218 LEVELS OP FRANCE. 



As thefe two co-ordina(es inlerfisfl each other at right angles,, 

 it is apparent that the method of geographers, for determining 

 the pofition of any place on the earth, is the fame as that by 

 which the poiition of a point on a plane is commonly deter- 

 mined. 

 jsnotexa<a, nft But this procefs, which would completely anfwer the views 

 account of the of geographers, if the terreftrial fphere were regular, ceafes 

 earth's furface. to be exacl when the irregularities and protuberances are con- 



fidered, with which the furface of this fpheroid is covered. 

 This true pofi- The poruif)n of any place depends in reality, according to 

 tionofaplaceiS([jjj| i^^jj on a third co-ordinate, which is fuppofed to 



»n a line pcrpen- .' i ' - rr 



dicularto that be drawn perpendicular to the point of interfeclion of the two 



aff.gned by gco- others. 



giaphers» 



This third co-ordinate ought to be taken vertically over the- 



place of which the pofition is to be rfcterrained, and its mea- 

 fure reckoned Irom the place ilfelf to its arrival at an imagi- 

 nary furface, produced by the revolution of a known curve 

 round tlie axis of the earth. 

 The level of the But it is Jinown, that if our globe were furrounded by a fluid 

 ?heSfu* mafs, all otiier force being fuppofed to be abfent but that of 

 fac?, from terreftrial gravity, the furface of this fluid mafs would be that 



whence to mea- ^f ^ fpherical folid, of which the mean furface of the fea, in 

 lure thole per- . _ , „ i- • r i 



pendiculars. its aclual Hate, reprelents a part : It appears then convenient 



to clioofe, for the third co-ordinaic here mentioned, that por- 

 tion of a vertical line palling through any place, which is 

 comprifed between that place and tiie mean furface of the 

 (ea, (uppofed to penetrate the globe and to be extended be- 

 neath the continent. 

 This h thcbeft, We have faid that the choice of this line would be conveni- 



thoughnotthe gj^j becaufe, in reality, the politiim of a point on the ter- 

 only method : _ •' ' ' 



reftrial furface oiay be determined by adopting any other fyf- 



tem of co-ordinaies ; for exauiple^ by fixing the pofition of 



this point, by three planes mutually interfecling at right angles ; 



But it is the but, befides the advantage of greater fimplicity in the expref- 



moftfimplc, and q^^ ^f {^g circular co-ordinales, they have moreover, that of 



IS Deiide3 gene- , ^ ^ . _ 



rally adopted. being generally adopted ; for the geographical charts, hither- 

 to pre[)ared, may be confidered as the proje6lion of the con- 

 The true pofi- tinents and iflands on the mean furface of the tea ; fo that there 

 tion of a place ^^^ remains, in order to render jjeoijraphy perfeft, to add to 



determined by - . ,• t. i 



annexing its the latitude and longitude ot all the places on the earth, the 



vertical 



