68 A Retrospect of Oceanography 



water at different temperatures, we may either take the series 

 made at the temperature nearest to that of the liquid, and 

 compare the two after making the necessary small corrections, 

 or we may construct a table by interpolation, giving the weights 

 required to immerse the hydrometer up to, say, every tenth 

 division of the stem in distilled water at different temperatures. 

 From such a table we should be able at once to find the weights 

 required to depress the hydrometer to the same scale divisions 

 as had been observed in the liquid, and from them obtain 

 the specific gravities. The table may, however, take another 

 form. The weight of distilled water displaced at every observa- 

 tion is known by the weight of the hydrometer and added 

 weights. If we know the volume of a cubic centimetre of 

 distilled water at all the temperatures covered by the experi- 

 ments, we have directly the volume of the immersed portion 

 of the hydrometer, and as such observations are made at 

 different temperatures, we obtain the volumes of the hydro- 

 meter at different temperatures, and its rate of expansion. 

 In constructing a table of the volumes of the hydrometer, 

 it should always be stated what factors have been used, so that 

 the absolute values depending on weighing alone can be 

 recovered. For all important or normal determinations, the 

 parallel series of observations in distilled water of the same 

 temperature should not be omitted. 



Assuming the correctness of our knowledge of the density 

 of distilled water of different temperatures, and deducing the 

 volume of the hydrometer from observations with it in distilled 

 water of known temperature, we obtain directly the volume 

 of a unit weight of the liquid, or its density; and for many 

 purposes this is convenient. 



The following table gives the results of determinations 

 of specific gravity of samples of Mediterranean water collected 

 in 1893 by H.S.H. the Prince of Monaco, which happened 

 to be made at identical temperatures with different hydro- 

 meters. The specific gravities given are each the means of 

 from nine to eleven separate observations on sea-water and 

 distilled water at the same time and at the same temperature. 

 The greatest difference between any pair of values is 3-3 in 



