226 THE SEA FISHERIES 



The hydrographical work may conveniently be considered first, 

 though it is only of recent years that its importance from a fishery 

 standpoint has been recognised. This work, which deals with the 

 physical and chemical properties of sea water, first came into 

 prominence as a result of the famous " Challenger " expedition. 

 For the present purpose it is sufficient to consider the constitution 

 and movements of the water of our northern seas. As will be 

 gathered from the preceding chapters, the British sea fisheries are 

 carried on in the waters of a submarine plateau, which forms a 

 part of the European continental shelf. Most of this shelf under- 

 lies the seas of North-Western Europe, where the depth is not 

 more than loo fathoms. The characters which are of importance 

 in tracing the movements of large masses of sea water are the 

 temperature, salinity, density and the gaseous contents, these 

 latter being principally oxygen, nitrogen, carbon dioxide and 

 sulphuretted hydrogen. The determination of these characters, 

 and particularly the temperature and salinity, simultaneously over 

 large areas at regular intervals is one of the chief features of the 

 hydrographical work of the international council. 



Samples of water are, therefore, collected on the research steamers, 

 provided by the various Governments. The instrument employed 

 is the Pettersen-Nansen water-bottle, which affords a very good 

 means of obtaining representative samples of sea water from various 

 depths. The " bottle " consists of a number of concentric cylinders 

 of a non-conducting material, ebonite. These cylinders enclose a 

 central space in which a deep-sea thermometer is placed. The 

 bottle is lowered to the required depth in an open condition and 

 then closed by means of a messenger, a weight which slides down 

 the wire to which the bottle is suspended. Both the central and the 

 concentric cylinders are consequently filled with water from the 

 depth to which the bottle was lowered, and as the loss of (or gain of) 

 heat is practically negligible during the time taken to haul in the 

 bottle, the thermometer in the central chamber gives the tempera- 

 ture. The water is then carefully run off and bottled for subsequent 

 examination at one of the shore laboratories. The determination 

 of the salinity is one of the most important subsequent examina- 

 tions. An estimation of the halogens present is made by.precipi- 

 tating these substances with nitrate of sUver. The total solids m 

 solution are then calculated by means of hydrographical tables, 

 and the salinity is expressed as the weight of soHd sahne matter per 

 thousand grammes of water. By means of these hydrographical 

 observations it is possible to determine the movements of large 

 bodies of water, and for the details of the results akeady obtained 

 the original treatises should be consulted. 



