PACIFIC EXPLORATION: L. J. BRIGGS 
401 
bulb by means of an inserted platinum wire and extending vertically 
downward, so that the point is approximately at the center of the bulb. 
The length of the mercurial column is about 74 cm. 
The flexible capillary permits a slight vertical movement of the observ- 
ing bulb with respect to the gas chamber. This movement is deter- 
mined by a micrometer screw of 1 mm. pitch which controls the motion 
of a carriage in which the observing bulb is rigidly mounted. The 
carriage slides on parallel rods mounted on a base which is rigidly 
cemented to the neck n of the gas chamber, so that the position of the 
bulb relative to the gas chamber is definitely determined by the screw. 
After the apparatus has been thoroughly cleaned and dried, pure 
mercury is introduced in excess of the amount required to fill the capil- 
lary and observing bulb. The apparatus is then highly evacuated 
through Si and ^2 and the observing bulb sealed at Si. Dry nitrogen is 
introduced into the gas chamber in excess of atmospheric pressure 
through a stop-cock temporarily sealed to the chamber at ^2. The 
micrometer head is then mounted, the protective casing of the gas 
chamber adjusted, and the whole apparatus placed in the ice-tank in a 
vertical position and surrounded with melting ice. A large U-type 
vacuum manometer is connected with the stop-cock of the gas chamber 
and the air exhausted from the connecting tube. After temperature 
equilibrium is attained, the stop-cock of the gas chamber is opened and 
the pressure of the nitrogen slowly reduced until the mercury stands in 
contact with the fixed point in the observing bulb when the carriage is 
near the middle of its range. The pressure in the gas chamber at this 
time is determined by reading the difference in level of the mercury 
surfaces in the manometer by means of a cathetometer. The observed 
pressure, corrected to mercury at 0°C., gives the difference in height of the 
two mercury surfaces in the gravity apparatus. The length of the 
column, less the scale reading, gives the 'constant' of the instrument, 
which is added to subsequent micrometer readings to obtain the true 
height of the mercurial column. 
When the above determination is made, the gas chamber connection 
is sealed off with a blow-pipe. The apparatus is now perfectly gas-tight, 
since it is entirely free from ground joints, stop-cocks and sealed-in plati- 
num connections, and the original openings at Si and ^2 have both been 
sealed with the blow-pipe. The apparatus is also readily transportable, 
since at ordinary temperatures the pressure in the gas chamber is sufii- 
cient to fill the observing bulb with mercury. It can then be tipped in 
any position. 
In observing, the apparatus is supported in a vertical position in 
