ON LIGHT. 



297 



theory help us in this difficulty, while at the same time 

 rendering an equally satisfactory account of the other 

 facts 1 To this we are enabled to reply in the affirma- 

 tive. Two equal sounds we know, under certain circum- 

 stances, can produce silence, as when the two strings 

 which, in a pianoforte, go to produce, when exactly in 

 unison, a uniform and liquid note ; if very slightly out 

 of tune, produce what are called beats, or a succession 

 more or less rapid (accordingly as the strings are more 

 or less discordant) of sound and silence. The same 

 tide-wave arriving at the same spots in the sea by two 

 courses of different lengths, results in producing no rise 

 and fall of the water at all, if the difference of path be 

 such that the high water of one portion shall reach 



Fig. 7. 



the place at the same moment with the low water of 

 the other. This is the case at a point in the North 

 Sea, midway between Lowestpft and the coast of Hol- 

 land, in lat. 52 27' N., long. 3 14' E. Its position was 

 pointed out by Dr Whewell from theory, and the fact 

 verified by Captain Hewett, R.N. 



(8 1.) This latter exemplification contains the essential 

 principle of the explanation in question, in nearly its 

 simplest state. If two waves, or rather two regular series 

 of equal waves all exactly like one another, and all 



