28 KAEL PEARSON 



DESCRIPTION OF PLATES. 



This memoir is accompanied by two abacs for which I have heartily to thank my assistant 

 Mr G. H. Soper. 



The first abac Plate I gives ^ir^. It has not been used in the text because interpolation from 

 the Tables gave slightly more accurate values, but its readings are quite sufficient for most practical 

 purposes. The maximum difierence we found in determining go-^ for the ten illustrations of this paper was 

 •0006, or a unit in the value of jO-,. read to two significant figures. 



The method of using it is as follows : Run along the horizontal giving the size of the population 

 (left-hand scale), until you meet the vertical giving the value |^ (1 + ij) (bottom scale) ; then follow the 

 45° line through that point till you reach the left-hand scale, take the horizontal through this point and 

 follow it, till you meet the vertical through |(1 -i- a^) (bottom scale), then again follow the 45° line to the 

 left-hand side, and from the point reached traverse the horizontal to the right-hand side of the diagram 

 where the scale gives the proper value of ocr,.. If, in traversing the 45° line we meet the top of the 

 diagram instead of the left-hand side, we follow the usual rule of such abacs, i.e. drop by a vertical 

 through the point to the bottom scale and run up the 45° line through that point to the left-hand scale 

 and continue as before ; the final value read for „(r,. on the right-hand scale has for each such drop to be 

 multiplied by 10. 



The second abac Plate II is entered by the value of log x^ on the left-hand scale and the value of ^<rr 

 on the bottom scale, the meet of the horizontal and vertical lines through these points determine 

 a contour the value of which in " probability correlation '' rp is recorded on the right-hand vertical scale. 



CAMBRIDGE: PRINTED BY JOHN rr AV ma io, ,^„ 



JUHJN CLAY, M.A. AT THE UNIVERSITY PRESS 



