ANALYSIS OF SEA- WATER FROM COOGEE. 57 



samples during the Challenger's voyage. The specific 

 gravity determinations of the present specimen were made 

 by a hydrometer (No. 6) presented to Professor Liversidge 

 by the designer J. Y. Buchanan, f.r.s. It is of the same 

 type as the particularly sensitive instruments used on the 

 "Challenger" and later on the "Princesse Alice." 



Buchanan gives a very instructive account of the use 

 and capabilities of these hydrometers in his "Retrospect of 

 Oceanography," 1 from which it may be of interest to give 

 the following brief summary. 



The instrument is made of glass and is provided with a 

 large cylindrical body (about 200 ccs. in volume) and a 

 very fine stem (about 3*5 mm. in diameter). In the manu- 

 facture the hydrometer is so weighted that at 30° 2 it floats 

 in distilled water up to a certain mark on the stem. To 

 make it float to the same mark in sea-water of the same 

 temperature, extra weights have to be added. The weight 

 of the hydrometer is also the weight of the water it dis- 

 places, hence we have the weights of equal volumes of sea 

 and distilled water at the same temperature ; the quotient 

 gives the specific gravity of sea- water (referred to distilled 

 water at the same temperature). Stem divisions serve to 

 avoid the use of very small weights. For the sake of con- 

 venience the specific gravity is usually expressed as a density 

 relation between sea- water at 15*56° C. and distilled water 

 at 4°. Such a " specific gravity" is denoted by 4^ 15*56. 



By means of such an instrument Buchanan claims that 

 the specific gravity of sea- water can be found to ± *00001. 

 As the salinity varies directly as the density, and the 



1 Eeprinted from the "Antarctic Manual," p. 901. 



2 In the instrument used (No. 6) the temperature corresponding to this 

 position of equilibrium was 29*1° C. — probably due to a slight contraction 

 in the body of this hydrometer — analogous to that of the bulb of a 

 thermometer on long standing. 



