110 THE ANTARCTIC MANUAL. 
The weight of the hydrometer is also the. weight of the water 
which it displaces. In the manufacture of the instrument its weight 
is so adjusted that in distilled water of 30°C. it floats with the water 
line at the lowest division, 0 mm.,on the stem. Let us assume that 
it does so exactly. Then the weight of the distilled water of 30°C., 
which it displaces, is 182°7830 grms. If now it be immersed in a 
saline solution, such as sea water, having a temperature of 30°C., it 
will not sink so deeply into it. In order to immerse it up to 0 mm. 
on the stem it will be necessary to add weight, let us say 4 grms., 
then we have at once the weights of equal volumes of distilled water 
and of the sea water, both at the same temperature 30°C.: they are 
182-7830 and 186°7830. The quotient, or 1°0218839, is the specific 
gravity of the sea water at 30°C., referred to that of distilled water 
of the same temperature as unity. Hence in order to get an accurate 
determination of the specific gravity of the sea water, we require to 
know only the weight of the instrument which floats at a given 
water line. In order to avoid the use of small weights which are 
necessary to make the hydrometer always float at the same water 
line, the stem has been divided into one hundred water lines, each 
1mm. apart. The external diameter of the stem is about 3 mm., 
so that the displacement in distilled water of the divided part is about 
lgrm. Let us imagine that it is so exactly. Let us now float the 
hydrometer in distilled water of 30°C.: it floats at Omm. Let 
us now add 0°5 and 1:0 grm. respectively, and we find that the 
immersion is increased to 50 and 100 mm. respectively, and the three 
weights of distilled water displaced are 182-7830, 183°2830 and 
183:7830 grms. respectively. Now let us float the instrument in 
the sea water of 30°C., and we require 4 germs. to make it float at 
0 mm. If we add 0°5 and 1:0 grm., these weights will produce 
slightly less additional immersions than they did in distilled water. 
Let the immersions be to 49 mm. and 98 mm. respectively. We have 
then the elements of three independent determinations of the specific 
gravity of the sea water at 30°C., referred to distilled water of the 
same temperature as unity. The first is the same as before, 
1°0218839. 
In the other two cases the displacements are not exactly the 
same. They could be made the same by adding about 0°01 and 
0°02 to the additional weights respectively. But it is inconvenient 
to have to work with these small weights. As 1 mm. of this stem 
corresponds to a displacement of 0°01 germ. distilled water it is 
evident that by subtracting 0°01 and 0°02 grm. from the weights of 
the hydrometer floating in distilled water we get the weights of dis- 
