790 
Proceedings of the Royal Society 
2. On the Tidal Yariation of Salinity and Temperature in 
the Estuary of the Forth. By Hugh Kobert Mill, D.Sc., 
and J. T. Morrison, M.A., of the Scottish Marine Station. 
(Plate XXVIII.) 
The observations which were described in papers read by one of 
us to this Society in January and April 1885,* and others since 
made, though as yet unpublished, serve to establish the condition of 
the Firth of Forth in its seaward reaches as to salinity and tem- 
perature. Between Queensferry and the Isle of May the increase 
in salinity, and the increase or decrease of temperature, are slight 
and gradual; there is little difference between bottom and surface, 
and tidal effects are insignificant. Between Queensferry and Alloa, 
on the other hand, the change in salinity per mile is much greater, 
and the tidal range is very considerable. The isolated observations 
which had been made in this region, and especially about Kincar- 
dine, seemed to indicate that the same regime held there as in the 
estuary of the Spey,f only the effects were on a reduced scale and 
somewhat modified. 
It was necessary to test the applicability of this theory, and we 
lately spent a week at Kincardine for this purpose, working from 
the sailing-boat “Paracelsus ” between Kincardine and Cambus. 
The words river, estuary, firth, are used with considerable latitude, 
and it is advisable in considering salinity to employ them with 
precisely definite meanings. This cannot be very easily done if the 
terms be used in reference to the geographical position of a river- 
system; but, if we may suppose that the river proper may be 
shortened by the intrusion of the estuary and lengthened by its 
withdrawal, a sufficiently precise meaning can be given. A river , 
then, is a stream of fresh water, which by gradual increase of 
salinity may merge into an estuary, or by an abrupt accession of 
salt water may run directly into a firth or sea. An estuary is the 
region where the tidal current effects a mixture of river water with 
* Mill, “ On the Salinity of the Firth of Forth,” Proc. Roy. Soc. Edin., vol. 
xiii. pp. 29-64 ; “ On the Temperature of the Firth of Forth,” Ibid . , pp. 157— 
167. See Chart of Firths at page 28. 
t Mill and Ritchie, “ Physical Conditions of Rivers entering a Tidal Sea,” 
Proc. Roy. Soc. Edin., vol. xiii. p. 460-485. 
