168 Captain Sabine’s experiments to determine the 
The correction for temperature is the reduction of the 
mean temperatures to 50 degrees, assumed as a convenient 
standard at which to compare the observations of the first 
voyage with each other, it being nearly the mean at which 
they were made ; the correction has been computed from the 
change in the length of the pendulum due to differences 
of temperature, the expansion of brass being considered 
-^■00220 inches per foot in 180 degrees of Fahrenheit. 
The correction for the height above the sea, is the part of 
a vibration lost in 24 hours by the pendulum in vibrating at 
such elevation instead of at the level of the sea, the force of 
gravity increasing inversely as the square of the distance 
from the centre of the earth. The differences of latitude 
being considerable between the stations at which the clocks 
have been set up, and the elevations being small, and differing 
but little from each other, it has not been deemed necessary 
to diminish this correction, by taking the geological character 
of the different stations into consideration ; the character is 
however noted wherever it was not previously well known. 
The correction for the buoyancy of the atmosphere has 
been computed in the manner explained by Captain Kater, 
in his Account of Experiments for determining the length of 
the Seconds Pendulum, published in the Philosophical Trans- 
actions for 1818; the specific gravity of the pendulum being 
considered 8,4. 
