MEASUREMEISTT OF SALINITY OF SEA WATER 6 



flow. Stir the water with an unpainted stick. Observe the tempera- 

 ture to the nearest tenth of a degree. Put the hydrometer into the 

 jar and steady it from spinning. Holding the top of the jar between 

 the thumb and first or second finger, allow the jar to hang freely, so 

 that the water surface is seen from below through the glass and the 

 liquid. Slowly lower the jar until the ellipse becomes a straight line, 

 which is the bottom of the water meniscus in the jar. The intersec- 

 tion of this straight line with the hydrometer scale will give the 

 correct reading of the hydrometer. This reading should be made 

 carefully to the nearest half division; that is, to one in the fourth 

 decimal place of specific gravity. 



Table 1. — Apparent specific gravity of sea icater 

 [As indicated by a glass hydrometer with no errors of graduation] 



Two or three readings should be taken, the hydrometer being dis- 

 turbed between readings, to insure against a reading made with the 

 bulb sticking to the side of the jar. It will be advisable to stir the 

 water and make a second temperature observation after the hydrom- 

 eter has been removed from the jar. The mean of the temperatures 

 taken before and after the hydrometer readings should be used. 



At the conclusion of the observations the hydrometer, thermometer, 

 stirring rod, and hydrometer jar should be well rinsed with fresh 

 water. 



From the specific-gravity and temperature observations the salinity 

 of the sample can be obtained conveniently by the use of the chart. 

 Figure 2, or by Table 1, below If the temperature is measured cor- 

 rectly to the nearest tenth of a degree centigrade and the specific 

 gravity to one in the fourth decimal place, the resulting value of 

 salinity will be correct to within one or two units in the third sig- 

 nificant figure. 



In using Figure 2, which will be more convenient than Table 1, 

 the point is found that corresponds to the observed temperature and 

 specific gravity, both corrected for instrumental errors. The posi- 

 tion of this point will show at once the first two significant figures 

 of salinity. The third figure is obtained by estimating the frac- 



