SIZE IN SHALLOW WATER 59 



in shoal water until they are about as high as the water is deep often 

 continue to travel onward thereafter over a comparatively level bottom 

 for considerable distances. 



In this case, the intermittent spilling along the tops of their crests 

 prevents any further increase in their heights either until their prog- 

 ress brings them into deepening water or until more active breaking de- 

 creases their heights to accord with the smaller depth, if their advance 

 carries them across some still shoaler bar. Shoal water also seems to 

 limit the sizes of waves in some other way, for often they are not as 

 high there as the water is deep, even in situations where the wind 

 strength and the fetch are enough for this. The distance, for example, 

 in Pamlico Sound, N. C, is long enough from shore to shore (40 to 

 45 miles) for the waves produced by the strong southwesterly winds 

 of summer to rise to 9 or 10 feet towards the end of their run, where 

 the water is 2 to 3 feet deeper than that over a considerable area. But 

 the local boatmen have informed us that the waves seldom, if ever, are 

 higher there than perhaps 6 to 7 feet, no matter how strong the wind. 

 And Pamlico Sound appears to be typical of similar situations else- 

 where in this respect. 



The explanation for this failure of the waves to rise higher in 

 shallow sounds lies, we believe, in the effect that shoal water has on 

 the lengths of the waves. Unfortunately, it is not yet known whether 

 the relationship between wave length and depth of water follows the 

 same curve for waves that are developed over shoal bottoms as it does 

 for those that advance from deep water into shoal. But it is at least of 

 the same order; i. e., waves in such situations grow more slowly in 

 length than they would in deep water, hence they are steeper, so that 

 they may commence to break sooner than they would otherwise. Esti- 

 mations of the waves to be expected in enclosed sounds under any par- 

 ticular combination of wind, fetch, and depth, must await theoretical 

 analysis of the subject, but the following rules appear to apply : (a) 

 the waves in such situations will never be much higher than the water 

 is deep and very likely will be considerably lower; (b) they will be 

 steep and will break along their crests throughout the greater part of 

 their runs, if the wind is strong. 



The preceding discussion of the waves in shallow sounds applies 

 equally to the inshore ends of the wave trains that are generated by 

 strong winds blowing parallel to open coasts in regions where the slope 

 of the bottom is more than usually gentle. The strong "Northers" that 

 sometimes strike the southwest coast of Florida illustrate this, for 

 while the fetch is long enough, theoretically, for a 30-mile wind to 

 produce waves 18 feet high by the time they have advanced the length 

 of the Peninsula from the offing of Apalachicola to the offing of Key 

 West, they cannot be more than 12 feet high, anywhere along the 2- 



