CHAP. VIL.] DEEP-SEA TEMPERATURES. 289 
liquid and slightly compressed air. <A small steel 
index, with a hair tied round it to act as a spring 
and maintain the index in any position which it 
may assume, lies free in the tube among the creo- 
sote at either end of the column of mercury. ‘This 
thermometer gives its indications solely by the con- 
traction and expansion of the liquid in the large full 
bulb, and is consequently liable to some slight error 
from the effect of variations of temperature upon 
the liquids in other parts of the tube. When the 
liquid in the large bulb expands, the column of mer- 
cury is driven upwards towards the half-empty bulb, 
and the limb of the tube in which it rises is graduated 
from below upwards for increasing heat. When the 
liquid contracts in the bulb, the column of mercury 
falls in this hmb, but rises in the limb terminating in 
the full bulb, which is graduated from above down- 
wards. When the thermometer is going to be used the 
steel indices are drawn down in each limb of the tube 
by a strong magnet, till they rest on each side on 
the surface of the mercury. When the thermometer 
is brought up, the height at which the lower end of 
the index stands in each tube indicates the limit to 
which the index has been driven by the mercury, 
the extreme of heat or cold to which the instrument 
has been exposed. 
Unfortunately, the accuracy of the ordinary Six’s 
thermometer cannot be depended upon beyond a 
very limited depth, for the glass of the bulb which 
contains the expanding fluid yields to the pressure 
of the water, and, compressing the contained fluid, 
gives an indication higher than is due to tem- 
perature alone. This cause of error is not con- 
U 
