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°, the assumed temperature, agrees with 55°, 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 × (B-b / B + b) = height of the hill in feet for the temperature of 55°. Add ⅟₄₄₀ of this result for every degree the mean temperature exceeds 55°; or subtract as much for every degree below 55°.
Fig. 45. Aneroid Barometer.
TO DETERMINE HEIGHTS OF OBJECTS.
By Shadows.
1. Set up vertically a stick of known length, and measure the length of its shadow upon a horizontal or other plane; measure also the length of the shadow thrown by the object whose height is required. Then it will be:—As the length of the stick’s shadow is to the length of the stick itself, so is the length of the shadow of the object to the object’s height.