Relating to the FIGURE of the EJRTH. ^ 



2: But though — , or fome fradl'ion not very different from 



it, fliould be admitted as the moft probable value of the com- 

 preffion, or ellipticity, as it is called, of the terreftrial fpheroid, 

 it ftill remains to be explained, why all the obfervations, confi- 

 dering the care with which they have been made, do not agree 

 more nearly with this conclufion. Among the caufes that may 

 be affigned for this inconfiflency, thovigh unavoidable miflakes^, 

 and the imperfedlion of inflruments, muft come in for a part, 

 there can be little doubt that local irregularities in the- diredion 

 of gravity have had the gx^eateft £hare in producing it. Of thefe 

 irregularities, that which arifes from the attraction of moun- 

 tains has had its exiftence proved, and its quantity, in one cafe, 

 afcertained, by the very accurate obfervations of the prefent 

 Aflronomer-Royal at Schehallien in Perthfhire. We may trace 

 the operation of this caufe in many of the degrees that have 

 been actually meafured. Thus, in the degree at Turin, when 

 divided into two parts, and each eftimated feparately, that which 

 was to the north of the city, and pointed toward Monte Rofa, 

 the fecond of the Alps in elevation, and the firft perhaps in 

 magnitude, was found greater in proportion than that toward 

 the ibuth, the plummet having been attradled by the mountain 

 above mentioned, and the zenith made of confequence to recede 

 toward the fouth. There are no doubt fituations in which the 

 meafurement of a fmall arch might, from a fimilar caufe, give 

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

 tive. 



But there is another kind of local irregularity in the direc- 

 tion of gravity, that may alfo have had a great effe<5l in difturb- 

 ing the accuracy of the meafurement of degrees. The irregula- 

 rity I mean is one arifing from the unequal denfity of the ma- 

 terials under and not far from the furface of the earth ; and 



this 



