734 



SCIENCE 



[N. S. Vol. XLII. No. 1090 



curves expressing the ratio of sp. gr. 

 15° C./15° C. to total salt and of sp. gr. 

 15° C./4° 0. to the same total salt content are 

 drawn on a scale of absolute units. The spe- 

 cific gravity determination made on the basis 

 of either of the two types of reference here 

 mentioned, is plotted on the perpendicular axis, 

 one unit on this axis being equal to one unit in 

 the fourth decimal place of the sp. gr. reading. 

 On the horizontal axis is plotted the corre- 



To determine the sp. gr. by either system of 

 reference of a sea-water solution containing 

 a known quantity of salts, reverse the process 

 just described. 



This diagram does not give results having a 

 degree of accuracy required for physical in- 

 vestigations, but is believed to be more accu- 

 rate than will be required for use in biological 

 work. The writer had only the convenience of 

 biologists in mind in preparing these notes. 





Oj <\J fti (\J fti C^ P5 C<j <Ti oj tlj (Tj 0)' CO 0)' Crj trf 0)* Bj. 



spending scale of salt contents, one unit on the 

 axis being equal to one unit in the second 

 decimal place when salt content is stated in 

 percentage. To use the table, the sp. gr. deter- 

 mined either as sp. gr. 15° C./15° 0. or as 

 15° C./4° C. is sought on the perpendicular 

 axis. The horizontal line on which this value 

 stands is traced to the point of intersection 

 with the line determined for the sp. gr. ratio 

 adopted. This point of intersection stands 

 directly above the point on the horizontal axis 

 at which the total salt is indicated. This can 

 be read with approximate accuracy by refer- 

 ence to the nearest given values. 



Any one wishing to work over a wider range 

 and with a greater degree of accuracy may 

 use the data of Knudsen^ in a similar way as 

 a basis for interpolation. 



For any one who is content with even 

 rougher approximations it may suffice to apply 

 a fairly accurate coefficient to the density read- 

 ings. Karsten^ has pointed out that when the 

 total salt content is divided by the number 

 represented by the first four decimal places in 



s Knudsen, ' ' Hydrographische Tabellen, ' ' Copen- 

 hagen, 1901. 



9 Karsten, G., WissensohaftUclie Meeresunter- 

 suclmngen, N. F. 1, H. I., 170, 1896. 



