CHAP, v.] PLOTTING 121 



Forty-five inches must be carefully measured, by the brass 

 diagonal scale, on to a pair of beam compasses, with the two 

 steel points shipped. Flattening the paper down by placing 

 the straight-edge close to the line A B, and putting weights 

 on it, with the centre A describe a short arc of circle D E, 

 scratching lightly the surface of the paper. Then moving the 

 straight-edge into the direction of C (which can be ascertained 

 roughly by a protractor), and again Aveighting it, make another 

 small scratch F G. With the assistance of a reading-glass, 

 and by means of a needle mounted in a handle, and spoken of 

 as the " Pricker," make a fine prick at the intersection of the 

 lines A B, D E, i.e., at H. 



Look out the versine of 107° 37' 15" (90+ half the required 

 angle), which is 1,302,717. This becomes -302717, which, 

 multiplied by 90, gives 27-244 inches as the chord. 



Measure this distance on the beam compass, and flattening 

 the paper as before, draw, with H as a centre, a short arc 

 K M crossing F G. The point of intersection is to be pricked 

 carefully as before, and the straight-edge can now be laid on 

 A and it, and the line ruled will be at exactly the angle re- 

 quired. This seems a tedious operation, but it is the only 

 way in which points can be got to go down satisfactorily, and 

 in the end much time will be saved. 



It may be noted here, that it is preferable to make a mark Steel 

 with a steel point instead of a pencil, from the practical difii- ^g used, 

 culty of measuring accurately the required distance on the 

 beam compass when the pencil-point is used, as, when the 

 pencil-point is cut sharp enough to make a fine line, it is almost 

 impossible to prevent breakage in applying it to the brass 

 scale divisions. It is also cleaner. In marking, the point 

 must be held sloping, so as only to impress, and not actually 

 to scratch the surface of the paper, which it will do if held 

 perfectly upright. 



Of course, if the paper is stretched on a board instead of 

 being loose on the table, the time and trouble of seeing the 

 paper flat is saved, but this is seldom used in our work. 



If the table of chords is available, look out the chord for 

 37° 14' 30" and multiply it by 4-5, as the table is made out 

 for a radius cf 10 inches. This will give the same quantity of 

 27-244 inches as found above. 



