244 CORALS AND CORAL ISLANDS. 
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 weather 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 rig- 
ging and hull. The cause of such a current is obvious. The 
sea is constantly pouring water over the reefs into the harbor, 
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 harbor-entrance, this being the only means of 
exit. There are many such cases about all the islands. Ina 
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, 
“Through the channels and among the inner reefs of the 
Australian reef-region,” says Jukes, ‘“‘they run sometimes 
with an impetuous sweep in the same direction even for two 
or three days together, especially after great storms have 
driven large quantities of water into the space between the 
outer edge and the land.” 
A current of the kind here represented will carry out much 
coral débris, and strew it along its course. The transported 
material will vary in amount from time to time, according to the 
force and direction of the current. It is therefore evident 
that the ground over which it runs must be wholly unfit for 
the growth of coral, since most zodphytes are readily destroy- 
ed by depositions of earth or sand, and require, for most spe- 
cies, a firm basement. Or if the flow is very strong, it will 
scour out the channels and so keep them open. The existence 
