AND OTHER WATER WAVES 95 



very different from that at the end of their long 

 journey. In the first place, the waves which would 

 have been noticed and measured from ship-board 

 would have comprised none of the length of even 

 the shortest seen at the Giant's Causeway. They 

 would have been the waves from 400 to 600 feet 

 long, with periods of less than 1 1 seconds, and they 

 could not have arrived at the Giant's Causeway 

 until later. But when they did arrive as arrive 

 they must it is evident that they gave rise to no 

 remarkable breakers, for the surf was decreasing 



two effects may be represented by the following scheme, in 

 which we take the front wave and place it at the back : 

 1234567 

 7123456 

 6712345 

 5671234 



4567123 

 3456712 



2345671 

 1234567. 



In each succeeding horizontal row the individual waves have 

 moved forward two places, but the group has advanced only 

 one. The late J. Scott Russell's early recognition of two wave 

 velocities appears to have been forgotten. He says, in his 

 Report on Waves, B. A. meeting at York, 1844 : " I have found 

 that the motion of propagation of the whole group is different 

 from the apparent motion of wave transmission along the 

 surface ; that in the group whose velocity of oscillation is as 

 observed, 3*57 feet per second, each wave having a seeming 

 velocity of 3*57, the whole group moves forward in the direction 

 of transmission with a much slower velocity." 



