On the use of the Marine Hydroyneter. 307 



specimens of their own country from foreign dealers. 1 have 

 received very complete specimens of the rare minerals of 

 Sweden in a present from Captain Svanenberg and Axel 

 Erdmann. All these, as I have already said, I have here in 

 Berlin. — {Mettheilungen der Natuforschenden Gesillschaft zu 

 Bern. N. bl and 58, January 1846.) 



On the Use of the Marine Hydrometer. By George Bu- 

 chanan, Civil Engineer, F.R.S.E., President of the Eoyal 

 Scottish Society of Arts. Communicated by the Royal 

 Scottish Society of Arts.* 



This is an instrument which I have found extremely useful 

 in inquiries connected with the prevalence of sea or river water 

 in different estuaries, wdth the view of determining the limits 

 of these waters in respect of the sea. This forms not only a 

 curious subject of investigation, but lias become of great 

 practical application in this country in connection with the 

 interests of the salmon-fisheries. These we know in rivers 

 are restricted in the modes of fishing, while in the sea they 

 may be carried on freely by any means of catching, such as 

 stake-nets or other fixed machinery. After the introduction 

 of this modern improvement in the fishing, the great question 

 arose, how to determine the limit between the river and the 

 sea, and by it to fix the point where the restrictions were to 

 be taken off, and the free use of fixed macliinery was to be- 

 gin. On this question much diversity of opinion has pre- 

 vailed ; and among other tests was that of the prevalence of 

 fresh or salt water. 



Having been engaged in various inquiries of this nature, 

 I found that for every purpose it was suflicient to test the 

 qualities of tlie waters by their specific gravities, this being 

 always an exact measure of the prevalence of sea or of fresh 

 water in any mixture. 



The specific gravities were accordingly measured by weigh- 

 ing each specimen in the usual way in a fine balance. But 

 this method being tedious, and ncai'ly inapplicable where a 



* Ifcud before tho Royal Scottish Society of Arts, .January 24, 1848.* 



