GRAVITATIONAL METHODS 



275 



KNIFE EDGE 



\ 



KNIFE 

 EDGE 



States Coast and Geodetic Survey. f Between that date and 1890, many 



observations were made with heavy pendulums about a meter long of the 



Kater (K), Repsold (R), Pierce (P), and other 



types. (See Figure 134.) The results of these 



determinations were not of sufficient accuracy to be 



of value in modern investigations of gravity and 



isotasy. 



In 1891, the Mendenhall invariable ^-second 

 (}i meter) pendulum apparatus was designed. This 

 equipment provided an air-tight case in which to 

 swing the pendulum under reduced air pressure, to 

 minimize air friction. An accurate chronometer was 

 substituted for the clocks previously used to measure 

 the period of vibration. These changes increased the 

 accuracy of the measurements and lowered the time 

 required for observation. Other improvements in the 

 design of the government pendulum apparatus have 

 been made since that date. By 1909, 47 stations had been occupied. 



The period 1909-1930 saw some 230 stations established and the devel- 

 opment of an interferometer method of determining the flexure of the 

 pendulum support. After 1920, wireless time signals broadcast from the 



V 



Fig. 134. — Sketch of 

 the Pierce (P), Repsold 

 (R), and Kater (K) pen- 

 dulums. The Pierce and 

 Repsold are reversible 

 pendulums. 





Fig. 135. — Pendulum prospecting equipment typical of the late 1920's. Left to right Two invar 

 pendulums; light source for photographic recording; pendulum case. (Courtesy of L. F. Athy.) 



t C. H. Swick, "Modern Methods for Measuring the Intensity of Gravity," U. S. Coast and 

 Geodetic Survey, Special Publication No. 69, 1921. 



