66 



The set-square is now held in this position and the apertures 

 at the ends of the line are pierced so as to make points on the 

 graph paper. The set-square is taken away and a fine line in 

 red mk is ruled on the graph : this will appear to coincide with 



Figs. 4 and 5. 



that part of the summational curve which is sensibly straight. 

 By inspection we find the points where curve and tangential 

 line begin to diverge, and half-way between them we may take 

 to be approximately the point of inflexion. This point is 

 marked and a perpendicular is dropped from it to cut the 

 horizontal axis. This latter point of intersection, read off on 

 the scale of lengths, gives the abscissa of the mode, or maximum, 

 of the frequency curve. 



The Points of Inflexion. 



Let the summational curve be supposed simple. There 

 will be two places on it where its curvature is greatest, and these 

 can be found graphically as follows : — 



Rule two parallel lines, about 1 cm. apart, on a transparent 

 set-square and rule another line perpendicular to both at about 

 the mid-points of the parallel lines. Graduate the lower line 

 in mms. Rotate the ungraduated line on the set-square on 

 the summational curve at the places of greatest apparent 



