SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 25 



of North America to San Francisco. This pattern of hydrometer was also used a good 

 deal in cable ships. 



In all work at sea three observations, or perhaps four, are quite sufficient. When 

 four observations are made, their arithmetical mean has theoretically two-thirds of the 

 value of the arithmetical mean of a series of nine. 



§ 8. As much misunderstanding seemed to exist not only regarding the qualifications 

 required for the successful practice of the hydrometric method, but also, to some extent, 

 of the principles on which the legitimacy of the method depends, I took occasion at the 

 meeting of the Sixth International Geographical Congress, held in London in 1895, at 

 which I read a paper entitled " A Retrospect of Oceanography during the last Twenty 

 Years," to deal with some of these misapprehensions. The following passage may be 

 quoted here with advantage : — 



" Many writers, in passing judgment on the hydrometer as an instrument for the 

 determination of the density of liquids, have only in their minds the hydrometer whose 

 indications are determined by comparison with another or standard instrument ; or by 

 immersion in solutions the densities of which have been otherwise ascertained. These 

 instruments have no greater value than that of more or less carefully constructed copies 

 of a standard, the method and the principle of the construction of which is not always 

 given. Rightly, therefore, they prefer the density as determined by weighing a vessel 

 filled with the liquid and comparing it with the weight of distilled water of the same 

 temperature filling the same vessel. The hydrometer which I constructed for the 

 Challenger Expedition, and used during the whole of it, is not a hydrometer 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 the distilled water of the 

 same temperature are determined directly. It is neither more nor less than a pykno- 

 meter, where the volume of liquid excluded up to a certain mark is weighed, instead ol 

 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 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 difiiculty of ascertaining the true tempera- 

 ture of its contents, which must be guessed, because it cannot be 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 be increased at will by 

 placing suitable small weights on the upper extremity of the stem. Their weight is 

 TRANS. ROY. SOC. EDIN., VOL. XLIX., PART I. (NO. 1). 4 



