380 



EEPOET — 1892. 



be used, two facts, and two facts alone, can be represented by the position 

 of each point on the surface. 



The practical appHcations of the two numerical facts represented by 

 each point must now be considered. In the case of the single fixed-point 



Fig. 8. 



jQ A 



a 



'.A 



1 



' 3 



o. 



system, some means of notifying each quantity equivalent to writing 

 upon it the thing represented was required. 



Suppose that in fig. 4 the height of the tide at different times had 

 been represented by the distance Oj A, Oi B, &c. Now, instead of 



/ 2 3 4 £ G 7 8 6 10 11N00U 1Z3-tS6T8 



to II It 



writing the hour of the tide on each distance fi-om the base line, or even 

 denoting these different hours by A, B, C, D, a much better plan is to 

 take the time as a second quantity, to be measured from the second point. 



