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## LEVELLING BY ANEROID BAROMETER

This should be used more for ascertaining relatively large differences
in altitudes than for purposes where any great nicety is required. For
hills under 2000 ft., the following rule will give a very close
approximation, and is easily remembered, because 55 degrees, the
assumed temperature, agrees with 55 degrees, the significant figures
in the 55,000 factor, while the fractional correction contains /two
fours/.

Observe the altitudes and also the temperatures on the Fahrenheit
thermometer at top and bottom respectively, of the hill, and take the
mean between them. Let B represent the mean altitude and b the mean
temperature. Then 55000 X B - b/B + b = height of the hill in feet for
the temperature of 55 degrees. Add 1/440 of this result for every
degree the mean temperature exceeds 55 degrees; or subtract as much
for every degree below 55 degrees.

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