INTERNATIONAL FISHERY INVESTIGATIONS 591 



from the surface down to the bottom — by no means a simple 

 proceeding. Then the salinity — or salt contents — is also to be 

 determined at corresponding situations. Temperature records 

 are obtained by the use of delicate thermometers which have 

 previously been standardised under the direction of the 

 Central Laboratory. In order to ascertain the temperatures 

 at the sea bottom and at intermediate levels it is necessary to 

 procure a sample of water from the required depth in such a 

 manner that its temperature does not appreciably rise while 

 it is being obtained. The Nansen-Pettersen water-bottle is 

 employed for this purpose, and the perfection of this instru- 

 ment is one of the most creditable results of the international 

 investigations. This apparatus is sent down to the required 

 depth open, and when it has reached the level from which it 

 is desired to obtain the sample it is closed by means of a 

 "messenger," and the water, contained in an insulated vessel, 

 is then brought on deck and the temperature is read on the 

 contained thermometer. Salinity is determined from water 

 samples collected in a similar way by ascertaining the amount 

 of a standard solution of silver nitrate required to precipitate 

 the contained salt. From tables prepared by the hydrographers 

 of the Central Laboratory the total amount of solid matter 

 contained in a unit quantity of the sea water is then calculated. 

 Other properties of sea water are of course observed, but 

 these two — the temperature and the salinity — are sufficient to 

 define the hydrographical constitution of the water studied. 

 Similar information is afforded by the identification of the 

 plankton, or the microscopical fauna and flora of the water. 

 By means of such observations it has been found that the 

 water in different parts of the sea possesses slightly different 

 properties. 



In the North European area we find usually in the sea 

 water which may be derived from three main sources: (1) Gulf 

 Stream water, the salinity of which is always over 35 per 1000, 

 and the temperature of which is relatively high ; (2) colder 

 and less dense water from the Arctic Sea ; and (3) fresher water 

 from the great rivers and from the Baltic Sea. Over wide 

 areas these various components may be traced, forming more 

 or less well-defined drifts or streams, and often well-defined 

 strata which lie on each other in the order of their specific 

 gravities. They may remain distinct without mixing or they 



