Figure 4.—BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago, from the survey by Captain Beechey, R.N.; the lagoon is choked up with reefs, but the average greatest depth of about twenty fathoms, is given from the published account of the voyage. Figure 5.—BOLABOLA, in the Society Archipelago, from the survey of Captain Duperrey in the _Coquille:_ the soundings in this and the following figures have been altered from French feet to English fathoms; height of highest point of the island 4,026 feet. Figure 6.—MAURUA, in the Society Archipelago; from the survey by Captain Duperrey in the _Coquille:_ height of land about eight hundred feet. Figure 7.—POUYNIPÈTE, or SENIAVINE, in the Caroline Archipelago; from the survey by Admiral Lutké. Figure 8.—GAMBIER ISLANDS, in the southern part of the Low Archipelago; from the survey by Captain Beechey; height of highest island, 1,246 feet; the islands are surrounded by extensive and irregular reefs; the reef on the southern side is submerged. Figure 9.—PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in the Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell; not nearly all the small submerged reefs in the lagoon are represented; the annular reef on the southern side is submerged. Figure 10.—KEELING, or COCOS ATOLL (or lagoon-island), in the Indian Ocean; from the survey by Captain Fitzroy; the lagoon south of the dotted line is very shallow, and is left almost bare at low water; the part north of the line is choked up with irregular reefs. The annular reef on the north-west side is broken, and blends into a shoal sandbank, on which the sea breaks. PLATE II.—GREAT CHAGOS BANK, NEW CALEDONIA, MENCHIKOFF ATOLL, ETC. Illustration: Plate 2 Figure 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey by Captain Moresby and Lieutenant Powell; the parts which are shaded, with the exception of two or three islets on the western and northern sides, do not rise to the surface, but are submerged from four to ten fathoms; the banks bounded by the dotted lines lie from fifteen to twenty fathoms beneath the surface, and are formed of sand; the central space is of mud, and from thirty to fifty fathoms deep. Figure 2.—A vertical section, on the same scale, in an eastern and western line across the Great Chagos Bank, given for the sake of exhibiting more clearly its structure. Figure 3.—MENCHIKOFF ATOLL (or lagoon-island), in the Marshall Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the Pacific;” originally surveyed by Captain Hagemeister; the depth within the lagoons is unknown.