256 



THE GRAMMAR OF SCIENCE 



along the surface of still water in opposite directions {a), 

 or in the same direction if the pursuing wave had the 

 greater speed. One of these waves would meet or over- 

 take the other {U) ; they would coalesce or combine (^), 

 producing in us for a time (which depends entirely on 

 their relative speeds) a new group of sense-impressions 

 differing totally from either individual group ; but they 

 would ultimately pass each other {d) and emerge with 



Wave II 



W --=:^ 



Wave II 



Fig. 19. 



their distinct individualities the same as of old {/). 

 Throughout the whole of this sequence the substrata of 

 the two individual waves are changing and for the time of 

 the combination their substratum is identical, and yet the 

 waves are able to preserve their individual characteristics, 

 so far as reappearing with them after combination is con- 

 cerned.^ Thus sameness of sense-impressions before and 



1 If analogy were to be sought to the sameness of total weight before, 

 during, and after combination, it might be found in the sameness of the 

 volume of fluid raised above the sea-level before, during, and after coalition. 

 Thus sameness of weight does not in conception necessarily involve sameness of 

 substratum. 



