218 Royal Society. 



waves that actually proceed along the coasts of the Atlantic : and 

 the modifications in their course and velocity are still more per- 

 ceptible in bays, gulfs, and narrower channels and inlets of the sea, 

 as well as in their progress along rivers. The author traces in detail 

 the effects which these different circumstances may be expected to 

 produce. He adverts to an important distinction which has frequently 

 been lost sight of, between the progressive motion of the tide-wave 

 and the actual horizontal motion of the water, or tide-current ; mo- 

 tions which do not bear any constant relation to one another. Hence 

 the change in the direction of the current does not invariably indicate 

 the rise or fall of the water. 



In the second section he examines the causes which have led to 

 inaccuracy in making observations on the tides ; the first of which is 

 dependent on the circumstance just mentioned, of the occasional 

 want of correspondence between the times of high and of slack water ; 

 the former referring to the moment of greatest elevation, the latter 

 to that when the direction of the current changes. The other causes 

 of error are derived from the change which takes place in the course 

 of the day in the moon's angular distance from the sun j from the 

 half-monthly inequality in the establishment, arising from the relative 

 position of the sun and moon during each lunation ; and from the 

 necessity that exists of making a correction for what may be termed 

 the age of the tide ; that is, the interval of time which has elapsed 

 between the period of the origin of the wave and the time of its actual 

 arrival at the place of observation. 



The third section, which forms the chief bulk of the paper, is oc- 

 cupied by a statement and discussion of the tide observations now 

 extant, and which the author has, with great industry, collected from 

 a variety of sources, both of published accounts, and of manuscript 

 documents preserved in the Admiralty. Commencing with the tide- 

 waves, first of the eastern and then of the western coasts of the At- 

 lantic, he follows them to the Northern sea, and to the different coasts 

 of the British islands, and of the German Ocean. He passes next to 

 the examination of those of the Southern Atlantic at Cape Horn and 

 the adjacent coasts ; thence tracing them, as far as the present im- 

 perfect data will allow, along the western shores of the American con- 

 tinent, to the central parts of the Pacific, and in their progress across 

 the Australian and Indian Oceans. He likewise examines the con- 

 dition of the tides in rivers, as to the magnitude and velocity of the 

 undulations, the occasional production of a high and abrupt wave, or 

 bore, and as to the influence of the natural stream of the river upon 

 the different periods of elevation or depression of the water. 



The fourth section contains general remarks on the course of the 

 tides, suggested by the preceding review of the phenomena they pre- 

 sent ; on the velocity of the tide- wave ; on the form of the cotidal 

 lines; on the currents which attend the tides ; on the production of 

 revolving currents; on the magnitude of tides; and on the constancy 

 of the cotidal lines. He adverts also to some peculiarities resulting 

 from interference, such as the differences of the two diurnal tides, and 

 occasionally the occurrence of single day tides. 



