Position of Strata in Stratified Rocks. 



377 



Postscript. 



As it is necessary that the protractor should stand perfectly 

 vertical, another mode of adjusting it to the centre of the instru- 

 ment must be devised, instead of that which is alluded to in the 

 paper, and exemplified by the instrument itself. For this pur- 

 pose two different methods may be adopted. The first is repre- 

 sented in the annexed figure ; the brass circle A B or a b, should 



bear as its central part a solid brass cylinder /" of at least two or 

 three lines in length, and one or two in diameter, it would be 

 received in a hole I' of the same diameter in the half circle cd. 



