SPECIFIC GRAVITY TESTS 53 



turbance caused by the slightest trace of dirt either 

 on the stem or on the surface of the water. 



There are three other methods of determining the 

 specific gravity which give accurate results. One 

 may weigh a known volume of water in a pyknometer ; 

 or one may weigh a body of known weight and volume 

 in the sample of water ; or, finally, one may use the 

 total immersion hydrometer. Of the first two the 

 pyknometer method is the better, but both require 

 an accurate balance and weights. The total immersion 

 hydrometer is shaped like an ordinary hydrometer, 

 with an ungraduated stem of about i inch or | inch 

 long. To use it, it is loaded with small platinum ring- 

 shaped weights, which are dropped on to the neck or 

 on to a platinum hook fastened to it, until it floats 

 in midwater. The system is then of the same specific 

 gravity as the water. It is very accurate, and once 

 the apparatus has been standardized the balance is 

 no longer required, and the whole outfit is easily port- 

 able. The hydrometer and pyknometers should be 

 made with the same accuracy and care, as regards 

 the quality of the glass, as an accurate thermometer, 

 in order to avoid uncertain changes of volume. 



In all specific gravity determinations the greatest 

 difficulty is keeping a constant and accurately known 

 temperature throughout. The writer has found an 

 Ostwald thermostat of great value in such work. 



Current Measurement. 



For the measurement of currents below the surface 

 the Ekman meter is probably the most convenient 

 apparatus (Fig. 10). It consists of a hollow spindle 

 turning freely by means of ball-bearings on a wire 

 passing through its centre, and bears on one side two 



