194 



It was only, therefore, at casual intervals that he could make use of 

 the means placed in his hands, and many interesting opportunities 

 were thus lost merely for want of adequate leisure to profit hy 

 them. 



The only stations, accordingly, at which the pendulum was swung 

 in a satisfactory manner, were, 1st, the Galapagos Islands in the 

 Pacific, lying 32 miles N. of the Equator; 2ndly, St. Bias, on the 

 N.W. shore of Mexico, in lat. 21^ N., and not far from California; 

 and, lastly, Rio de Janeiro, the capital of Brazil, in lat. 22'55' S. 



The ellipticity of the earth, resulting from the experiments made at 

 the Gallipagos, is -nTT-mr ; but it must be observed that, as the nature 

 of the ground on which the pendulum was here swung, was totally 

 different from that of the stations at which it was swung in England, 

 this result is not to be depended upon as giving a true expression 

 for the ellipticity ; since whenever a comparison is to be made be- 

 tween different stations on which the same pendulum has been 

 swung, the circumstances ought always to be as nearly alike as pos- 

 sible in all that relates to the density of the ground. At the Gala- 

 pagos, however, every thing was volcanic, and the islands were sur- 

 rounded by a deep sea, whereas at the English stations the rocks 

 were mostly primitive, and were surrounded by land. At the second 

 station, San Bias, there were two series of experiments made, the first 

 by Capt. Hall, which gave an ellipticity of 3U g.3 1, and the second by 

 Mr. Henry Foster, one of his officers, nrr.-B-Tr. These two series, how- 

 ever, were performed under circumstances so different, as to account 

 sufficiently for the want of exact agreement in the results. During 

 the first the temperature was steady, the sky always clear, and the 

 rate of the clock uniform ; but before the second series was well 

 commenced the rainy season set in with great violence, the sky be- 

 came overcast, few stars could be seen, the temperature fluctuated, 

 and the rate of the clock became unsteady. The relative degree of 

 credit, therefore, which attaches to the two series, is as 397 at 47, 

 the amount of the respective factors, or nearly as 8 to 1, and we 

 may assume the correct ellipticity at WT.T. At Rio de Janeiro, by 

 Captain Hall's series, the ellipticity comes out ->nrr-7Tr> and by Mr. 

 Foster's -y-n-i-.TT. The sum of the factors in this case being 148 and 

 304, or nearly as 261 in favour of Mr. Foster's experiments, the 

 correct ellipticity may be taken at -3-5-^.1-5- 



Captain Hall is of opinion that it would contribute materially to 

 the determination of this question if the same pendulum were to be 

 swung at places resembling the Galapagos, in geological character, 

 such as the Canaries and Azores, Madeira, St. Helena, the Isle of 

 France, and various islands in the Eastern Ocean ; besides swinging 

 it at stations resembling those of Captain Kater in this country, 

 especially at the Cape of Good Hope, and at a port in the Falkland 

 Islands, which happen to lie in the correspondent latitude to that of 

 London, and afterwards as a check on the adjacent main land of 

 Patagonia. 



Captain Hall announced his intention of instituting, in the mean 



