APPARATUS AND APPLIANCES USED. 
51 
creosote acts on a small quantity of mercury in the 
tube, which rises or falls as the creosote expands or 
contracts. 
In each of the tubes above the mercury is a small 
metallic index, having a hair attached to it, which, 
pressing against the glass tube, acts as a spring, and 
keeps the index in its place, so as to be read off and 
recorded. 
It is evident that the bulb of the thermometer 
would be exposed to the pressure of the water as 
well as the temperature ; to prevent this, an additional 
bulb is blown outside the bulb of the thermometer ; 
this is partially filled with spirit, which is boiled 
before it is hermetically closed, so that' it contains 
in addition to the spirit some spirit vapour. The 
pressure now acts on the outer and not on the 
inner bulb, which ^s therefore only affected by 
temperature. These thermometers are tested by 
hydraulic pressure, from two to three tons on the 
square inch, and are considered trustworthy up to 
3000 fathoms. 
On commencing the operations of sounding, the 
weighted sounding rod, the water bottle, and the 
thermometers are suspended to the line, and lowered 
from the sounding-bridge by reversing the engine 
for 500 fathoms; the line is then let go and allowed 
to run out freely. As it runs out, the exact time of 
each 100-fathom mark entering the water is regis 
tered and set down in its appropriate column in a 
