XIV. Researches on the tides. —Twelfth series. On the laws of the rise and fall of the sea's surface during each tide

Abstract

The subject of the present communication is different in its nature from those of previous memoirs on the tides presented by me, and printed by the Society; since it refers, not to comparison of the times and heights of different tides, but to the rate of the rise and fall of the surface of the water in successive stages of the same tide. This inquiry has often been prosecuted at particular places by naval observers, and is of very material importance to navigation. For even supposing the time and height of high water to be known, it is still often requisite, for nautical purposes, to know the height of the water at a given interval before or after the moment of high water. And this inquiry may be the more useful, inasmuch as the laws of rise and fall of the surface are nearly the same at all places; the differences being, for the most part, of such a kind as can be ascertained and allowed for without much difficulty. Hence these laws, once stated, will be applicable on every coast; and the knowledge of them may supersede those laborious trains of observation which have often been instituted in order to ascertain the laws at particular places. The materials of the present investigation (which is principally founded upon ob­servation) are the following: —Five months’ tide observations made at Plymouth, in which, besides the time and height of high and low water, the time of the surface passing two lines above the level of mean water was carefully observed; these latter observations being made, at my request, by direction of the Lords Commissioners of the Admiralty: —Three months’ observations (taken out of a larger series) made at Liverpool, under the direction of Capt. Denham, R. N., in which the height of the surface was noted every half hour: —and twelve months’ observations made at Bristol by Mr. Bunt, by means of his tide-gauge. The latter observations were reduced by Mr. Bunt himself; the others were discussed under my direction by Mr. Dessiou and Mr. Ross, of the Hydrographer’s Office, with their usual care and skill.