Figure 14. — Vacuum chamber for use with 

 the Kater pendulum. Of a number of 

 extraneous effects which tend to disturb the 

 accuracy of pendulum observations the most 

 important is air resistance. Experiments re- 

 ported by the Greenwich (England) observa- 

 tory in 1829 led to the development of a 

 vacuum chamber within which the pendu- 

 lum was swung. 



Expedition of 1838-1842, on the procurement of 

 pendulums for this voyage. Wilkes ordered from the 

 London instrument maker, Thomas Jones, two un- 

 usual pendulums, which Wilkes described as "those 

 considered the best form by Mr. Baily for traveling 

 pendulums," and which Baily, himself, described as 

 "precisely the same as the two invariable pendulums 

 belonging to this [Royal Astronomical] Society," 

 except for the location of the knife edges. 



The unusual feature of these pendulums was in 

 their symmetry of mass as well as of form. They were 

 made of bars, of iron in one case, and of brass in the 

 other, and each had two knife edges at opposite ends 



equidistant from the center. Thus, although they 

 resembled reversible pendulums, their symmetry of 

 mass prevented their use as such, and they were rather 

 equivalent to four separate invariable pendulums. 36 



Wilkes was taught the use of the pendulum by 

 Baily, and conducted experiments at Baily's house, 

 where the latter had carried out the work reported on 

 in 1832. The subsequent experiments made on the 

 U.S. Exploring Expedition were under the charge of 

 Wilkes, himself, who made observations on 11 separate 

 occasions, beginning with that in London (1836) and 

 followed by others in New York, Washington, D.C., 

 Rio de Janeiro, Sydney, Honolulu, "Pendulum Peak" 

 (Mauna Loa), Mount Kanoha, Nesqually (Oregon 

 Territory), and, finally, two more times in Washing- 

 ton, D.C. (1841 and 1845). 



Wilkes' results were communicated to Baily, who 

 appears to have found the work defective because of 

 insufficient attention to the maintenance of tempera- 

 ture constancy and to certain alterations made to the 

 pendulums. 37 The results were also to have been 

 included in the publications of the Expedition, but 

 were part of the unpublished 24th volume. For- 

 tunately they still exist, in what appears to be a 

 printer's proof. 38 



The Kater invariable pendulums were used to 

 investigate the internal constitution of the earth. 

 Airy sought to determine the density of the earth by 

 observing the times of swing of pendulums at the top 

 and bottom of a mine. The first experiments were 

 made in 1826 at the Dolcoath copper mine in Corn- 

 wall, and failed when the pendulum fell to the bottom. 



36 One was of case brass and the other of rolled iron, 68 in. 

 long, 2 in. wide, and K in. thick. Triangular knife edges 2 in. 

 long were inserted through triangular apertures 19.7 in. from 

 the center towards each end. These pendulums seem not to 

 have survived. I here is, however, in the collection of the 

 U.S. National Museum, a similar brass pendulum, 37 \ in. 

 long (fig. 15) stamped with the name of Edward Ktibel (1820- 

 96), who maintained an instrument business in Washington, 

 D.C, from about 1849. The history of this instrument is 

 unknown. 



7 See Baily's remarks in the Monthly Notices of the Royal 

 Astronomical Society (1839), vol. 4, pp. 141-143. Sec also letters 

 mentioned in footnote 38. 



3S This document, together with certain manuscript notes on 

 the pendulum experiments and six letters between Wilkes and 

 Baily, is in the U.S. National Archives, Navy Records Gp. 37. 

 These were the source materials for the information presented 

 here on the Expedition. We arc indebted to Miss Doris 

 Ann Esi li and Mr. Joseph Kudmann of the staff of the U.S. 

 V1l1n11.1l \ln 1 lui ( 1 ] 1 i 1 1 l; 0111 attentinn to this i-arly Ameri- 

 can pendulum work. 



318 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



