature was observed more frequently. 
The boat was anchored in the channel where the depth varied from 
3 to 5 fathoms, according to the tidal phase ; samples of water 
were collected by means of a bottle, the stopper of which was 
pulled out by a string when it was sunk to the proper depth ; tem- 
perature was observed with the Scottish-frame Negretti and Zam- 
bra’s thermometer. Every hour, or half hour if the change was 
rapid, a series of temperature observations at distances of every 2 or 
3 feet from the bottom upwards was made, and if a considerable 
difference appeared between two positions, one or more intermediate 
observations served to define it exactly. Observations of density 
were made in like manner with a small hydrometer graduated 
directly to read “specific gravity at 60° E.” from 1*000 to 1*030; 
and samples were frequently kept to have their density carefully 
determined with the delicate hydrometer on shore. Observations 
were continued for from two to seven hours at a time, and were 
arranged so as to include the most interesting phases of the tide at 
positions between Kincardine and Cambus. 
* The inlet must be a narrow one relatively to its length, otherwise it is 
a bay or bight. The Moray Firth, for instance, is not a firth in our meaning 
of the word. 
