"Z^O 



ILLUSTRATIONS OF 



207. The tendency of the primary ft 



to b 



ftraight in the horizontal dire6lion, and- 

 bent in the vertical, is a phenomenon 



which points very dired:ly 

 whence it has arifen. 



the 



fes from 



n. A furface of limple cur- 

 farface ftraight in one dire6lion, is 



what 



very much- 



um 



i^efpecl; to the meridian, may be i 

 iticlined to , one another. A flra 

 and well in Cornwall, and one that does the fame at the 



r 



eaft end of the Altaic chain, will be in planes, which, if 

 produced, Vv^ould cut one another at right angles. All 

 this is fufficiently plain from the doarine of the fpherc,, 

 and is mentioned here, merely as a caution to prevent 

 too haily conclulions from being drawn from any corre- 



among; the flrata of remote 



tries. 



For the fake of thofe who would deduce the medium 

 bearing of any body of flrata from a number of obfer- 

 vatlons, it may be proper to take notice, that the true 

 average is not to be found by fimplj taking an arith- 

 metical mean among all the obfervations. A more ex- 

 ad waj is to work by the traverfe table, as in keep- 



reckoning, (fuppoling 

 y), and to compute from 



the 



amount 

 calling 



vided 



or wefcing. 

 e fum of al 



i fum 



form 



tl\ IS 



he latter, di- 

 the tang-ent of 



fehe angle which the general diredion of 

 with the meridian. 



/^ 



