114 THE ANTARCTIC MANUAL. 
for the second or lighter liquid. The difference is 0°005640 grm., 
and this corresponds to the immersion of the whole of the stem, 
which is divided into 100 mm. ‘Therefore the difference of density 
corresponding to a difference of immersion of 1 mm. is 0°000056. 
If we consider a difference of immersion of one-tenth of a millimetre, 
it corresponds to a difference of density of 0°0000056. This example 
gives at once an idea of the capability of the instrument. 
Additional weights—In order to be able conveniently to increase 
the weight of the hydrometer, a set of weights is supplied, of such a 
form that they can be placed on the top of the exposed stem. They 
are made of wire, in the form of spirals and of rings. The following 
are the particulars of a set: 
-0 and 3°0 grms. respectively. 
nd 0°5 grms. respectively. 
, 0°5, 0-2, 0-1 and 0°05 grms. respectively. 
0:1, 0°1, 0-1 and 0-1 grms. respectively. 
Spirals of brass . 7:0 
Rings of brass =) 10) 
Spirals of aluminium 1:0 
Rings of aluminium 0-1 
This set of weights has been designed so that, for varying the 
weight in distilled water, one aluminium spiral, which will carry the 
lighter aluminium rings as required, will be sufficient, and for vary- 
ine the weight in sea water, one brass spiral will be sufficient, rings 
of brass or aluminium being added as required. 
An example has already been given of a determination of specific 
gravity when parallel series of observations are made in the sea water 
and in distilled water of the same temperature. In the following 
example the volume of the hydrometer up to every division of the 
stem, and for all temperatures, from 0° to 30° C., is taken as known. 
It has been deduced from a number of series of observations on dis- 
tilled water in the way just described. The sample of sea water has 
the temperature of the air in the room or laboratory. This is essential. 
If it is inconvenient to wait until the water has acquired this tempe- 
rature then a coarser and Jess valuable hydrometer should be used. 
After the temperature of the water has been observed, the 
hydrometer is floated in it, with sufficient additional weight, say 
5 erms., to immerse the stem to one of the lower divisions of the 
stem—let us say 3; then let 0°4 grm. be added and let it be im- 
mersed tu 42, and for the third observation let a further weight of 
0:4 grm. be added, and let it now float at 80. The hydrometer is 
then removed, dried and replaced in its box, and the temperature of 
the water isagain taken. It must not have varied more than 0:2 C., 
in which case the mean between the temperature at the beginning 
and that at the end can be used. 
