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. 



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