42 



A B C OF THE STEEL SQUARE 



on the outside of the blade, as shown iri Fig. 

 20, it will be proof that the square is correct. 

 It may be somewhat difficult to make a meas- 

 urement of this kind on the instrument itself, 

 with sufficient accuracy to be beyond dispute. 

 I suggest, therefore, that the square be laid flat 

 upon an even surface, like a drawing table, and 

 that with a chisel-pointed pencil lines be scribed 



Fig. 20. 



along the tongue and along the blade. Mark 

 accurately the distance of 9 inches from the 

 heel up the tongue, and 12 inches from the heel 

 along the blade. Then measure diagonally and 

 see if the distance is exactly 15 inches. 



In what has preceded there has been a sugges- 

 tion that the error due to lack of precision in 

 measurement is diminishes if the figures are in- 



