58 C. J. WHITE. 



density can be found with such accuracy, it follows that 

 such a form of hydrometer is peculiarly suitable for rapid 

 and accurate sea-water work, especially on board ship, 

 where the extra refined methods of a well equipped labora- 

 tory are inapplicable. Moreover, "in a sea-water whose 

 specific gravity is 1*03000, 1 in the fifth place represents 

 iroTo of the total solid contents, so by careful use of the 

 hydrometric method the salinity to one part in 75,000 of 

 water, or differences of 1 grain per gallon can be deter- 

 mined." 



Certainly as far as hydrometric work goes, these instru- 

 ments seem to approach the limit of accuracy and delicacy, 

 yet the most sensitive hydrometer has an inherent defect, 

 i.e., the uncertainty of the exact resting point of the partly 

 submerged instrument, due in part and presumably chiefly 

 to capillarity. In the present case the possible error from 

 this cause was of the order of 1 mm. of the stem. Thus 

 for specially accurate work other methods must be sought. 



A practical application of the instrument to the determ- 

 ination of the specific gravity of the Coogee sample will 

 exemplify its method of use. 



Vol. of hydrometer to mm. of stem at 22° O. = 185*01384 cc. 

 Volume of stem to 5 mm. ... ... ... '05810 



Total immersed volume 185*07194 



Weight of hydrometer + weights displacing 



same volume of sea-water at 22° C. = 189*5871 gms, 

 .*. Weight of 1 cc. at 22° O = 1*02445 



Tabular correction to reduce to 15*56 *00164 



.*. 4 S 15*56 = 1*02609 



For greatest precision a direct chemical method offers 

 the most satisfactory solution of the problem — such as the 



