problem can be solved by means of Plate VII, the use of v/hich will 

 be explained when discussing the practical procedure. 



Distance from which Observed Swell Comes; 

 Travel Time; Velocity of Wind which Produced the Swell 



If the height and the period of the swell are observed it is 

 possible to find approximate values of the distance to the end of 

 the generating area from which the swell came, of the travel time 

 of the swell, and of the wind velocity in the generating area. In 

 Plate VIII the coordinates are the height of the swell (in feet) 

 and the period of the swell (in seconds). The plate contains three 

 families of curves: full-drawn curves giving the distance to the 

 generating area in nautical miles, light dashed curves giving the 

 travel time from the generating area in hours, and heavy dashed 

 curves giving the wind velocity in the generating area in knots. 



The values which can be derived from the plate are only 

 approximate because the height and period of the swell depend also 

 upon the ratio between wave velocity and wind velocity (C/U) at 

 the end of the fetch. The graph is constructed for C/U = 0.8, 

 corresponding to average conditions, and gives too high values if 

 C/U is smaller and too low values if C/U is larger. However, 

 variations in C/U between 0.7 and 0.9 will not introduce errors 

 exceeding 10 per cent, but errors will also arise from inaccuracies 

 in the observations of height and period of the swell and from lack 

 of knowledge as to changes caused by following or opposing winds. 

 The values read off from the graphs may therefore be 25 per cent 

 in error. 



31 



