1887.] Dr H. R. Mill on Salinity and Temperature of Firths. 261 
have not been able to obtain hitherto ; but in my work on the 
Spey with Mr Ritchie, on the Forth with Mr Morrison, and on the 
Dornoch Firth with Mr F. M. Gibson, I have fully tested the 
methods of studying the problem by means of observations of 
salinity and temperature. Salinity determinations by means of a 
very delicate hydrometer are certainly best in all cases ; but in 
many, especially at certain periods of the year, the thermometer 
gives an equally exact picture of the state of things, with far less 
trouble and the cost of much less time. The collection of one 
sample of water from a given depth, the bottling of it, determining 
the density, calculating and reducing the result, occupies by my 
method nearly 25 minutes, and cannot be finished on the spot 
where observations are being made. No less-exact determination 
of density is of permanent value, and it is obvious that the results 
obtained cannot be ascertained in time to be of service in directing 
the course of the observations. But the temperature can be found 
simultaneously at three or more different depths, and the correct 
result arrived at in rather less than five minutes ; hence, any sudden 
change or apparent anomaly may be detected and investigated at 
once. The combination of both methods is certainly best, but 
wherever the river water is a few degrees warmer or colder than 
that of the sea, I should emphatically recommend the use of the 
thermometer as the chief instrument for investigating the flow of 
the tidal currents. 
No reference has been made in the foregoing to Dr Gibson’s 
analysis of water samples collected in the region under consideration 
in 1883, and discussed in his Fishery Board Report. Our joint plan 
of work for 1886 comprised the collection of samples for chemical 
analysis and gravimetric determination of density. About 50 
specimens of water were collected, and the analysis is now pro- 
ceeding, under Dr Gibson’s supervision. 
I have to thank Dr Gibson for many suggestions in carrying out 
the part of the joint work in which I am more immediately con- 
cerned, and for his permission to publish separately the resume of 
the observational results obtained. 
