Figure 5. — An apparatus for the practice measurement 

 of the length of the pendulum devised on the basis of a 

 series of preliminary experiments by C. M. de la Conda- 

 mine who, in the course of the French geodetic expedition 

 to Peru in 1 735, devoted a 3-month sojourn on the island 

 of Santo Domingo to pendulum observations by Mairan's 

 Method. In this arrangement, shown here, a vertical rod 

 of ironwood is used both as the scale and as the support 

 for the apparatus, having at its top the brass pendulum 

 support (A) and, below, a horizontal mirror (O) which 

 serves to align the apparatus vertically through visual 

 observation of the reflection of the pointer projecting from 

 A. The pendulum, about 37 inches long, consists of a 

 thread of pite (a humidity-resistant, natural fiber) and 

 a copper ball of about 6 ounces. Its exact length is 

 determined by adjusting the micrometer (S) so that the 

 ball nearly touches the mirror. It will be noted that the 



came to be supplemented by corrections for irregulari- 

 ties of terrain and by different types of isostatic 

 reduction. 



La Condamine, who like Bouguer was a member of 

 the Peruvian expedition, conducted his own pendulum 

 experiments (fig. 4). He experimented in 1735 at 

 Santo Domingo en route to South America, 18 then 

 at various stations in South America, and again at 

 Paris upon his return to France. His pendulum con- 

 sisted of a copper ball suspended by a thread of pite. 

 For experimentation the length initially was about 12 

 feet, and the time of swing 2 seconds, but then the 

 length was reduced to about 3 feet with time of swing 

 1 second. Earlier, when it was believed that gravity 

 was constant over the earth, Picard and others had 

 proposed that the length of the seconds pendulum be 

 chosen as the standard. La Condamine in 1747 

 revived the proposal in the form that the length of the 

 seconds pendulum at the equator be adopted as the 

 standard of length. Subsequently, lie investigated the 

 expansion of a toise of iron from the variation in the 

 period of his pendulum. In 1755, he observed the 

 pendulum at Rome with Boscovich. La Condamine's 



clock pendulum would be obscured by the scale. La 

 Condamine seems to have determined the times of 

 coincidence by visual observation of the occasions on 

 which "the pendulums swing parallel." (Portion of plate 

 1, Memoires publies par la Societe francaise de Physique, vol. 4.) 



18 ' 11 uu r.s M. de la Condamine, "De la mesure <lu pcndule 

 a Saint Domingue," Collection de memoires, vol. 4, pp. 3-16. 



310 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



