200 On the Comtruction of Sea Harbours. [Sept. 



any direction can neither sufficiently still the interior water, nor 

 prevent the surf from accumulating at the entrance. For the 

 completion of the former purpose, a secondary pier, b, is built, 

 nearly at right angles to the main pier, extendmg to the shore 

 from such a distance within the head of the main pier, and leav- 

 ing an opening for the entrance of such a length, that the line 

 h a b of the most leeward heavy surf, points to the outside of the 

 secondary pier^s head. These are the inclosing piers, and others 

 for mooring vessels, 8cc. may be built in the basin thus rendered 

 smooth. 



To relieve the main pier from as much of the surf as possible, 

 Avhen the abrupt side extends further into the sea, a small cover- 

 ing pier, e, juts out from the extreme point of the land so far 

 that a line, e d, drawn from its sea-end to the head of the main 

 pier, makes an angle of 45° with the direction of the last pier. 

 More than this, the main pier cannot be covered, without inter- 

 fering too much with the lines of approach ; but it keeps back 

 the surves striking under angles of 15° to 45°, which, when 

 strong, are the most dangerous surves. 



Every surf acting against the secondary pier rebounds to the 

 shore, so that a covering pier is not wanted to protect the 

 entrance from its reverberated water. But heavy surves on a 

 long range of lee-shore force a strong lateral agitation into the 

 harbour. This can be considerably weakened by a very small 

 pier, c, placed at such a distance from the secondary pier, that its 

 sea-end shall not reach the line e d, when continued to the 

 shore. This pier also lessens the quantity of surf in the space 

 between it and the secondary pier, by and so secures a safer 

 retreat than the exposed shore, to vessels failing to enter the 

 harbour. But a greater length of smooth shore is very desira- 

 ble, as in particular gales vessels are sometimes driven past the 

 pier, c. Hence also harbours gradually extended have obtained 

 additional works, which make the inclosed lee-shore and 

 entrance more or less similar to those represented by fig. 4, when 

 the dotted pier, g, is added. In this state, vessels can take 

 shelter within either head ; but if driven too far in, they can 

 reach the moorings on the innerside of the piers, or retire to the 

 shore, which is much smoother than the exterior shore. 



In situations where the main pier begins at the extreme pomt 

 of the abrupt side of a creek, or extends further into the sea; or 

 where it commences from a straight part of 

 the main shore, stretches directly into the 

 sea, and then turns to be parallel to the shore ; 

 a covering pier would be too expensive ; and 

 to prevent the reflected surf from accumulat- 

 ing too much at the sea end, the main pier, 

 as in fig. 5, is built in parts not exceeding 100 

 yards each, having angles of 25° to 30°, with 



Fig. 5. 



