Seconds Pendulum, and the Ellipticity of the Earth. 251 



and with all his calculations. If the equatorial pendulum be 

 39*01, the ellipticity computed from the pendulum at London 

 will be -00325; but if it be 39-01568, the ellipticity will be 

 •00349. 



When two places are but a little different in latitude, the pen- 

 dulum at one may be deduced from the pendulum at the other 

 with considerable accuracy, even supposing that f is known 

 approximately only. Now if we deduce the pendulum at 

 Jamaica from those at Madras and San Bias, we shall find 

 39-03247 and 39*03051 instead of 39*03510 the length by 

 experiment: and, in like manner if we compare Drontheim 

 with Unst and Stockholm, the pendulum at the first place 

 will be 39*17949 and 39-17789, instead of 39-17456. This 

 indeed proves nothing directly against the accuracy of the 

 experiments ; for it may be caused by irregularity in the dis- 

 tribution of gravity. But it certainly ought to have some 

 weight, since we are not forced to have recourse to vague rea- 

 soning about local attraction, in order to reconcile with one an- 

 other, and with a regular figure, so many other experiments, 

 made by different observers, and extending over a great por- 

 tion of the surface of the globe. It likewise deserves to be 

 noticed that an invariable pendulum was carried from London 

 to Jamaica, by Mr. Campbell : and, upon the authority of ex- 

 periments made with it, the seconds pendulum at Jamaica, is 

 stated in the Mecanique Celeste, liv. 3 me , § 42, to be to that 

 at Paris as 0*99745 to 1 ; from which the pendulum at Jamaica 

 comes out 39*02952 instead of 39*03510. Making every al- 

 lowance for inaccuracies in Mr. Campbell's experiments, 

 there must remain doubts about the true length of the pen- 

 dulum at Jamaica. 



Sept. 30, 1826. J. Ivory. 



Postscript. — In the 43d Number of the Quarterly Journal 

 of Science, just published, there is a notice of experiments 

 made with an invariable pendulum by M. de Freycinet in a 

 voyage round the world. It is greatly to be regretted that the 

 Journalist has not put us in possession of the facts of the ex- 

 periments; for certainly the two columns of numbers in p. 144, 

 give no information about the relative lengths of the pendulums 

 at the different places. There are eight new stations, and of 

 these, two are already in my table on good authority ; three are 

 dismissed by the writer in the Journal, as good for nothing, 

 or not suited to his purpose ; and of the remaining three, no- 

 thing can be said at present, as it is necessary to take the facts 

 from the original work of M. de Freycinet. 



Oct. 3, 1826. J. I. 



2 I 2 XXXVIII, Notice 



