378 Prof. NECKER on the Position of Strata in Stratified Rocks. 



And the protractor g h would have at its central part a hollow 

 tube /, fitting exactly to the cylinder /" of the brass circle a b. 



The second method, which is still better, and has' been sug- 

 gested to me by Mr ROBISON, is that of adapting the brass cylin- 

 der at the centre L or / of the protractor. This cylinder should 

 be made to enter into a brass cylindrical tube occupying the 

 centre /' of the half circle c d, and projecting under the half 

 circle to a length equal to the thickness of the brass circle A B 

 or a b. This tube would be received in a hole of equal size cor- 

 responding to the central part I" of the circle a b. 



