96 



Mr. Ivory on the Ellipticity of the Earth 



In the latter part of the table the results of some other com- 

 binations of the experiments are set clown. In particular 

 Sierra Leone, Trinidad, Bahia, and Jamaica, are each com- 

 bined with Greenland and Spitzbergen, the most northerly 

 stations, these being the cases in which the total variation of 

 the pendulum is greatest and most likely to cover the irregu- 

 larities of observation. We may conclude, that in no good 

 combination of the experiments, will the ellipticity come out 

 greater than -00349. We may safely adopt, as the true mean 

 ellipticity, the quantity -00333 obtained from the first six com- 

 binations in the table. 



Captain Kater has determined the length of the seconds 

 pendulum at seven different stations between Dunnose in the 

 Isle of Wight, and Unst the most remote of the Shetland 

 islands, extending through 10° of latitude. The total variation 

 of the pendulum between the extreme stations is only -035 in., 

 which is much too small for safely computing the earth's el- 

 lipticity, on account of the great irregularities of observation 

 to which such operations are liable. But although Captain 

 Kater's experiments, when taken by themselves, are insuffi- 

 cient for determining the ellipticity of the earth, they are not 

 the less valuable, because they may be combined with the ex- 

 periments of others made at a proper distance from them. 

 The following table contains the ellipticities obtained by com- 

 bining the length of the pendulum determined by Captain 

 Sabine at Maranham, with Captain Kater's experiments. The 

 pendulum at Trinidad combined with the same experiments 

 will give very nearly the same results. 



Stations. 



Latitude. 



Pendulum. 



Ellipticity. 



Unst 



Leith Fort . . . 



Clifton 



Arbury Hill . . 



Shanklin Farm 



60 45 28 N. 

 57 40 59 

 55 58 41 

 53 27 43 

 52 12 55 

 51 31 8 

 50 37 24 



Inches. 



39-17146 

 39-16159 

 39-15554 

 39-14600 

 39-14250 

 39-13929 

 39-13614 



Mean . . 



•00329 

 •00327 

 •00328 

 •00332 

 •00328 

 •00332 

 •00331 



•00329 



In the article "Pendulum" in the Supplement to the Encyclo- 

 paedia Britannica, M. Biot has given us the lengths of the pen- 

 dulum as determined by himself and other French philoso- 

 phers at eight different stations, extending through 22° of la- 

 titudes from Formentera to Unst. The seconds pendulum is 



expressed 



