CHEMICAL AND PHYSICAL NOTES, 115 
Let this mean temperature be 20° C., and we have for the first 
determination— 
Volume of body of hydrometer at 20°C. . . . « 183°4191 cc. 
Volume of immersed stemS3mm. . . «. . ss 0°0315 cc. 
Total immersed volume . . . . « 183°4506 ce. 
Weight of displacing “hydrometer Day Uh . 187-7830 grma. 
Density or weight of 1 c.c. of the sea water at 20° Gis 1°023616 
For the second determination we have— 
Volume of body of hydrometer at 20°C. . . « «= 1834191 ce. 
Volume of immersed stem 42mm. . fe chs is oi 0°4383 c.c. 
Total immersed volume . . . 183°8574 c.c. 
Weight of displacing hydrometer . . . 188-1830 grms. 
Density or weight of 1 ¢.c. of the sea water at 20° C.. 1°023522 
For the third determination we have— 
Volume of body of hydrometer at 20°C. . 1. . . 183-4191 ce. 
Volume of immersed stem 80 mm. . ‘ ar 0° 8384 ¢.c. 
Total immersed volume . » » . 184°2575 ec. 
Weight of displacing hydrometer  . . . . 188°5830 grms. 
Density or weight of 1 c.c. of the sea water at 20° om 1°023475 
The mean of the three densities is 1°023538. 
This is the density at 20° C.; or it is the specific gravity at that 
temperature, that of distilled water at 4° C. being unity. 
Having obtained the density at 20° CO. it will be required to find 
the density which the water has at some other temperature; for 
instance, the standard temperature which for all the Challengcr waters 
was taken as 15°'56 C., or 60° Fahr. The reason for the adoption of 
this standard temperature was that, when the Challenger sailed, the 
most trustworthy tables for the purpose were those of Hubbard, and 
he used 60° Fahr. as his standard temperature. It is, however, also 
the most suitable temperature to which to reduce observations of 
density made in temperate regions, because about as many observa- 
tions will have to be corrected up as have to be corrected down. 
Looking to the fact that the ocean waters contained between the two 
tropics are of greater volume than all the remaining waters on the 
globe taken together, it would be more suitable for a universal 
common temperature of reduction to take the mean temperature of 
the air at sea between the tropics. This would be about 22°°5 C.,* 
* See ‘A Retrospect of Oceanography during the last Twenty Years,’ by J. Y. 
Buchanan, F.R.S., in the Report of the Sixth International Geographical Congress, 
held in London, 1895, where this subject is discussed. 
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