. DARKNESS FEOM TWO LIGHTS. 199 



wave twice the size of either; but if the one 

 wave should be just so far before the other, that 

 the hollow of the one coincided with the elevation 

 of the other, and the elevation of the one with 

 the hollow of the other, the two waves would 

 obliterate or destroy one another, the elevation 

 as it were of the one filling up half the hollow 

 of the other, and the hollow of the one taking 

 away half the elevation of the other, so as to 

 reduce the surface to a level. These effects will 

 be actually exhibited by throwing two equal 

 stones into a pool of water, and it will be seen 

 that there are certain lines of a hyperbolic form 

 where the water is quite smooth, in consequence 

 of the equal waves obliterating one another, while, 

 in other adjacent parts, the water is raised to a 

 height corresponding to both the waves united. 



In the tides of the ocean we have a fine 

 example of the same principle. The two immense 

 waves arising from the action of the sun and 

 moon upon the ocean produce our spring-tides by 

 their combination, or when the elevations of each 

 coincide ; and our neap-tides, when the elevation 

 of the one wave coincides with the depression of 

 the other. If the sun and moon had exerted 

 exactly the same force upon the ocean, or pro- 

 duced tide waves of the same size, then our neap- 

 tides would have disappeared altogether, and the 

 spring-tide would have been a wave double of the 

 wave produced by the sun and moon separately. 

 An example of the effect of the equality of the 

 two waves occurs in the port of Batsha, where 

 the two waves arrive by channels of different 

 lengths, and actually obliterate each other. 



Now, as sound is produced by undulations or 

 waves in the air, and as light is supposed to be 



