I 



DISTURBANCES IN THE OCEAN. 173 



volume of steam developed by the entry of water into a 

 volcanic vent, as the water was heaped above the disturbed 

 area, water might run in radially towards this spot. 



Supposing a primary wave to be formed in the ocean 

 by any such causes, then the falling of this will cause a 

 second wave to be formed, existing as a ring round the 

 first one. The combined action of the first and second 

 wave will form a third one, and so the disturbance, start- 

 ing from a point, will radiate in broadening circles. 

 During the up and down motion of these waves, the 

 energy which is imparted to any particle of water will, 

 on account of the work which it has to do in displacing its 

 neighbours, by frictional resistance, gradually grow less 

 and less, until it finally dies away. The waves which are 

 the result of this motion v/ill also grow less and less. 



If a series of sea waves were produced by a single dis- 

 turbance, we see that these will be of unequal magnitude. 

 Now, for small waves, the velocity with which they travel 

 depends upon the square root of their lengths ; but with 

 large waves, like earthquake waves, the velocity depends 

 upon the square root of the depth of water, and these 

 latter travel more quickly than the former. 



If, therefore, we have a series of disturbances of un- 

 equal magnitude producing sea waves, which, from the 

 series of shocks which have been felt upon shores subse- 

 quently invaded by waves, seems in all probability often 

 to have been the case, it is not unlikely that the waves of 

 an early disturbance may be overtaken and interfered with 

 by a series which followed. 



These considerations help us to understand the appear- 

 ance of the records on our tide gauges, and also the 

 phenomena observed by those who have recorded tidal 

 waves as they swept inwards upon the land. For instance, 

 we understand the reason why sea waves, as observed at 



