CHAP, v.] PLOTTING 123 



factured, should be cut to an edge, not a point, and the 

 straight-edge being placed in position, and weighted to keep 

 it in contact with the paper throughout its length, the flat side 

 of the pencil is placed against it, and tried at both points, to 

 see whether the line will pass truly over them. Care must 

 then be taken to hold the pencil in the same position while 

 drawing the whole line. 



In laying off by chords an angle over 60°, or a little under Angles 

 60°, it will be found best to mark off 60° first, and measure °'''' ^° 

 the remainder of the angle from the 60° prick. This is done 

 by drawing short arcs with the radius used, from the station 

 from which it is desired to lay off the angle, and from the 



Fig. 22. 



I? 



1^ 



radius prick (H in last figure), the intersection of these must 

 be pricked off as 60°, and another short arc being drawn with 

 the originating station as centre, the chord of the difference of 

 the angle from 60° is measured from the 60° prick to the last 

 short arc, as in Fig. 22 (above). 



This is done not from any incorrectness of the principle if 

 the angle were laid off at once, but because it is inconvenient 

 to be measuring long distances as chords, as there is a greater 

 chance of some little inequality of the paper causing error, 

 and also, the longer the chord measured, the more acute will 

 be the angle between the two intersecting arcs, and conse- 

 quently the greater the difficulty of pricking in accurately at 

 the intersection. 



