done on board H.M.S. ' Challenger. 3 595 



1-00000 and 1-03400 can be observed. It is not necessary that these 

 weights should be accurate multiples of the weight of the table ; it is 

 sufficient if they approach it within a centigramme, and their actual 

 weight be known with accuracy. The weights of the table and weights 

 in actual use are :— 



Weight of table 0*8360 gramme, 



Weight of weight No. 1 0-8560 



II 16010 „ 



„ „ III 24225 grammes, 



IV 31245 „ 



V 40710 „ 



VI 48245 „ 



For oceanic waters the hydrometer is always used with the table and 

 either No. IV. or No. V. weight. 



When the mechanical part of the construction of the instrument 

 was finished, with the exception of the closing of the top of the stem 

 (which instead was widened into a funnel-shape large enough to receive 

 the ordinary decigramme weights), the calibration of the stem was effected 

 by loading the stem with successive weights, and observing the conse- 

 quent depressions in distilled water of known temperature. This done, 

 the top was sealed up and the instrument carefully weighed. The ex- 

 pansion of the body with temperature was determined in a similar 

 manner by reading the instrument in distilled water of various tem- 

 peratures. The coefficient of expansion of the glass was then found 

 to be 0-000029 per degree Centigrade. 



For using this instrument at sea about 900 cub. centimetres of sea-water 

 are taken, and the containing cylinder placed on a swinging table in a 

 position as near the centre of the ship as possible. The observation with 

 the hydrometer, loaded with the necessary table and weight, is then 

 effected in the ordinary way, the accuracy of the readings being but little 

 affected by rolling; pitching, however, is found to have a distinctly 

 disturbing effect ; and when it is in any way violent, it is advisable to 

 store the specimen of water till the weather improves. 



The temperature of the water at the time of observation is deter- 

 mined by one of Geissler's "normal" or standard thermometers, gradu- 

 ated into tenths of a degree Centigrade; and it is essential for the 

 accuracy of the results that the water, during the observation of the 

 hydrometer, should be sensibly at the same temperature as the atmo- 

 sphere, otherwise the changing temperature of the water makes the 

 readings of both the hydrometer and the thermometer uncertain. At low 

 temperatures (below 10° or 12° C.) a tenth of a degree makes no sensible 

 difference in the resulting specific gravity ; but at the high temperatures 

 always found at the surface of tropical seas, rising sometimes to 30° C, 

 the same difference of temperature may make a difference of 3 to 4 in 

 the resulting specific gravity. 



