52 THE WATER 



has made it a rule to take a surface sample every ten 

 miles ; but, on the other hand, an Atlantic liner could 

 do very valuable work by arranging for an observation 

 every four hours. 



Chemical and Physical Examination. 



Sea water consists of a solution of various salts 

 dissolved in pure water. The ratio these salts bear to 

 one another is generally so constant that, for most 

 oceanographical purposes, we may consider that it 

 remains unchanged while the total amount present 

 varies. It follows, therefore, that if we determine one, 

 say the chlorine, we can calculate the total weight of 

 all, and also the specific gravity. Tables have been 

 drawn up from time to time to enable this to be done 

 quickly ; the most recent are Knudsen's, which were 

 calculated on the basis of a number of careful experi- 

 ments for the International Council for the Exploration 

 of the Sea, and as it is of the greatest importance that 

 all modern hydrographical researches should be strictly 

 comparable with one another, the methods and stan- 

 dards of the Council should be followed accurately. The 

 work of analysis can only be carried out by, or under 

 the supervision of, a skilled chemist. Its details are 

 too elaborate to find a place in this elementary hand- 

 book, but the Society will be glad to advise in the 

 matter. 



A sea water can also be examined by direct de- 

 termination of its density at an exactly known tem- 

 perature. It should be stated here that the ordinary 

 hydrometer of partial immersion, in which a portion 

 of the stem appears above water, is absolutely useless 

 for modern work, and should be discarded. The 

 chief cause of its want of reliability is the great dis- 



