14 On the Comtruciion of Sea Harbours* [July^ 



times in a minute. Any increase to this rate would injure ves- 

 sels in the ebb tide, when they strike the ground. 



b. The surf is raised, and driven forward by the wind, and of 

 course takes its direction. If a part of the surf be stopped by 

 the outside of the pier, the adjoining part continues to move 

 forward, leaving the water on the innerside undisturbed, except 

 by lateral agitation, for a given distance 

 forward. Let fig. 3, represent this posi- 

 tion. 1, 2, are parts of the piers of a 

 harbour, rejecting the surf. Let b, 1, 

 be the direction of the wind, and a b d o. 

 line of waves moving in this direction. 

 The part a b, is stopped by the pier ; but 

 the part b d continues to move forward, 

 say to h ; beyond this point, the waves 

 lengthen out towards the pier 2 ; and 

 are limited on this side by the curved 

 line h I. Thus while the exterior sea 3 is covered with a high 

 ^urf, the part 4 has only the lateral agitation ; and the harbour 

 5, being acted on by the same force only through the entrance 

 h 1, is nearly quiescent. 



The passing surf affects the harbour 5 least in the direction 

 a c? ; as the sea withia this hue, in the parts 3 and 4, is compa- 

 ratively smooth : and most when coming in the line 0,2 ; as 

 then the part 4 exterior to the line 2,0 is equally covered with 

 the part 3, by the surf; and of course a stronger lateral agita- 

 tion is forced through the entrance. The surf indeed almost 

 enters the harbour when driven in the line c h, and would dis- 

 quiet it more than in either of the former courses ; but that a 

 gale in this line, making only a small angle with the lee shore, 

 never raises a high surf at the harbour, and the surf is least in a 

 gale directly into the entrance h i ; for as it faces the calmest 

 quarter, or, in other words, as the lee shore, at no great dist- 

 ance, projects into the sea, and covers the entrance, this gale 

 comes over too small a range of sea, to force an injurious surf 

 into the harbour. This description developes the mode of 

 forming a harbour that rejects the surf. 



c. When a wave strikes a pier at right angles, it rebounds 

 directly back; but if it strike obliquely, the angle of rebound is 

 equal to that of percussion. In oblique percussion, the reflected 

 surf is greater at the leeward than at the windward end of the 

 pier, by the amount of such surf collected through the whole, or 

 a part, of the length, according to the strength or frequency of 

 the impinging waves. The surf that strikes the pier at an angle 

 of about 25°, sends the greatest quantity of reverberated water 

 to the leeward end ; and that impinging under an angle of 45°, 

 disturbs the sea in front, to the greatest distance forwards. 



The terms leeward and windward ends are used relative to the 



