110 



THE VOYAGE OF H.M.S. CHALLENGER. 



Geissler's " normal " or standard thermometers, graduated into tenths of a degree centi- 

 grade ; and it is essential for the accuracy of the results that the water, during the 

 observations of the hydrometer, should be sensibly at the same temperature as the 



Fig. 38. — Method of using the Hydrometer. 



atmosphere, otherwise the changing temperature of the water makes the readings of both 

 the hydrometer and the thermometer uncertain. 



Collection of Samples of Ocean Water. — Water from the surface was collected in the 



