122 ABC OF THE STEEL SQUARE 



of the board to this. Lay off this length on the 

 three-ply at B D (Fig. 82) ; take the width B C 

 (Fig. 84) on the blade, rise at C on the tongue; 

 find the hypothenuse, and apply with a pair of 

 compasses at Fig. 82 with B as a center cutting 

 at C. Then apply at D as a center, cutting 

 at A. Now find the hypothenuse C to D (Fig. 

 84), and apply the compasses as before, with 

 D and B as centers, cutting at A and C. Con- 

 nect up the points where the arcs intersect to 

 B and D ; this is the face of the inclined prism, 

 and contains the true shape of rail. Continue 

 the tangent line C B, 3 inches or whatever is 

 required for the shank, and square the joints of 

 the lines B and D. In order to locate the major 

 axis the horizontal trace is now required. Stand 

 the bed block on the plan (see Fig. 83). Run 

 the blade of the square down until it touches the 

 board; mark this, and remove the block. It will 

 be seen that the bed block has not got the 3 

 inches allowed at the bottom, but the horizontal 

 trace is as easily found with as without the 

 allowance ; all that is required in the former case 

 being to turn the blade to B (still keeping the 

 heel at the top of the bed block), make a mark 

 where the square touches and lay on the square 

 as shown at Fig. 83. Mark at the blade, and 

 glide back the square until the tongue touches 



