During their formation, these islands are fraught with danger to ships, for sometimes, in parts of the ocean where charts show deep water, the sailor finds an unexpected coral reef, and, before he is aware, the good ship runs on this living wall and becomes a wreck. Many a noble vessel goes to sea well appointed and with a good brave crew, but never more returns;--who knows how many such have, when all on board thought themselves secure, been dashed to pieces suddenly, and lost upon the coral reefs of the Pacific? These circular islets of coral never rise more than a few feet above the surface of the sea, but there are many other islands in the South Seas-- some of which have been thrown up by the action of volcanoes, and are wild, rugged, mountainous, and of every conceivable shape and size. The busy corallines before mentioned are so numerous in the South Seas that they build their coral walls everywhere. As they have an objection apparently to commence building in shallow water, they are obliged to keep off the shore a distance of a mile or more, so that when they reach the surface they enclose a belt of water of that width, which is guarded by the reef from the violence of the waves, and forms a splendid natural harbour. Almost every South-Sea island has its coral reef round it, and its harbour of still water between the reef and the shore. It would seem as if the beneficent Creator had purposely formed those harbours for man's convenience, because narrow openings are found in all the reefs, without which, of course, the sheltered waters within could not have been entered. These openings are usually found to occur opposite valleys where the streams from the mountains enter the sea. It is therefore supposed that fresh water kills the coral insects at these places, thus preventing the reef from forming an unbroken circle. Low islets are usually formed on each side of the openings on which a few cocoa-nut trees grow; so that the mariner is thus furnished with a natural beacon by which to guide his vessel clear of the reef safely into the harbour. One of the most interesting of the larger islands of the Pacific is Otaheite (now spelt and pronounced Tahiti), at which Captain Cook arrived on the 4th of April 1769. It had been discovered, however, nearly two years before the date of his visit--as the next chapter will show. CHAPTER FIVE. DISCOVERY BY CAPTAIN WALLIS OF OTAHEITE OR TAHITI. The beautiful island of Tahiti was discovered by Captain Wallis in the year 1767. It was on a bright day in June when he first saw it from the deck, but when his vessel (the _Dolphin_) came close to it, a thick mist descended like a veil and shut it out from view of the impatient mariners, who were compelled to lie to until the mist should clear away. At length it rolled off, and disclosed one of the most lovely and delightful scenes that could be imagined.