Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2- Appendix): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

294 AFPliXDIX. ing her power, and the wave is travelling toward the west, another wave is approaching from the Pacific — a wave which has been retarded in its passage— and its crest passes through the Indian archipelago, while the water would otherwise be falling at the western part of Torres Strait. At the same time, a derivative* wave moving north- ward along the west Australian coast, combines vdth the Pacific wave to raise a high tide about the north-west coast of Australia, where, if it were not for these auxiharies, there would be low water at that time. Six hours afterguards, one body has ebbed toward the Pacific — the other southward, toward the then comparatively low ocean, south of Australia, and what — if Torres Straits were blocked up ; and the water prevented from falling away toward the south would be a high tide, is, in fact, low water. The tides in the two northern bays are derivatives, and move northward. High water taking place at one time — within an hour — all along the east coast of Africa, shows that the rise of sea, or tide-wave, there moves westward or eastward, and the times of high water at the islands are farther confirmations ; for the wave is at Chagos and at the Mauritius three or four hours before it is high water on the African coast. The KeeUng time shows that there the water rises longer, in consequence of that part of the ocean being affected by the advancing swell of the Pacific. The only remaining particular case which I now recollect is that of the south coast of Australia — from King George Sound to Spencer Gulf — a large space of sea, in which there is very little rise of tide and even that little very irregular. As the high water moves westward from the meridians of that great bay, a tide moves southward from the Indian archipelago, where it is high water just as it should be low in the bay men- tioned : hence there is a filling, or flowing, from one wave, while an- other is retreating. In this wide expanse, affected by derivative tides from three adjoining oceans, we cannot but expect irregularities ; either very high tides, caused by combination — or httle or no tide, in conse- quence of mutual destruction — one tide ebbing from, whUe another is flowing toward the same place. Throughout these remarks I have intentionally omitted to say much of the sun's action, because, though very inferior, it is simi- • Derived from a great southern wave passing westward.