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

APPENDIX. 207 At the mouths of hvrge rivers it sometimes happens that salt water is actualty running up the river, underneath a stream of fresh water which still continues to run down. This I have witnessed in the river Santa Cruz. Of course intermixture takes place gradually, though by slow degrees. The height of waves may be here mentioned, with reference to rollers or other undulations of water however caused. Large waves are seldom seen except where the sea is deep and extensive. The highest I have ever -witnessed myself were not less than sixty feet in height, reckoning from the hollow between, perpendicularly to the level of two adjacent waves : but from twenty to thirty feet is a common height in the open ocean during a storm. I am quite aware of, and have long been amused by the assertion of some persons, whose good fortune it has been not to witness really large waves — that the sea never rises above twelve or fifteen feet — or, that no wave exceeds thirty feet in height, reckoning in a vertical line from the level of the hollow to that of the crest. In H. M. S. Thetis, during an unusually heavy gale of wind in the Atlantic, not far from the Bay of Biscay, while between two waves, her storm try-sails were totally becalmed, the crest of each wave being above the level of the centre of her main-yard, when she was upright between the two seas. Her main-yard was sLxty feet from the water-line. I was standing near her tafFrail, holding by a rope. I never saw such seas before, and have never seen any equal to them since, either off Cape Horn or the Cape of Good Hope. Calculations of tides, applicable to the method of following- out New- ton's general principles, adopted by Mr. Whewell and most persons whose opinions on this subject all men respect — are equally applicable to the view here taken. In either case the time of high water, and rise of tide on a certain day, is ascertained at a given place exjierimentally : and as the causes of that tide are the moon and the sun ; changes in their position with respect to the earth will operate changes in the tides, which, as to time and quantity, will depend upon the above data, and the positions of earth, moon, and sun. The variation of tide is what we liave to deal with in ordinary calcu- lation, not the original movement. c c