164 MR J. Y. BUCHANAN ON THE 



contraction B between the ballast bulb and the body of the instrument, 5 centimetres ; 

 the body from B to C, 13 centimetres; the stem, from C to F, 14 centimetres, making 

 the total length over all 32 centimetres. On the stem a length of 10 centimetres, DE, 

 is divided in millimetres, and numbered at each centimetre, 0, 1, 2, ... 10, from 

 below upwards; the lowest division, 0, is 1 centimetre from the junction of the stem 

 with the body of the instrument at C, and the highest division, 10, is 3 centimetres 

 from the upper extremity of the instrument at F. The external diameter of the body 

 of the instrument is 37 millimetres, and that of the ballast bulb 32 millimetres. The 

 external diameter of the stem is 3'5 millimetres, and the internal diameter 2"5 milli- 

 metres. As the ballast used is lead shot, and the load of this shot has to be frequently 

 altered, the internal diameter of the contraction at B as well as that of the stem must 

 be such that shot can be added to or removed from the instrument without trouble. 

 G is the button on the cane of white enamel glass which is suspended in the axis of 

 the stem, and, by affording a white background, enables the scale which is etched on 

 the glass to be seen with facility. 



The glass shell of the hydrometer, as it came from the glass-blower, was first 

 weighed approximately, and the weight so found was 40 "5 grams. It was then loaded 

 with No. 10 lead shot so that it floated in distilled water of 19'5° C. with the zero division, 

 which is the lowest on the scale etched on the stem, exposed above water. 95 '6 grams 

 of shot were required for this purpose. 



The hydrometer so loaded was found to weigh exactly 136 "1022 grams in air. 



The atmospheric conditions were as follows : — 



Barometer, 760 "3 mm. 

 Temperature of air, 19"0° C. 

 Relative humidity, 70 per cent. 



Whence the weight of 1 c.c. air = 0*001208 gram. 



Taking the specific gravity of glass to be 2"5, that of lead to be 1 r35, we have :— 



For the volume of 40 '5 grams glass . . . . . 16*2 c.c. 



For that of 95*6 grams lead . . . . 



And for the total volume of lead and glass 

 The volume of 136"1 grams brass weights is 



Whence the balance of volume for correction is 



8-4 



24-6 

 17-0 



7-6 



The weight of 7 '6 c.c. air under the above conditions is 0*00912 gram. 



Therefore the true weight in vacuo of the glass and shot is 136*1022 4- 0*00912 = 

 136*11132 grams. To this has to be added the weight of the air which the hydrometer 

 contains This is arrived at by floating the instrument in distilled water and by adding 

 suitable weights at the top of the stem, so as to immerse it up to the 50-mm. division 

 on the scale. The weight of distilled water so displaced is equal to the sum of the 

 weights of the glass, the shot, the small external weights added, and that of the air 



