ON LIGHT, 299 



crests of the one series shall reach it at the same identi- 

 cal moment with the crests of the other ; the two series of 

 crests conspiring and being superposed on each other 

 will produce crests of double the -height of either singly : 

 while, on the other hand, if the difference of channels be 

 such that the crests of the first series shall reach it simul- 

 taneously with the troughs (or lowest depressions) of the 

 second ; the one will destroy the other, and there will be 

 neither elevation nor depression at their joint point of 

 arrival. In the former case, supposing the two channels 

 thenceforward to unite into one (as in the annexed figures, 

 which require no explanation further than that the series 

 of cross lines represent the crests of the waves), the two 

 series when they reunite in a channel c D, as in Fig. 8, the 

 exact size of the initial one, A B, will form a joint series 

 exactly similar to that in A B, which will run on in that 

 channel thenceforward ; but in the latter, as in Fig. 9, there 

 will be produced no waves at all, and the water in c D 

 will (except just close to the point of junction, where 

 some kind of eddy will be formed) remain undisturbed. 



(82.) Accepting the term "wave" in its most general 

 sense, in whatever way we suppose it propagated, whether 

 by alternate up-and-down movements of the successive 

 particles, as in water-waves by transverse lateral ones, 

 as in a stretched cord wagged horizontally or by direct 

 to-and-fro vibration, as in the air-waves, in which sound 

 consists or in any more complex manner, the same 

 considerations evidently apply. If two sets of exactly 

 equal and similar waves can by any previous arrange- 

 ment be made to arrive simultaneously at the "entrance' 1 



