112 THE ANTARCTIC MANUAL. 
above example, the specific gravity is 185:783 + 181:783 = 
1°0220042, in place of 1°0218845, showing a difference of 1°97 in 
the fifth decimal place. An error of 1 centigramme would therefore 
affect only the seventh, and an error of 1 milligramme, only the eighth 
decimal place. 
Exception having, at one time or another, been taken to the prin- 
ciple of the absolute-weight hydrometer, the writer is convinced that 
it is not superfluous to direct attention to the above elementary 
arithmetical considerations. 
It is not always convenient to make parallel observations in dis- 
tilled water in the case of every sea water examined. Therefore, for 
common use, the results of a number of series of observations of the 
hydrometer in distilled water of different temperatures have been taken 
and interpreted in the above sense as giving the displaced volumes for 
the divisions of the stem, and the temperatures in question. Kopp’s 
table of the thermal dilatation of distilled water has been used. It 
is unnecessary to go into all the details of this calculation. The 
results for this hydrometer, No. 16, taken as example, are given in 
the following pages. 
The stem is divided into millimetres for a length of 100 mm. 
Its external diameter is about 3 mm. Its displacement is ascer- 
tained by floating the hydrometer in distilled water of given tempe- 
rature, and adding successively weights, increasing by 0-lgrm. The 
result of the determinations is given in the following table :— 
Interval on Stem. Volume of Interval. 
Millimetres, Cub. centimetres. 
0 to 10 0:1049 
10 20 0°1036 
20 30 0°1065 
30 40 0°1031 
40 50 0°1014 
50 60 0°1077 
60 70 0°1069 
70 80 0°1043 
80 90 0°1047 
90 100 0°1006 
0 to 100 1:0437 
From this table giving the volume of successive lengths of 
10 mm. of the stem, a table is to be constructed giving the volume 
of the immersed portion of the stem for every millimetre on it. 
There will be 101 entries in this table. The first will be 0 mm. and 
0 c.c., the eleventh 10 mm. and 0°'1049.c.c., and the last 100 mm. and 
1°0437 e.c. 
The volume of the hydrometer, when immersed just up to the 
