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PACIFIC EXPLORATION: L. /. BRIGGS 
the cork-covered ice tank and completely covered with fragments of 
melting ice 1 to 3 cm. in thickness, dropped loosely into the tank. The 
fixed point is observed through a glass tube introduced through the ice 
and is illuminated through a similar tube on the opposite side. The 
observing tube is equipped with a low power lens. The ice tank is 
25 cm. in diameter and is covered with cork lagging 7.5 cm. thick. 
At sea the tank is swung from gimbals, the outer gimbal ring being 
suspended from 4 spiral steel springs hung from the four corners of a 
frame of 1-inch galvanized pipe, the four legs of which are screwed to 
the deck. Movable weights on the bottom of the tank serve to adjust 
the apparatus to a vertical position, which is determined by means of a 
sensitive level on the head of the instrument. When the apparatus is 
mounted on the open deck, an effective wind shield is essential, the 
supports of which must be entirely independent of the apparatus. 
Theory. — Let us determine through a series of observations the height 
of the mercurial column, at some station where g is accurately known. 
Let us now consider the apparatus to be transported to another station, 
where g is greater. The mercurial column will be depressed, compressing 
the gas. The observing bulb is accordingly lowered until the fixed 
point is again in grazing contact with the mercury surface. The volume 
of mercury in the bulb and capillary is now the same as at the first station, 
since the slight flexure of the capillary produces no appreciable change 
in its volume. Therefore the volume of mercury in the gas chamber 
is also the same as in the first observation. In other words, the volume 
of the gas is constant when the upper mercury surface is in grazing con- 
tact with the fixed point, and is independent of the position of the bulb. 
Since the temperature is constant, the pressure p of the gas is always 
the same at the time of taking an observation. We have then 
P =pgh =gih (1) 
in which g and represent the acceleration of gravity at the two sta- 
tions, h and hi the corresponding heights of the mercury column, and 
p the density of mercury. The latter is constant, since the mercury 
is always at the temperature of melting ice. Equation (1) then reduces 
to 
g/gi = h/h (2) 
or, the height of the column is inversely proportional to the acceleration 
of gravity. If the height h is measured at some station where g is known, 
the acceleration of gravity at any other station may therefore be de- 
termined simply by measuring hi. 
