deduced from Capt. Sabine's Experiments. 325 



cipal datum on which the calculation turns, namely, the mean 

 equatorial pendulum of the tropical stations, is, in every case, 

 the same. 



Although the observations we have made are sufficiently 

 obvious, yet we may add in confirmation of them, that if any 

 of the pendulums near the equator be left out, so as to alter 

 the mean of the equatorial pendulums, the ellipticity will also 

 be changed. Thus Captain Sabine, by a calculation in which 

 all his 13 stations are included, finds the equatorial pendulum 

 equal to 39-01568, and the ellipticity, to -00346. But if we 

 leave out the stations at St. Thomas and Ascension, the 11 

 remaining stations will give 39*01374 and "00340, for the like 

 quantities. Further, if we leave out St. Thomas, Ascension, 

 Sierra Leone, Bahia and Jamaica, the equatorial pendulum 

 will be 39*01213 and the ellipticity '00336, approaching very 

 nearly to the values usually received. And if we add to 

 Captain Sabine's 13 stations the experiments made by other 

 observers near the equator, for instance, at Madras and Rio 

 Janeiro, the effect produced will be the same, that is, the 

 equatorial pendulum and the ellipticity will be both lessened. 



There is an assertion of Captain Sabine at p. 353 of his 

 work, which it will not be improper to notice. " If each of 

 the tropical stations, which I have visited, be severally com- 

 bined with each of the stations within 45° of the pole, no one 

 result, amidst all the irregularities of local attraction, will be 

 found to indicate so small a compression as that of previous 

 reception." Now this is strictly correct with respect to five 

 of his tropical stations ; but I have shown, in this Journal for 

 August last, p. 96, that if the pendulums at Maranham and 

 Trinidad be combined with Captain Kater's seven experi- 

 ments, the resulting ellipticity will be '00329 at a mean, which 

 can hardly be accounted different from '00327, the quantity 

 adopted by the French philosophers. 



It follows from all this discussion that the ellipticity assigned 

 by Captain Sabine is obtained only when all his tropical pen- 

 dulums, unmixed with any other experiments near the equa- 

 tor, are combined with the northern experiments. When- 

 ever the mean length of the equatorial pendulums is altered, 

 either by leaving out some, or by adding the experiments 

 made by others near the equator, the ellipticity will undergo 

 a change. If the mean equatorial pendulum of the tropical 

 stations be 39*0156, the ellipticity will be '00346; otherwise, 

 it will have a different value. Now it must be allowed that 

 Captain Sabine's seven tropical pendulums are exceedingly 

 irregular and anomalous ; and no confidence can be placed 

 in a mean deduced from so few experiments liable to such 



objections. 



