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

282 APPENDIX. the moon upon a body of water, by which any portion is attracted towards her before she is vertically over it, as well as after she has passed to the westward of the meridian of that portion. But little attention appears to have been paid to a consideration of the momentum acquired by any great body of water moved from the position it M'ould occupy if undisturbed, and to the consequences of that momentum, when the water returns from a temporary displace- ment. And there seems to be a difficulty in altogether reconciling the statement that " tides are diminished by diffusion,* with the manner in which the great tides of the Northern Atlantic are supposed to be caused — a supposition which is mainly dependent upon theprinciple of " forced vibrations or oscillations. "f In consequence of similar ideas, excited by the facts previously mentioned, the following questions were inserted in the Geographical Journal for 1836 : " It may appear presumption in a plain sailor attempting to oiFer an idea or two on the difficult subject of ' Tides ;' yet, with the utmost deference to those who are competent to reason upon the subject, I will venture to ask whether the supposition of Atlantic tides being principally caused by a great tide-wave coming from the Southern Ocean, is not a little difficult to reconcile with the facts that there is very little tide upon the coasts of Brazil, Ascension, and Guinea, and that in the mouth of the great river Plata there is little or no tide .' " Can each ocean have its own tides, though affecting, and being affected by the neighbouring waters ? " Can the mass of an ocean have a tendency to move westward as well as upward, after and towards the moon as she passes ? If so, after the moon has passed, will not the mass of that ocean have an easterly inclination to regain that equilibrium (with respect to the earth alone) from which the moon disturbed it (sun's action not here considered) ? " In regaining its equilibrium, would not its owa momentum carry it too far eastward ; and would not the moon's action be again approaching ? " Can one part of an ocean have a M'estward tendency, while another part, which is wider or narrower, from east to west, has an eastward • Whewell's Essay, p. 217. + Herscliel's Astronomy, Cab. Cyo. p. 334.