92 



THE VOYAGE OF H.M.S. CHALLENGER. 



at the same temperature at a given depth, if, in the process of turning, the mercury, 

 instead of separating at the point D as it is intended to do, separates at a point 



somewhat higher, the amount of mercury which will be deposited in the recording 

 column will be less than it should be, and consecpiently the instrument will show a 

 lower temperature than really exists, and the three occasions on which the Negretti & 

 Zambra gave a lower reading than the protected Six may be readily accounted for in this 

 manner.- That the instrument in the majority of cases gives a higher reading than the 

 protected Six thermometer must be due to one of two causes ; either the pressure of 

 the water outside the tube as the thermometer descends is sufficient to close entirely 



