S34 



DR. GREGORY S STRICTURES ON DON RODRIGUEZ. 



lium, and 



agronomical 



theory. 



trial admeasurements, those which are deduced from astrono- 

 mical theory, and the oscillations of pendulums, correspond 

 The only infe- very nearly. Thus, Laplace's deduction of the compression. 



renccs which f rom t h e lengths of pendulums in different latitudes, is ---- 1 . 

 agree are those ° r 333 7h 



from pendu- (See Puissant, Topographs, &c. p. 66.) Clairault's well known 



modification of Newton's theorem, derived from the diminu- 

 tion of gravity, gives -5--^. The phenomena of the precession 

 of the equinoxes and the nutation of the earth's axes, give 

 ^l f for the maximum limit. A lunar inequality in longitude 

 depending upon the earth's ellipticity, and expressed by — 

 20 /A 987 sin. ft of the moon in longitude, requires the com- 

 pression to be between ^ 4 and ^j.^.^.^., but nearest the latter 

 limit. And a lunar inequality in latitude, depending also on 

 the compression, and expressed by — 24 ;/ 6gi4 sin. J) , requires 

 the compression to be between -j 1 ^ and - J ^^. 1 -> still leaning to 

 the latter limit. So that the ratio of the earth*s axes, as 

 deducible from these independent theoretical considerations, 

 lies within much narrower limits than we can get in any 

 other way. But this does not affect the truth of the preceding 

 remarks. It serves principally to shew, that whatever may 

 have been the derangements of the terrestrial spheroid since 

 its original formation, they are not such as have differently 

 affected the several phenomena occasioned by its aggregate 

 attraction : while a very slight consideration of the effects of 

 the deluge, of earthquakes, of volcanic operations,.of extensive 

 dislocations of strata, &c. may serve to convince us, that, 

 however regular the earth might once have been in its general 

 shape, there is now no reason to expect that " very regular" 

 surface from which Don Rodriguez persuades himself there 

 ought to be no essential deviation. 

 A 5d reason of 3. Don Rodriguez is farther confirmed in his opinion, that 

 the Don's re- ihere must be an error in the observations, especially at Arbury 

 Hill, of * f nearly 5 seconds," because he thinks no such 

 anomaly as that can fairly be ascribed to the effect of local 

 attractions. He does not deny " that irregularities of the earth 

 and local attractions may occasion considerable discrepancies ;" 

 yet be does not believe they can ever produce a deviation of the 

 magnitude just specified. Here again he is at war with the 

 decisions, I believe, of all preceding philosophers who have 



directed 



Remarks 00 

 this fact. 



futed. 



