THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY. 



[NEW SERIES.] 



MAY 1828. 



LI. On the Ellipticity of the Earthy as deduced from Experi- 

 ments with the Pendulum ; and on the Formula employed for 

 obtaining it. By William Galbraith, Esq. A.M.* 



HPHE number of accurate experiments made with the pen- 

 -"- dulum to determine the figure of the earth, is now consi- 

 derable, and the discussion of them has been somewhat exten- 

 sive. From these it appears that discrepancies in the length 

 of the pendulum by experiment in a given latitude may be 

 considerable, amounting to above 0*005 of an inch, arising 

 from the unequal action of the strata immediately under the 

 station, for which no allowance can be easily made. From 

 these considerations, it is evident combinations of pendulum 

 experiments may be employed to bring out any required ellipti- 

 city, at least within certain limits, unless a judicious selection 

 be made of those placed under analogous circumstances. To 

 obviate these, the mean of a number of experiments in the same 

 latitude, but in very different longitudes, distributed as regu- 

 larly as conveniently can be, over the same parallel, should be 

 chosen, comprehending, if possible, every variety of geologi- 

 cal basis. From this method of proceeding, local irregularities 

 would be so far counteracted as to bring out results somewhat 

 satisfactory; and until this is accomplished, perhaps other arti- 

 fices may be advantageously employed to obviate as far as pos- 

 sible these unavoidable and troublesome incongruities. Among 

 other considerations, perhaps a basis of a certain specific gra- 

 vity might be chosen as a standard ; and by comparing the 

 lengths of the pendulum on various bases of different specific 

 gravities, an approximate rule might perhaps be obtained by 

 which, with considerable exactness, the lengths of the pendu- 



* Communicated by the Author. 

 New Series. Vol. 3. No. 17. May 1828. 2 T lums 



