MATHEMATICAL INSTRUMENTS. 



45 



The face of the glass centre-piece, on which the lines are 

 drawn, is placed as nearly even with the under surface of the 

 instrument as possible, that no parallax may be occasioned 

 by a space between the lines and the surface of the paper. 



By help of the vernier, the protractor is graduated to 

 single minutes, which, taking into consideration the numer- 

 ous sources of inaccuracy in this kind of proceeding, is the 

 smallest angular quantity that we can pretend to lay down 

 with certainty. Price, $18.00 to $40.00. 



Fig. 74, 



