66 A Retrospect of Oceanography 



meter in the above sense : it does not give comparative results ; 

 it gives absolute ones. By its means, the weights of equal 

 volumes of the solution and of distilled water of the same 

 temperature are determined directly. It is neither more nor 

 less than a pyknometer, where the volume of liquid excluded 

 up to a certain mark is weighed instead of that included 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 hydro- 

 meter. On the other hand, the volume of the hydrometer 

 can safely be made many times larger than that of the pykno- 

 meter, the dimensions of which must always be kept small 

 on account of the difficulty of ascertaining its true temperature, 

 which is always a matter of guesswork, 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 ascer- 

 tained with every required accuracy. 



Again, for every determination with the ordinary pykno- 

 meter, 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. 

 The stem of the instrument is divided over a length of o-i 

 metre into millimetres, and its diameter is chosen so that 

 100 millimetres of it will displace 0-9 to I cubic centimetre; 

 the total volume of the instrument is intended to be about 

 180 c.c., and the glass-blower who supplies them generally 

 fulfils this specification very closely. He loads the instrument 

 so that it floats at o millimetre in distilled water of 30 C. 

 The small weights used are in the form of spirals of aluminium 



