The increasing passage of the southern commerce through Torres Strait, had attracted the notice of the British government to the peculiar perils of the navigation. The Strait is one of difficult passage from the state of the currents, reefs, &c., and the difficulty was enhanced by the imperfect nature of the charts. Along the east coast of Australia, and as far to the north as New Guinea, an immense ridge of coral rock extends; and through the gaps in this barrier reef, vessels must find their way to the Torres Strait. The two government vessels, the Fly and the Bramble, were sent out to make a survey of the barrier reef. The especial objects of the expedition being--the survey of the eastern edge of the great chain of reefs--the examination of all the channels through the barrier reef, with details of those which afford a safe passage--and the erection of beacons on their outer islands as guides to the navigation. The commanders of the vessels were directed to give marked attention to all circumstances connected with the health of the crews, the climate, temperature, products, and science; and especially the phenomena of magnetism. A geologist and a zoologist were added to the expedition, the whole under the command of Captain Francis Blackwood. In order to make the subsequent details more intelligible, we give a brief abstract of the voyage. The Fly, with her tender the Bramble schooner, sailed from Falmouth, April 11, 1842, and made the usual course to the Cape, touching at Teneriffe on the way, where a party ascended the Peak, and determined its height to be twelve thousand and eighty feet above the sea. Reaching Van Diemen's Land in August, and Australia soon after, they sailed from Port Stephens December 19, to commence their survey. After an examination of the Capricorn Group, they commenced the survey of the northern part of the great barrier-reef, up to the Murray Islands. In the next year, they erected a beacon on Raines Islet to mark the entrance of a good passage through the reef. The rest of the year was spent in surveying Torres Straits. They remained thus occupied till the beginning of 1845, when they sailed for Europe, and anchored at Spithead in June 1845, after an absence of three years. The result of those investigations was, a large accession to our previous knowledge of the sea to the eastward of Australia, now become important from our settlements; and a survey of five hundred miles of the great chain of coral reefs which act as the breakwater against the ocean. We have heard much of coral islands, certainly the most curious means of increasing the habitable part of the world; in fact, a new insect manufacture of islands. They are of all sizes. We give the description of a small one of this order in the Capricorn Group, an assemblage of islands and reefs on the north-east coast of Australia, so called from the parallel of the Tropic of Capricorn passing through them.