Mendenhall half-second pendulums.
Figure 2 of William Bowie's "Investigations of Gravity and Isostasy"
Image ID: theb1330, NOAA's Historic Coast & Geodetic Survey (C&GS) Collection
Photo Date: Ca. 1895
Credit: NOAA Central Library
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of the U.S. Department of Commerce, National Oceanic & Atmospheric
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April 30, 2013