118 THE ANTARCTIC MANUAL. 
that ¢ncluded up to a similar mark. In the pyknometer, the internal 
surface per unit of length of the stem can be made smaller than the 
external surface per unit of length of the stem of the hydrometer. 
On the other hand, the volume of the hydrometer can safely be made 
many times larger than that of the pyknometer, the dimensions of 
which must always be kept small on account of the difficulty of as- 
certaining its true temperature, which must be a matter of guess- 
work, because it is not measured directly. The temperature of 
another mass of liquid is measured, and the two are assumed to be 
identical. With the hydrometer, the liquid being in large quantity 
and outside of the instrument, its temperature can be immediately 
ascertained with every required accuracy. 
“ Again, for every determination with the ordinary pyknometer, 
the weight of the liquid contained in it has to be determined by a 
separate operation of weighing. With the hydrometer the weight of 
the liquid displaced, being always equal to its own, is determined 
once for all by repeated series of weighings, where every refinement 
is used to secure the true weight of the instrument. This weight can 
then be increased at will by placing suitable small weights on the 
upper extremity of the stem. Their weight is also most carefully 
determined once for all, so that at any moment the total weight of 
the displacing instrument is accurately known. 
“In determining the density of a sea water in an expedition, only 
the absolute-weight hydrometer should be used. The samples of 
water should be stored in the laboratory where the observations are 
going to be made, and they should have sensibly the same tempe- 
rature as the air while the observations are being made. If the 
motion of the ship is at any time too violent for it to be convenient 
to make the observations, then a sufficient supply of bottles should 
be at hand to keep the samples until the motion becomes less without 
interfering with the collection of other samples. When the water is 
in the cylinder, its temperature is carefully taken with a trustworthy 
thermometer, which must be divided into tenths of a Centigrade 
degree, The thermometer is then removed, and the hydrometer 
immersed and loaded with small weights, until the water-level rises 
to one of the lower divisions of the scale. It is unnecessary to point 
out that the water in the cylinder must be at rest, that the stem of 
the hydrometer must be sheltered from wind, that everything must 
be clean, and that the ordinary precautions usually observed in every 
physical or chemival laboratory are to be observed. Having obtained 
the first reading, further small weights are added by steps of 0°1 grm. 
yntil at least nine observations haye heen obtained. Sometimes it is 
