of the Fishery Board for Scotland. 



399 



where 4 S t stands for the specific gravity of standard water at temperature t 

 measured by a hydrometer graduated so that in pure water at 4° the 

 reading is lOOO-O; and 17 . 5 S t stands for the specific gravity of the same 

 standard water at temperature t measured by a hydrometer graduated so 

 that in pure water at 17° -5 the reading is 1000-0. 



The formula adopted in preparing the following table, used in making 

 reductions for the present Report, is 



Tables of Specific Gravities of Standard Sea Water compared with Pure 



Water at 17°-5 C. 



17 . 5 S 17 . 5 = 1026-8. 



t° 



it 



t° 



ir<5 S t o 



t° 





0 



1025-9 



» 



1028-5 



18 



1026-7 



1 



1029-4 



10 



1028-4 



19 



1026-4 



2 



1029-4 



11 



1028-2 



20 



1026-2 



3 



1029-3 



12 



1028 0 



21 



1025-9 



4 



1029 2 



13 



1027-8 



22 



1025-6 



5 



1029-1 



14 



1027-6 



23 



1025-4 



6 



1029-0 



15 



1027-4 



24 



1025 1 



7 



1028-8 



16 



1027-2 



25 



1024-8 



8 



1028-7 



17 



1027-0 



26 



1024-5 



In order to obtain comparable results, the water tested must be brought 

 always to the same temperature, or else a calculation be made by means 

 of the above table to reduce the specific gravity observed to what it would 

 have been in the same water at the standard temperature. The standard 

 temperature is taken at 17° "5, the temperature of pure water at which the 

 hydrometer reads lOOO'O when plunged in it, and at this temperature 

 the table gives a specific gravity of 1026-83, which is that of the sea-water 

 chosen as a standard at 17° '5. 



This is a purely arbitrary standard, but it has been Used in the 

 'Challenger' report, and is accordingly retained here. The 'Challenger' 

 figure really is 4 S 15 . 56 = 1026-0, i.e., a hydrometer, graduated to read 

 1000-0 when immersed in pure water at maximum density, will read 

 1026 0 when floated in this standard sea-water at 60° F. or 15° -56 C. 



The method of using the table is as follows : — Suppose the reading of 

 the hydrometer to be 1026-2 and the temperature of the water to be 

 10-°C. If the water had been the standard sea- water the specific gravity 

 would have been 1028*4 — 2 - 2 higher. Hence, had the temperature of 

 real water been 17° '5, the specific gravity would have been 2 - 2 less than 

 the standard value of 1026*8, i.e., it would have been 1024-6, which 

 accordingly is entered in the tables as the specific gravity of the water 

 tested. 



After three years' experience of specific gravity observations it seems 

 desirable to consider how far such difficult and expensive work should be 

 continued, taking into account the very small sum of money at the dis- 

 posal of the Board for carrying out these important physical observations. 

 If a more adequate grant for this work cannot be obtained, as it is 

 2 C 



