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. 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.