Relating to the FIGURE of the EARTH: 19 



north, it will probably be determined how far the diftributioa 

 of the ftrata of this country afFedts the direcflion of gravity. It 

 will indeed be curious to remark what irregularities take place 

 on advancing into the denfer ftrata of the north. The limeftone 

 and fandftone ftrata of the middle part of the Ifland will fuc- 

 ceed to the chalk of the fouth, the primitive and denfer ftrata 

 ftill occupying the weft, at leaft at intervals, as in Wales, Cum- 

 berland, and Galloway. Further to the north, that is, beyond 

 the Tay, the ftrata become entirely primitive, moft of them of 

 the denfeft kind, and in the interior of the Ifland, with a very 

 few exceptions, continue the fame to its moft northern extre- 

 mity. In the furvey of Britain, therefore, feveral fituations 

 muft occur where the plummet, pafTmg from lighter to denfer 

 ftrata, ought to give indications of fome irregularities in the 

 direction of the gravitating force. It will be feen hereafter 

 how far thefe conjedlures are verified by experience. 



21. A REMARK, that is in no danger of being reckoned hypo- 

 thetical, is, that the concluGon derived from the comparifon of de- 

 grees of the meridian, with degrees of the circle perpendicular to 

 it, becomes of neceifity more liable to error as we advance into 

 higher latitudes. The reafon is, that whatever error is commit- 

 ted in determining the magnitude of D' — D, muft be multi- 

 plied into the fquare of the fecant of the latitude, in order to 



give its full effed in changing the value of the fradlion -. For 



it has been fliewn, that ~ = ^-( ^ ^,° ) fee '<p ; now, if we fup- 



pofe the error committed in afcertaining D' — D to be in all 



cafes the fame, the error of the fradion ~° - will alfo be in.. 



all cafes nearly the fame, the denominator D' being but little af- , 

 feded either by tlie fuppofed error, or by the change of lati- 

 tude. But this error, which may thus be confidered as a conftant ' 



C 2 quantity, 



