204 CORALS AND CORAL LS LANDS. 



presented may be taken as those of an independent witness, 

 as they were written out before the publication of his work. 



There are usually strong tidal currents through the reel 

 channels and openings. These currents are modified in 

 character by the outline of the coast, and are strongest wher- 

 ever there are coves or bays to receive the advancing tides. 

 The harbour of Apia, on the north side of Upolu, affords a 

 striking illustration of this general principle. The coast at this 

 place has an indentation 2,000 yards wide and nearly 1,000 

 deep, as in the accompanying sketch, reduced from the chart 

 by the Expedition. The reef extends from either side, or 

 cape, a mile out to sea, leaving between an entrance for ships. 

 The harbour averages ten feet in depth, and at the entrance is 

 fifteen feet. In this harbour there is 

 a remarkable out-current along the 

 »"' ^ ^ bottom, which, during gales, is so 



n strong at certain states of the tide 



that a ship at anchor, although a 

 wind may be blowing directly in the 

 harbor, will often ride with a slack 

 cable; and in more moderate wea- 

 ther the vessel may tail out against 

 the wind. Thus when no current 

 but one inward is perceived at the 

 surface, there is an undercurrent acting against the keel and 

 bottom of the vessel, which is of sufficient strength to 

 counteract the influence of the winds on the rigging and 

 hull. The cause of such a current is obvious. The sea 

 is constantly pouring water over the reefs into the harbour, 

 and the tides are periodically adding to the accumulation; 

 the indented shores form a narrowing space where these 

 waters tend to pile up : escape consequently takes place along 

 the bottom by the harbour-entrance, this being the only means 

 of exit. There are many such cases about all the islands. In 

 a group like the Feejees, where a number of the islands are 

 large and the reefs very extensive, the currents are still more 

 remarkable, and they change in direction with the tides. 



HARBOUR OF APIA, UPOLU. 



