Dr. Gregory's Strict ures on Don Rodriguex. 18? 



affected by hills and valleys, is also very manifest. Pro- 

 testor Playfair, after describing the irregularities thus oc- 

 casioned in the degree at Turin, adds, " There arc, ??o doubts 

 siiuaiions in which the measurement of a small arc 

 mitrht, from a similar cause, give ihe radius of curvature of 

 the meridian inJiriUe, or even negative." See Edinburgh 

 Transactions, vol. v. p. 3. And Dr. Maskelyne, after 

 treatina of Mason and Dixon's degree in North America, 

 eavs, " Mr. Henry Cavendish having investiirated several 

 rules for finding the attraction of the iiie<.]uaiities of the 

 earth, has, upon probable suppositions of the distance and 

 height of the Alleganv mountains from the degree mea- 

 sured, and the depth and declivity of the Atlantic Ocean, 

 computed what alteration might be so produced in the 

 length of rhe degree; and finds that it may have been di- 

 ftiifiiihed by 60 or 100 toises by these causes. He has also 

 found, by similar calculations, that the degrees measured 

 in l;aly, and at the Cape ot Good Hope, may be very seji- 

 silly affected by the attraction of hills, and defect of the 

 attraction in the Mediterranean Sea and Indian Ocean." 

 Phil. Trans, vol. Iviii., or New Abridgement, vol. xii. 

 p. 379. 



With respect to the third cause of irregularity Puissant, 

 Geodisie, 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 ju^t quoted, (a 

 memoir, it should be recollected, which was written Jive 

 years before the remarkable anomalies in the English mea- 

 sures were known,) affirms that "from suppositions no 

 way improbable, concerning ihedensity and extent of masses 

 of varying strata ^)eneath the surface, he has found, that 

 the errors thus produced may easily amouvt to Itn or twelve 

 seconds.'\ "This cause of error (as he justly remarks) is 

 formidable, not only because it may go to a great extent, 

 but because there is not any visible mark by which its ex- 

 istence may always be distinguislied." 



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

 j'lumb-line, neither of whici) is correctly appreciable in all 

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

 ble but important} and the concurrent effect of all may, 

 douljtlcss, be very considerable. Yet Don Rodriguez is 

 unwilling to attribute a deviation of four or five seconds to 

 any or all of these pauses. 



4, This writer infers that mistakes must have occurred 

 in the obbervaUons, befauie the sum of other " errors will 



bi 



