Since 



The Sea-water and its Physical and Chemical Properties 43 



S, X 10-3 



and 

 then 



"35. 0. = 0-97264, 



A^^^ = 0-02736 



^.,t= 1 + 



X 10- 



1 + a, X 10-3 



1 + a, X 10-3 



The values of the three terms J j,,, 6,,,^ and S,,^, can be given in short tables from which 

 the anomaly can be found correct to five decimal places. The same accuracy can be 

 obtained by accurate graphical methods or with the ingenious slide rule of Sund (1929). 



The usual method for determining the density in oceanography is by calculation 

 from the temperature, the salinity and the pressure. The physical methods of de- 

 termining density such as the hydrostatic weighing and the pycnometer are unsuited 

 for oceanographic purposes, but the hydrometer has however often been utilized in 

 oceanography. Some very troublesome sources of error present with the ordinary 

 stem hydrometer have been discussed in detail by Krummel (1900), Buchanan (1884) 

 and Nansen (1900). They originate from insufficient attention to temperature differ- 

 ences between the instrument and the water sample and within the water sample 

 itself, the variable wetting of the instrument (traces of oil on the surface), the air 

 content of the water sample and not least to the variable capillary rise of the water 

 in the stem of the instrument which is often difficult to allow for. With proper use this 

 instrument gives values for a^ correct to two units in the second decimal place. Nan- 

 sen (1900) avoided the errors due to varying surface tension at the stem by using a 

 "hydrometer of total immersion" in which the ffoat is balanced in the water sample by 

 the addition of suitable weights. This method gives a^ correct to the third decimal 

 place (SvERDRUP, 1929). Since work with small weights is inconvenient on board 

 ship O. and H. Pettersson (1929), used a diff"erent method of loading a float hydro- 

 meter which is very simple and requires no handling of the float. A fine chain is sus- 

 pended from the float (chain hydrometer) so that the length of chain supported above 

 the bottom is a measure of the density. 



Another method for the direct determination of the density which has been used in 

 older investigations (Pulfrich refractometer) utilizes the difference in refractive index 

 of the water sample from that of distilled water. This is measured either by the Hall- 

 wach method or by interferometry. The first method was used by Krummel (1889) on 

 the "Plankton" Expedition and later in 1892 by Drygalski on the Greenland Expedi- 

 tion. The interference method is more sensitive, although it requires suitable labora- 

 tory work to give the desired accuracy. (Askania Interferometer, Bein, Hirsekorn and 

 Moller, 1933, 1935). This interference method has been developed to give greater 

 precision and will give the density to the third decimal place in a^. 



As well as the optical refractivity it is also possible to use the electrical conductivity 

 for the determination of densities. This method has several times been recommended 

 but has seldom actually been used. A survey of these experiments has been given by 

 Bein (1936). An instrument suitable for routine use was first developed by the Bureau 

 of Standards in Washington (Thuras, 1918; Wenner, 1930). It was in continual use 

 by vessels of the Ice Patrol in the North Atlantic Ocean from 1921 and was used by the 



