84 



U. S. COAST AND GEODETIC SURVEY 



following: will be determined by the first numbered scale graduation 

 above the surface of the water, and if this is less than 10, it should 

 be prefixed by a zero (0) . The last figure, representing 

 the fourth decimal place in the reading, is determined 

 by the smallest subdivisions, each of which represents 

 a change of 0.0002 in density. If the density of the 

 water is less than unity, the printed figures 1.0 in the 

 form should be stricken out and the reading as illus- 

 trated in figure 30 substituted. 



265. The water whose density is to be tested is poured 

 into the jar provided for the purpose and the hydrome- 

 ter floated in the same. A thermometer is then placed 

 in the jar to obtain the temperature for correcting the 

 density reading. A sufficient time should be allowed to 

 elapse to permit the hydrometer, thermometer, and re- 

 taining jar to acquire the same temperature as the water. 

 In reading the hydrometer the eye should be brought 

 to the level of the surface of the water and the reading 

 taken which appears to coincide with the level surface. 

 After using, the jar and instruments are to be carefully 

 cleaned to prevent the accumulation of salt. 



266. Reduction of density. — The density of sea 

 water as observed depends not only upon the amount 

 of solube matter contained in a unit volume but also 

 upon the temperature of the water at the time of obser- 

 vations. It is. therefore, necessary to reduce the ob- 

 served densities to some standard temperature in order 

 that they may be comparable and indicate the amount 

 of matter held in solution. 



267. The table on pages 86-87 gives a series of differ- 

 ences to be applied to the observed densities in order 

 to reduce tliem to a standard temperature of 15° C. 

 The table, which is based upon data given in appendix 6 - 

 of the United States Coast and Geodetic Survey Keport 

 for 1891. is applicable to readings of a hydrometer 

 standardized at a temperature of 15° C. with reference 

 to unit density at 4° C. If the hydrometer used is 

 standardized at some other temperature or refers to 

 another unit density, a further correction will be neces- 

 sary. The tabular differences include the correction 

 due to the expansion or contraction of the hydrometer 

 itself as well as the change in the density of the water 

 arising from changes in temperature, and should be 

 applied according to sign to the observed hydrometer 

 readings. 



268. The differences in the table, which are expressed 

 in ten-thousandths of a unit, are given for each whole 

 degree of temperature from 0° to 35° C, and for each 

 change of 0.0010 in the density from unity to 1.0310. 

 For observed densities less than unity, the top line of 

 the table may be used without material error. The fol- 

 lowing example illustrates the use of the table : Suppose 



a hydrometer reading of 1.0244 has been taken when the temperature 



^-: 



0.9960 

 0.9962 

 0.996t 

 0.9968 

 : 0.9968 

 ; 0.9970 

 : 0.9972 

 0.9974 

 0.9976 

 0.9978 

 0.9980 

 0.9982 

 0.9984 

 0.9986 

 0.9988 

 0.9990 

 0.9992 

 0.9994 

 0.9996 

 0.9998 

 1.0000 

 1.0002 

 1.0004 

 1.0006 

 1.0008 

 1.0010 

 1.0012 

 1.0014 

 1.0016 

 1.0018 

 1.0020 

 1.0022 

 1.0024 

 1.0026 

 1.0028 

 1.0030 

 1.0032 

 1.0034 

 1.0036 

 1.0033 

 1.0040 

 1.0042 

 1.0044 

 1.0046 

 1.0048 

 1.0050 

 1.0052 

 1.0054 

 1.0056 

 1.0068 

 1.0060 

 1.0062 

 1.0064 

 1.0066 

 1.0068 

 1.0070 

 1.0072 

 1.0074 

 1.0076 

 1.0078 

 1.0080 

 1.0082 

 1.0084 

 1.0086 

 1.0088 

 1.0090 

 1.0092 

 1.0094 

 1.0096 

 1.0098 

 1.0100 

 1.0102 

 1.0104 

 1.0106 

 1.0108 

 1.0110 



Figure 30. — 

 Graduati o n s 

 of hydrometer 

 .scale. 



