10 



THREE CKUISES OF THE "BLAKE. 



space enough being left for the compression of the outer bulb. 

 The correction of each thermometer for pressure is ascertained 

 by experiment.^ 



A great objection to the use of the Miller-Casella for obtain- 

 ing deep-sea temperatures is found in the fact that they will 

 register only the lowest temjDerature ob- 

 tained. The most common form of deep- 

 sea thermometer now in use is a modifica- 

 tion of Negretti and Zambra's instrument, 

 of which a figure is here given ; the propel- 

 ler is constructed on the same principle as 

 that of the Sigsbee water-cup, and holds the 

 thermometer with the bulb (h) downward 

 till the line is drawn up. On being freed 

 from the stopper (/') the thermometer as- 

 sumes the position given in the figure, and 

 the column of mercury w^hich has become 

 freed from the bulb by the tilting on the 

 axis (j)) indicates the temperature at the 

 point where it was turned over. When tem- 

 peratures are taken in a district where they 

 diminish gradually from the surface to the 

 bottom, as is generally the case in the trop- 

 ics, this defect is not of great consequence. 

 When, however, we seek to ascertain the 

 temperatures of the Gulf Stream, or to 

 work in the Arctic regionSj where the cold- 

 est water may be nearest the surface, we 

 are unable, of course, to determine the posi- 

 tion of the coldest or warmest intermediate 

 layers of water. The only perfect method 



/ -- 



Fig. 12. — Neg^etti-Zarabra 



Thermometer, itaUan of taking Serial tcmpcratures is one which 

 records them continuously on deck during 



Tflodel. 



1 Professor Tait, who has made au ex- into the sea is pressure, and that the 



haustive investigation of the corrections correction to be applied to them is that 



to be applied to the deep-sea thermom- given by Captain Davis and Professor 



eters of the " Challenger," came to the Miller, of somewhat less than half a de- 



concliision that the only cause which is gree Fahrenheit for every mile under 



active in affecting them when let down the sea. 



