MR J. Y. BUCHANAN ON THE 



PAR. PAGE 



Full specification is given of the hydrometer and the accessory weights which were 

 used in all the determinations made during the voyage of the Challenger. 



7. Importance of constancy of temperature in the practice of the hydrometric method. This 23 



was secured in the Challenger by her construction, and by the climate of the seas in which 

 she cruised. The laboratory door was always locked while specific gravity observa- 

 tions were being made. Table VIII. gives the instances of duplicate observations on 

 identical samples of water with the same hydrometer differently weighted made in situ 

 during the voyage. In voyages later than that of the Challenger hydrometric observa- 

 tions were usually made in triplicate, and, later still, in series of nine. 



8. Quotation from the Report of the Sixth International Geographical Congress held in 25 



London in 1895, dealing with misunderstandings regarding the principle of the instru- 

 ment and the qualifications required for the successful practice of the hydrometric 

 method. 



SECTION II. 

 The Principle and Construction of the Closed Hyrdrometer. 



9. Experiments assumed to be made in vacuo at the sea-level in lat. 45°. True weight of the 26 



hydrometer determined and its approximate volume determined by immersion in distilled 

 water. Determination of weight of hydrometer in air and allowance for buoyancy 

 of hydrometer when being weighed, and of exposed stem when floating in the 

 liquid. 



10. Developments of the above for multiple observations. 27 



11. Preparation of Accessory Weights. These weights are made of wire, the heavier of brass 29 



and the lighter of aluminium. The pressure which they exert on the hydrometer, 

 when in use, is due to their weight in air. 



12. Exposed Stem. The effect of buoyancy of the exposed stem is, in practice, almost 30 



inappreciable. 



13. Final Determination of the Weight of the Hydrometer. This is effected at different dates 31 



and under different meteorological conditions, from which the true weight in vacua 

 is obtained. A table gives the exact weights in vacuo of hydrometers Nos. 17, 

 21 and 3. 



14r. Experiments for the determination of the displacement of the hydrometer in distilled water, 33 



and description of Tables, A, in which these results are recorded. 



15. Correction for the Departure of the Mean Reading from 50 mm. 35 



16. Correction for Temperature. 35 



17. Tables A^ to Aj give details of the determination of the displacement in distilled water of 36 



hydrometer No. 17 at 15°, 19-5°, 23° and 26° C. ; and of hydrometer No. 21 at 19-5° C. 

 Table B gives the observed weights of hydrometers Nos. 17, 21 and 3 when floating at 

 the 50-mm. mark in distilled water at various temperatures. 



SECTION III. 



Determination of the Specific Gravity of a Saline Solution. 



18. Details of tliree series of observations made with hydrometer No. 17 in a solution of 1/8 CsCl 44 



in 1000 grams of water at 19'5° C. are given in Table C, which is arranged in the same 

 manner as Tables A. 



