40 A B C OF THE STEEL SQUARE 



Draw a straight line, AB, say three feet in 

 length, as shown in Fig. 19. This may be done 

 by a straight-edge. Use a hard pencil sharp- 

 ened to a chisel point. With the compasses, us- 

 ing A and B as centers, and with a radius long- 

 er than one-half of AB strike the arcs CD and 

 EF. Then with the straight-edge draw a 

 straight line, GH, through the intersection of 

 the arcs. If the work is accurately done the re- 

 sulting angles AOH, HOB, BOG, and GOA will 

 be right angles. Lay the square to be tested 

 onto one of these angles, as shown in the illus- 

 tration, and with a chisel-pointed pencil scribe 

 along the blade and along the tongue. If the 

 lines thus drawn exactly coincide with those first 

 drawn it is satisfactory proof that the square 

 is accurate, and in the same way the square 

 may be placed against one or the other of these 

 right angles in a way to test its interior angle. 

 The method shown in Fig. 19 anticipates the 

 use of another tool besides the square in making 

 the test- A right angle, however, may be drawn 

 for the purpose described by a method which 

 uses only the square, and which does not re- 

 quire the services of any other tool, or what is 

 the same thing, consider the tool itself to be the 

 figure drawn, and then measure for the purpose 

 of determining the accuracy of the figure. 



