256 



DR. GREGORY S STRICTURES ON DON RODRIGUEZ. 



Plavfii iron Hi e 

 attraction of 

 hills, &c. 



Maskelyne. 



Cavendish. 



Puissant on 

 local attrac- 

 tions. 



Playfair on 



after describing the irregularities thus occasioned in the degree 

 at Turin, adds, " there are, no doubt, situations in which the 

 measurement of a small arch might, from a similar cause, give 

 the radius of curvature of the meridian, ivjhrite, or even nega- 

 tive." See Edinburgh Transactions, vol. v. p. 5. And Dr. 

 Maskelyne, after treating of Mason and Dixon's degree in 

 North America, says, " Mr. Henry Cavendish having inves- 

 tigated several rules for finding the attraction of the inequa- 

 lities of the earth, has, upon probable suppositious cf the dis- 

 tance and height of the Allegary mountains, from the degree 

 measured, and the depth and declivity of the Atlantic ocean, 

 computed what alteration might be so produced in the length 

 of the degree ; and finds that it may have been diminished by 

 60 or 100 toises by these causes. He has also found, by similar 

 calculations, that the degrees measured in Italy, and at the 

 Cape of Good Hope, may be very sensibly affected by the 

 attraction of hills, and defect of the attraction in the Medi- 

 terranean Sea and Indian Ocean." Phil. Trans, vol. Iviii. or 

 New Abridgment, vol. xii. p. 578. 



With respect to the third cause of irregularity, Puissant, 

 Geodesie, p. 137, remarks, that " anomalies in the latitudes, 

 are, doubtless, produced by local attractions which change the 

 direction of the apparent vertical." And Professor Playfair, 

 in the excellent memoir I have just quoted, (a memoir, it 

 should he recollected, which was written five years before the 

 remarkable anomalies in the English measures were known) 

 affirms., that " from suppositions no way improbable, con- 

 cerning the density and extent of masses of varying strata 

 beneath the surface, he has found, that the errors thus produced, 

 may easily amount to ten or twelve seconds.'" " This cause of 

 error, (as he justly remarks) is formidable, not only because 

 tt may go to a great extent, but because there is not any visible 

 mark by which its existence may always be distinguished." 



Here, then,, are three sources of deflection from the true 

 plumb-line, neither of which is correctly appreciable in all 

 circumstances, yet of which each may be not only percep- 

 tible, but important j and the concurrent effect of all may, 

 doubtless, be vory considerable. Yet, Don Rodriguez is un- 

 willing to attribute a deviation of 4. or 5 seconds, to any, or all, 

 of these causes. 



4. This 



