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

288 APl'ENDIX. imperceptible, except l)y its effects, there can be little doubt after reflecting how small a lateral movement of an ocean would cause immense commotion at its boundary, in consequence of the slight elasticity of water, when free to move. Now let the moon be supposed to move from M in fig. 2 to M in fig. 3. Tlie highest point of the water would then be transferred from F to K, during which transfer the water must fall at F and rise at G : and so of other points. In this manner when the moon causes a tide by her direct attraction, a wave or swelling, whose crest is above the natural level of the sea, moves westward, until it is stopped by a barrier of land. But when it recedes from that barrier, how is the excess of the wave above the height of the sea (when uninfluenced by the moon) transferred to the other side of the same ocean ? There is no return wave : if there were, islands intermediate would have an ebb, and a flood tide, every six hours ; four floods in twenty-four hours ; but they have, on the contrary, six hour tides, alternate ebb and flow, twice in twenty-four hours, like those of the shores of continents, though generally smaller in amount. Water cannot rise in one place unless it falls in another ^it does fall on one side of an ocean, while it rises on the other — how then is the fluid transferred .'' There is only one way which is by the mass oscillating. In the former case when the moon passed over, it was a libratory movement, in this latter it is an oscillation. If it is shoviTi, as I believe, that the ocean oscillates, we see that there are two principal causes of tides — one the direct raising of water by the moon : and the other, oscillation excited by that tem- porary derangement of the natural level of the sea. From the preceding facts and deductions combined with the com- monly received laws of fluids and gravitation, the following conclu- sions may be drawn : 1 Every large body of water is affected by the attraction of the moon, and sun, and has tides caused by their action. 2. Bodies of water are not only raised, or accumulated, vertically, by the attraction of the moon and sun ; but they are also drawn laterally by them. 3. When a large body of water is prevented from continuing a horizontal movement, it rises until whatever momentum it had acquired ceases ; and then it sinks gradually.