METHODS OF OCEANOGRAPHICAL RESEARCH 259 



salinity its course can be roughly traced as it moves north- 

 wards. The physical state of the water and its chemical 

 composition have an extremely important bearing on the 

 life contained therein, and the temperature, density and 

 saltness of the water are being kept continually under 

 observation by those engaged in marine biological research 

 in different regions. 



The chief instrument for studying the temperature of 

 the sea is the thermometer. The study is a fairly simple 

 matter when it is only necessary to find the temperature 

 of the water at the sea surface. Water is merely dipped up 

 in a wooden bucket and an accurate thermometer placed 

 in it for two or three minutes ; it is important to use a 

 w^ooden bucket rather than a metal one, because metal is 

 such a good conductor of heat that in a short time it may 

 materially affect the temperature of the water contained 

 in the bucket and so lead to errors. 



But it is quite another matter when we wish to know 

 the temperature at fifty, 100 or even 1,000 fathoms. The 

 w^ater down to a certain depth becomes cooler as one goes 

 deeper, and the first difficulty to be overcome therefore is 

 that if a thermometer is lowered, say to fifty fathoms, 

 it will pass through warmer and warmer water as it is 

 hauled to the surface, and by the time it is examined it 

 will be registering quite a different temperature from that 

 actually occurring at fifty fathoms. This difficulty is 

 overcome by using specially designed bottles that are 

 thoroughly insulated. In addition, by means of a weight 

 known as a " messenger " which can be sent down the 

 wire, the bottle, which is lowered open at both ends, can 

 be closed (Plate 92). On its downward journey the water 

 merely flows through it, so that when it is closed it takes a 

 sample of water from that depth at which closing took 

 place. Owing to the insulation the water sample thus 



