SECT. XIII. INTERFERENCE OF WAVES. 99 



moon would have produced separately a phenomenon depending 

 upon the interference of the waves or undulations. 



A stone plunged into a pool of still water occasions a series of 

 waves to advance along the surface, though the water itself is 

 not carried forward, but only rises into heights and sinks into 

 hollows, each portion of the surface being elevated and depressed 

 in its turn. Another stone of the same size, thrown into the 

 water near the first, will occasion a similar set of undulations. 

 Then, if an equal and similar wave from each stone arrive at the 

 same spot at the same time, so that the elevation of the one 

 exactly coincides with the elevation of the other, their united 

 effect will produce a wave twice the size of either. But, if one 

 wave precede the other by exactly half an undulation, the eleva- 

 tion of the one will coincide with the hollow of the other, and the 

 hollow of the one with the elevation of the other ; and the waves 

 will so entirely obliterate one another, that the surface of the 

 water will remain smooth and level. Hence, if the length of 

 each wave be represented by 1, they will destroy one another at 

 intervals of |, |, , &c., and will combine their effects at the inter- 

 vals 1, 2, 3, &c. It will be found according to this principle, 

 when still water is disturbed by the fall of two equal stones, that 

 there are certain lines on its surface of a hyperbolic form, where 

 the water is smooth in consequence of the waves obliterating each 

 other, and that the elevation of the water in the adjacent parts 

 corresponds to both the waves united (N. 160). Now, in the 

 spring and neap tides arising from the combination of the simple 

 solilunar waves, the spring tide is the joint result of the combi- 

 nation when they coincide in time and place ; and the neap tide 

 happens when they succeed each other by half an interval, so as 

 to leave only the effect of their difference sensible. It is, therefore, 

 evident that, if the solar and lunar tides were of the same height, 

 there would be no difference, consequently no neap tides, and the 

 spring tides would be twice as high as either separately. In the 

 port of Batsha, in Tonquin, where the tides arrive by two channels 

 of lengths corresponding to half an interval, there is neither high 

 nor low water on account of the interference of the waves. 



The initial state of the ocean has no influence on the tides ; for, 

 whatever its primitive conditions may have been, they must soon 

 have vanished by the friction and mobility of the fluid. One of 

 the most remarkable circumstances in the theory of the tides is 



p 2 



