Dr. Gregory* s Strictures on Don Rodr7guez. 185 



Laplace, -it p. 56. of his " Exposition :" " Les degres da 

 nord ft de France donnent -y^ pour I'ellipticite de la terre, 

 que les tlesires di.' France tt de I'equatear donnent egale k 

 ■g-h-: 'I paroit done qiK: la terre est sensiblcment differenta 

 d'nn ellipi'oide. II v a n)enie lien de croirc qu'elle n'est pas 

 un solide de }eooliition,(^i que ses dt-nx liemisplicres ne sont 

 pas seniblabics de chaqne cote de I'etiLiateur." 



It is curious, however, to observe that notwithstanding 

 this exirerne want of nniforinity, in the results furnished 

 by terrestrial admeasurements, those wliicli are deduced 

 from astronomical theory, and the oscillations ot pendu- 

 lums, correspond very nearly. Thus, Laplace's deduction 

 of the compression Irom the lengths of pendulums in dif- 

 ferent latitudes, is -g^-rg-. (See Puissant, Topograpltie, Sec. 

 p. GQ.) Clairaut's well known modification of Newton's 

 theorem, derived from the diminution of gravity, gi^es-^-J-^. 

 The phenomena of the precession of the equinoxes and 

 the nutation of the earth's axes give -g-J^ for the maximum 

 limit. A lunar inequality in lonaimdc depending upoa 

 the earth's ellipticiiy, and expre'^ssed by —10"^S1 sin S 

 of the moon in longitude, requires the compression to be 

 between -jJ-^ and a-oiirFJ but nearest the latter limit. And 

 a lunar inequality in latitude, depending also on the com- 

 pression, and expressed by — 24-"()9i4 sm J, requires the 

 compression to be between -r^ and tttV-b-' ^''" leaning tr» 

 the latter limit. So that the ratio ot the earth's axes, as 

 deducible from these indi. pendent theoretical consideraiions, 

 lies within much narrower limits than wc can get in any 

 other way. Rut this does not affect the truth ot the pre- 

 ceding remarks. It serves principally to show that, what- 

 ever may have been the derangements of the terrestrial 

 spheroid since its original formation, they are not si;ch as 

 bave differently atlected the several phaenomena occasioned 

 by its a<ri!regate atiraction : while a very slight considera- 

 tion of the effects of the deluge, of earthquakes, of volcanic 

 operations, of extensive dislocations of strata, Sec. may strve 

 to convince us, that however regular the earth nughi once 

 have bcenin its general shape, there is no\v "" reason to 

 €xpcct that " very regular" surface from which Don Ro- 

 driguez persuades himself there ought to be no essvniial de- 

 viation. 



3. Don Rodriguez is further confirmed in his opinion 

 that there must be an error in the observations, especially at 

 Arbury Hill, of <' nearly five seconds," because lie thinks 

 JIG such anomaly as that can faiily b« asciibed to the etTcct 

 of local attractions. He does not deny " iliat irregularities 



of 



