HYDROGRAPHIC INVESTIGATIONS 57 



deep-water basins on either side of it all this, of course, being 

 in addition to the detailed exploration of the sea areas in 

 immediate proximity to the European coasts. 



The Wyville-Thomson ridge is known to separate two 

 oceanic depressions with widely different physical conditions, 

 and consequently a different fauna. Reference to these 

 investigations is made elsewhere. 



The International Hydrographic investigations have for 

 their object the investigation of the physical characters of 

 sea-water in this extensive area and even beyond it (Fig. 7). 



The chief physical characters which it was essential to 

 determine at the outset are (i) the temperature, at the surface 

 and at intermediate depths ; (2) the salinity that is, the weight 

 of solid saline matter in 1,000 grammes of sea-water; and (3) 

 the nature and abundance of the gases (oxygen, nitrogen, 

 carbonic acid, and sulphuretted hydrogen) dissolved in it. 

 Other properties of sea-water are also of importance, but the 

 hydrographic condition of a given volume of sea-water can 

 usually be determined from the three characters given above. 

 These characters vary from time to time in the same region 

 e.g., the water on the Dogger Bank has different temperature 

 and salinity in summer and winter; and also the water in 

 different areas, such as the Faroe-Shetland Channel and the 

 Cattegat, varies considerably. The determination of both 

 temperature and salinity, simultaneously over the whole area, 

 at periodic intervals was one of the first tasks of the Inter- 

 national Council. 



Each exploring vessel makes a special hydrographic cruise 

 at regular stated intervals, and the vessels start as near as 

 possible simultaneously. At the commencement of the inves- 

 tigations the cruises were made quarterly, the months chosen 

 being February, May, August, and November. Each country 

 had a special area allotted to it for investigation, and in the 

 case of Great Britain these areas were again subdivided. Each 

 of these areas and subareas was marked off in lines to be 

 traversed by the research steamer on its quarterly cruise. On 



