n 



*¥ 



I 



= 1 





Figure i g, [hum pendulums used in early work 

 at the U.S. Coast and Geodetic Survey. Shown on 

 the left is the Peirce invariable; center, the Peirce 

 reversible; and, right, the Repsold reversible. 

 Peirce designed the cylindrical pendulum in 1881- 

 1882 t<> stud) the effect of air resistance according 

 to the theory of (i. G. Stokes on the motion of a 

 pendulum in .1 viscous field. Three examples ol 

 the Peirce pendulums are in the U.S. National 

 Museum. 



the proposal made bj Fayc and theoretically 

 pursued b) Peirce to swing two pendulums from the 

 same st.mil with equal amplitudes and in opposite 

 phases, but that the proposal was not practicable. 

 He concluded that for absolute determinations ol 

 gravity, the Bessel reversible pendulum was highly 

 appropriate il one swung two exemplars oi different 

 weight from the same stand for the elimination ol 

 flexure. Prof, von < tppolzer's important report n 

 nized that absolute determinations were less ao 

 than relative ones, and should be < ondui ted 01 

 special places. 



I he discussions initiated by Pi emonsn ation 



nl the flexure of the Repsold stand resulted, finally, 

 in the abandonment of the plan to make absolute 

 determinations of gravit) at all stations with the 

 n\ ersible pendulum. 



Peirce and Defforges Lavariable, 



Reversible Pendulums 



The Rcpsold-Besscl reversible penduluui \\a* de- 

 d and initially used to make absolute deta 

 tions of gravity not only at initial stations such as 

 Kew, the observatory in Paris, and the Smithsonian 

 Institution in Washington, D.C., but also at stations 

 in the field. An invariable pendulum with a single 

 edge, however, is adequate for relative deter- 

 minations. As we have seen, sucli invariable pendu- 

 lums had been used l>\ Bouguer and Rater, and .liter 

 the experiences with the Repsold apparatus had been 

 recommended again 1 >> Baeyer for relative deter- 

 minations. Hut an invariable pendulum is subject to 

 uncontrollable changes of length. Peirce proposed 

 to detect such changes in an invariable pendulum in 

 the field l>v combining the invariable and reversible 

 principles. He explained his proposal to Faye in a 

 Utter dated July 23. 1880, and lie presented it on 

 September 16, 1880, at the fourth session of the sixth 

 General Conference of Die Europ'dische Gradmessung, 

 in Munii 



As recorded in the Proceedings of the Conference, 

 Peirce wrote: 



But I obviate it in making my pendulum both invariable 

 and reversible. Every alteration of the pendulum will 

 be revealed immediately 1>\ the change in the difli 



of the tWO periods of oscillation ill the two positions. 



Once discovered, it will be taken account of by means 

 of new measures of the distance between the two supports. 



73 Op. cit. (footnote 67) 



PAPER 44: DEVELOPMENT OF GRAVITY PEND1 LUMS IN THE 19TH CENTURY 



127 



