34 University of California Publications in Zoology [Vol. 15 



of a sinker suspended in water was developed by one of us, G. F. 

 MeEwen. Although the apparatus was not fully ready for use until 

 after all the samples herein tabulated were tested, a discussion of the 

 method is given for comparison with those just described. 



A small cylindrical glass sinker (s) , having a volume of about 25 e.c. 

 and being suitably weighted and hermetically sealed, is suspended by 

 a fine silver wire (0.09 mm. in diameter) from one arm of a Sartorius 

 chemical balance (sensitive to 0.1 mmg.) in a cylindrical funnel (c) 

 which is used for holding the water. A similar glass funnel (/) is 

 supported above the balance case and a tube {t), provided with a 

 valve (v), leads from the neck of this fimnel through the side of the 

 balance case and serves to carry the water into the cylindrical funnel 

 (c). In order to prevent the formation of air-bubbles the extremity 

 of the tube is tapered to an inside diameter of one millimeter and is 

 so arranged as to direct the stream against the wall of the funnel (c). 

 From the neck of this fiuinel a drainage tube (fZ) passes through the 

 side of the balance ease. 



The manner of operation is as follows: The funnel (/') is filled to 

 a certain height with the water sample and after allowing it to stand 

 until all the air-bubbles have been expelled the valve (v) is opened, 

 allowing the water to flow into the funnel (c) until the top of the 

 sinker is covered by about three-fourths of an inch of water. The 

 valve (v) is then closed and the next sample placed in the funnel (/'). 

 A thermometer graduated to tenths of a degree, which is always kept 

 suspended between the sinker and the wall of the fimnel (c), is then 

 read and the beam of the balance released for weighing. As soon as 

 the oscillation ceases the balance scale is read, after which the beam 

 is raised. As a check, the thermometer is again read and the weighing 

 repeated. If the two results agree the water is immediately drained 

 through the tube (cZ) into the bottle from which it was taken. After 

 the tubing has been thoroughly flushed with some of the water of the 

 next sample, everything is in readiness for the next test. 



In calibrating the sinker the theoretical formula was reduced to 

 the following simpler form, which is accurate to five parts in a million 



providing the specific gravity, S-, lies between 0.99 and 1.025 and the 



temperature at the time of weighing lies between 10° and 30° C : 



S^ =c+a{20°—6°)+k{w—w') 

 4 

 where c is a constant depending upon the value of «•', a and k cali- 

 bration constants, depending upon the sinker used, 6 the temperature 



