26 Mr. F. Galton on The Just-Perceptible Difference, [Jan. 27, 



intervals, in either direction, horizontal or vertical. Half an hour's 

 practice suffices to learn the numbers. The figures 0, 8, and 9 do 

 not enter into any of the couplets in the schedule, the remaining 51 

 couplets in the complete series of 100 (ranging from 00 to 99), con- 

 tain 21 cases in which 0, 8, or 9 forms the first figure only ; 21 cases 

 in which one of them forms the second figure oniy ; and 9 cases in 

 which both of the figures are formed by one or other of them. These 

 latter are especially distinctive. This method has five merits — 

 medium, short, or very short steps can be taken according to the 

 character of the lineation at any point ; there is no trouble about 

 orientation ; the bearings are defined without a protractor, the work 

 can be easily revised, and the correctness of the records may be 

 checked by comparing the sums of the successive small co-ordinates 

 leading to a point of reference, with their total value as read off 

 directly. 



A method of signalling is also in use for military purposes, in 

 which positions are fixed by co-ordinates, afterwards to be connected 

 by lines. 



[F. G.] 



