of the air resistance against the wave form, (2) the wave velocity 

 (period) Increases continuously. 



"FRONT " OF ADVANCING ENERGY 



Figure 10. Advance of m'-ave energy in time t_ from 

 a source into still water. A very small amount of 

 the energy has advanced the distance _Ct_. The region 

 of rapid increase, "the front," has advanced the dis- 

 tance C_ t. 

 2 



When the problem is treated analytically it is not necess- 

 ary to introduce any new constants. The travel time of the waves 

 and the decrease in wave height can be obtained as special solu- 

 tions of the fundamental equation which was discussed in the sec- 

 tion on the groiATth of waves. 



The results of this ana],ysls are presented in Plate VI. The 

 coordinates are the wave period at the end of the fetch, T-^,, and 

 the distance of decay , D, that is, the distance which the waves 

 travel through areas of calm. The main part of the graph contains 

 two sets of curves. One set gives the factor by which the wave 

 height at the end of the fetch, IL-,, must be multiplied in order to 

 find the height o:" the swell at the end of the distance of decay, 

 Hp. The other set gives the travel tl.ae, t (in hours) for the 



'D' 



29 



