ON THE ELLIPTICITY OF THE EAETH. 465 



the amount of the true reduction to a vacuum was no 

 doubt unusually large in consequence of its particular 

 form, and therefore it can be no guide in regard to the 

 similar reduction required in other experimental forms, 

 which must in every case be ascertained by special ex- 

 periment. This is one of many points which require 

 to be investigated, before measurements of the absolute 

 length of the seconds pendulum, by different methods 

 and dissimilar apparatus, can be assumed with confi- 

 dence as giving identical conclusions. 



The variations in the rate of the pendulum at the 

 stations of Captain Kater and myself are shown in the 

 following tabular view. They are expressed in refer- 

 ence to the rate in London (86,400 seconds in a mean 

 solar day), in the house which was the common base 

 station of both experimentalists. The pendulum being 

 by its mechanical construction invariable in its length 

 (excepting only from the effects of changes of tempera- 

 ture), the variations in its rate are the direct results of 

 observation at the several stations, requiring only three 

 corrections to be applied : 1. for differences of tem- 

 perature ; 2. for differences of the density of the air ; 

 3. for differences of elevation above the level of the sea. 

 For the first and second, the corrections have themselves 

 been determined experimentally ; and for the third, the 

 correction has been calculated from the duplicate pro- 

 portion of the distances of the station and of the sea- 

 level from the earth's centre, diminished by a uniform 

 factor of 0-67. (Phil. Trans., 1819, p. 354; and Pend. 

 Exp., p. 332.) Of the three corrections, that for the 

 variations of temperature is the one of which the ac- 



VOL. iv. HH 



