4v0 
Miscellanea 
inclination of the scale is adjusted so that, when it moves along the T-square, the zero-point will 
pass over the mean il and the point 5 over the point (S". The units of the scale will then give 
fifths of the standard deviation. Once the inclination has been adjusted, the T-square may be 
moved iq> or down if necessary to complete the plotting. The verticals are then drawn in, and 
the value of ?/q marked off on the central ordinate. The T-square being now placed parallel to 
the base, the scale and square are adjusted so that the zero point lies over the base and the point 
10 over the top of y^. The tabular values of e"^^' then give the remaining ordinates directly 
from the scale readings. 
Even the trouble of extracting or reading the tabular values may however be eliminated, if a 
special scale be engraved, in which the distances of the divisions from the left-extremity are pro- 
portional to the ordinates e"^^'. The inclination of the moving blade being once adjusted as 
above, the remaining ordinates are plotted direct from the divisions of the scale without 
interpolation. 
As a matter of practice I have thought it best for the scales to be made, as in fig. 2, with the 
joint like an ordinary carpenter's rule, such a joint being stiff" and wearing fairly well. The centre 
portion of each edge of the rule is bevelled for plotting, but the ends are left thick to prevent 
the rule slipping under the T-square or straight-edge. Four scales can be engraved on such a 
rule. 
A 
B 
Fig. 1. AB is a straight-edge or T-square. 
It is evident that the principle is applicable to a variety of curves, and I have had special 
scales engraved for plotting ellipses (the scale being given by equidistant ordinates in a quadrant 
of a circle), arcs of parabolas (such as are i-equired in drawing parabolic girders, bending 
moment curves, etc.), curves of sines or cosines, and the normal curve of errors. The scales were 
made for me by A. G. Thornton, King Street West, Manchester. For statistical work, three 
plain scales of different units and divisions and a normal-curve scale form a convenient set. 
With the i^lain scales, curves of any sort are readily plotted with any desired ratio of length to 
breadth, e.g. for reproduction on a given page or lantern slide. 
