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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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