22 MR J. Y. BUCHANAN ON THE 



difference between the successive weights was rather less than that required to immerse 

 the divided portion of the stem. 



On rare occasions multiple observations were made on a single water, using ordinary 

 decigram weights, but this was found to be very inconvenient. Nevertheless, the 

 advantage of multiple observations was clearly perceived, and provision for their being 

 made was included in the specification of all later instruments. Also, the system of 

 numbering the centimetres on the stem was altered. In the Challenger instrument 

 the number 10 marks the lowest division on the stem, and marks the highest. In 

 all later instruments the lowest division is 0, and the centimetres are numbered 

 1, 2, 3, . . . 10 upwards. 



In every determination of the specific gravity of a sample of water, the weight 

 of the volume of it which was displaced by the hydrometer floating in it at an 

 observed division on the scale was represented by the sum of the weights of the 

 hydrometer, the table, and the accessory weight used. 



The volume of the water so displaced by the hydrometer was arrived at as 

 the result of an extensive series of observations made with it in distilled water 

 at different temperatures. 



The relation between the weight and the volume of a mass of distilled water 

 at all ordinary temperatures, as determined by Kopp, was accepted as correct, and 

 was used in reducing the observations made with the hydrometer in distilled water 

 so as to arrive at the volume of its body, that is, the whole of the hydrometer 

 below the lowest division on the stem at all ordinary temperatures. Its rate of 

 thermal dilatability was taken to be constant within the limits of temperature 

 considered, and its probable value was obtained by taking the mean of all those 

 observed. 



The final result was stated by giving the volume of the body of the instrument 

 up to the lowest division on the stem at 0° C. as V, and the rate of its dilatability, 

 dV/dt, as e. 



Thus the full specification of the hydrometer, that is, the glass instrument alone, 

 is furnished by four data. 



For the hydrometer used in the Challenger they are : — 



Weight Mi vacMo of the hydrometer . . W 160 '2 128 grams. 

 Volume of body of hydrometer up to lowest 



division on stem at 0' C. . . V 160-277 c.c. 



Rate of expansion of body per ° C. . . e 0*00455 c.c. 



Total volume of divided stem (100 mm.) . v 0*8650 c.c. 



The specification of the set of accessory weights which were used with this 

 hydrometer is as follows : — 



No. . . . 

 Weight in grams 



0. 



I. 



II. 



III. 



IV. 



V. 



VI. 



0-8360 



0-8560 



1-6010 



2-4225 



3-2145 



4-0710 



4-8245 



