16 
THREE CRUISES OF THE “ BLAKE.” 
eee 
i 
Thermometer, 
model. 
EIS 
| K LO 
Fig. 12. — Negretti- Z ambra 
Italian 
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- nulio for obtain- 
ing deep-sea temperatures is found in the fact that they will 
register only the lowest temperature 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 (b) downward 
till the line is drawn up. On being freed 
from the stopper (f) the thermometer as- 
sumes the position. given in the figure, and 
the column of mercury which has become 
freed from the bulb by the tilting on the 
axis (p) 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 regions, 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 
of taking serial temperatures is one which 
records them continuously on deck during 
1 Professor Tait, who has made an ex- 
haustive investigation of the corrections 
to be applied to the deep-sea thermom- 
eters of the “Challenger,” came to the 
conclusion that the only cause which is 
active in affecting them when let down 
into the sea is pressure, and that the 
correction to be applied to them is that 
given by Captain Davis and Professor 
Miller, of somewhat less than half a de- 
gree Fahrenheit for every mile under 
the sea. 
