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. 289 4. The momentum acquired by a body of water in thus sinking back to the position it should occupy, with reference to the earth's attraction only, carries it beyond that position to one from which it has a tendency to recoil again — and so to keep up an oscillation until brought to rest by the friction of its bed. (Attraction of the moon and sun not considered.) 5. The recurring influences of the moon and sun are checks on these oscillations, and prevent their taking place more than once between each separate raising of the water in consequence of their attractions. 6. Different zones (or widths measured by latitude) of an ocean, may move differently, each having waves and oscillations at times differing from those of an adjoining zone, in consequence of one having more or less longitude, depth, or freedom from obstacles than another. 7. Original waves and oscillations combine with, and modify one another, according to their relative magnitude, momentum, and direction. 8. The natural tendency of tide-waves, and oceanic librations, is from east to west ; and of oscillations, from west to east, and east to west also : but derivative waves or oscillations move in various directions according to primary impulse, and local configuration of the bed of rn ocean. Conformably to these conclusions, I will now try to explain a few of the more remarkable anomalies of tides, in various parts of the world : taking it for granted that the reader is acquainted vfith existing works on the subject, especially those of Mr. Whewell,* and the brief but comprehensive and explanatory view taken by Sir J. Herschel in his treatise on astronomy. •} I mentioned that between Callao and the western shores of the Pacific, in the parallel of about 12° south, no comparison of times can be trusted ? Why not ? may be asked. Four or five hours west of Callao, there is a multitude of islands which checks the libratiou of the ocean. Another tide wave forms westward of them, on a small scale, and it is by this second tide, altered by derivative tides * Published in the Philosophical Transactions. + Cabinet Cyclopaedia. A Treatise on Astronomy, chap, xi., pp. 334, 5, 6, 7, 8, 9.