AND OTHER WATER WAVES 123 



the wind is shown by the arrow B. Here, 

 moreover, at about 4-ioths the distance from 

 centre to edge, the strongest winds are usually 

 developed. The line of advance of B will, there- 

 fore, be the line along which the greatest wave- 

 development will occur. There is not any constant 

 relation between the rate of advance of a cyclone 

 and the velocity of the winds locally developed 

 within its area. Considering only the critical posi- 

 tion B in the cyclone, the rate of advance of the 

 cyclone is, from our present point of view, simply 

 the rate of advance of the locus of the force which 

 is there creating the waves ; we have to do, in 

 fact, with waves created by a travelling disturbance . 

 Let us consider groups of simple harmonic waves 

 of different lengths, and therefore different speeds, 

 to be already formed and to be travelling together 

 (and therefore superimposed upon one another) 

 as forced waves pressed upon by the wind. This 

 wind, however, in a progressive cyclone is a 

 " travelling disturbance," and while it will, to some 

 extent, increase all the waves beneath it which it 

 can press upon at all, the waves which move slower 

 than the travelling disturbance are being left behind 

 all the time. On the other hand, all waves whose 

 velocity is greater than that of the travelling dis- 

 turbance run ahead of it, and are no longer subject 



