APPENDIX. 295 



lar to that of the moon. Perhaps the Otaheite tide may be purely- 

 solar; this, however, is not at all certain. 



It appears to me probable, that many important currents are caused 

 by the tidal libration or oscillation of the sea. As the earth turns 

 only one way, the moon is continually puUing, as it were, in one 

 direction, and to this cause, I think, most of the greater currents 

 may be traced. Wind, evaporation, and the variable weight of the 

 atmosphere may each have a share in moving the waters horizontally ; 

 but there are many facts which lead to a conclusion that the moon 

 and sun are principal agents in causing currents.* 



Having alluded to the effect of atmospheric pressure on the ocean, 

 I will take this opportunity of mentioning that the chief cause of 

 water rising on the shore before hurricanes, or gales of wind, may 

 be the lightened pressure on the surface of the sea, indicated by the 

 mercury being low in a barometer. This is very remarkable at the 

 Mauritius and in the river Plata, at both which places the water 

 rises unusually before a storm, while at the same time the mercury 

 falls. As the column rises, so the water falls again. I have in- 

 stanced those places as being well known, and alFected very little by 

 tide : but the fact has been observed by me in many places during 

 the Beagle's voyage, and I have besides collected the testimony of 

 others respecting it. 



These causes may materially affect the height of tides and the 

 strength of currents. In the wide but shallow Plata, the depth of 

 water and nature of current varies in extraordinary accordance with 

 the barometer. 



Another cause of the water rising before a high wind, or storm, 

 as well as of a ground swell, of rollers, or of that disturbed tumul- 

 tuous heaving of the sea, sometimes observed while there is little or 

 no wind at the place, may be the action of wind on a remote part 

 of that sea ; an action, or pressure, which is rapidly transmitted, 

 through a fluid but slightly elastic, to regions at a distance. 



I have collected many instances of rollers, or a heavy swell, or a 

 confused ground swell being felt at places, where not only there was 

 no wind at the time, but to which the wind that caused the move- 



• A continued stream may be produced by a succession of impulses, 

 as a rotatory system of waves may " be kept in constant circulation by 

 impulses received from the adjacent tides." — See Whewell in Phil. 

 Trans. 1836, p. 299. 



