Figure 26. — Vacuum receiver within which the Mendenhall pendulum is swung. The pres- 

 sure is reduced to about 50 mm. to reduce the disturbing effect of air resistance. When 

 the apparatus is sealed, the pendulum is lifted on the knife edge by the lever q and is 

 started to swing by the lever r. The arc of swing is only about 1°. The stationary mirror 

 is shown atg. The pendulum shown in outline in the center, is only about 9.7 inches long. 



of the chronometer. The relative intensity of gravity 

 was determined by observations with the first 

 Mendenhall apparatus at Washington, D.C., at sta- 

 tions on the Pacific Coast and in Alaska, and at the 

 Stevens Institute, Hoboken, New Jersey, between 

 March and October 1891. 



Under Supt. Mendcnhall's direction a smaller, ',- 

 second, pendulum apparatus was also constructed 

 and tested, but did not offer advantages over the 

 }i-second apparatus, which therefore continued in 

 use. 



In accordance with Peirce's theory of the flexure 

 of the stand under oscillations of the pendulum. 

 determinations of the displacement of the receiver 

 of the Mendenhall apparatus was part of a relative 



determination of gravity by members of the Coast and 

 Geodetic Survey. Initially, a statical method was 

 used, but during 1908-1909 members of the Survey 

 adapted the Michelson interferometer for the determi- 

 nations of flexure during oscillations from the shift of 

 fringes. M The first Mendenhall pendulums were 

 made of bronze, but about 1920 invar was chosen 

 because of its small coefficient of expansion. About 

 1930, Lt. E. J. Brown of the Coast and Geodetic 

 Survey made significant improvements in the Menden- 

 hall apparatus, and the new form came to be known 



8< W. H. Burger, "The Measurement of the Flexure of 

 Pendulum Supports with the Interferometer," Report of the 

 Superintendent of the U.S. Coast and Geodetic Survey for 1909-10 

 (Washington, I'M l ). app. no. 6. 



334 



li I I I TIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



