An Account of the Bell Rock Light-House / Including the Details of the Erection and Peculiar Structure of That Edifice; to Which Is Prefixed a Historical View of the Institution and Progress of the Northern Light-Houses

by Robert Stevenson

To account for these in and off shore tides, or central and marginal streams, would be interesting. But to what cause shall we ascribe them? Sir Isaac Newton, and the other eminent philosophers who have followed that great man in considering the theory of the tides, confine their attention chiefly to an explanation of the influences of the sun and moon, and the laws of gravitation, in affecting the waters of the ocean, leaving it to the result of experience and observation to account for such anomalies as those to which we now allude. It is noticed by Adair, Mackenzie, and other nautical surveyors, that the tides run longer upon the shore than in the offing, and the advantage of working a ship with in and off shore tides, is familiar to every mariner. The existence of these opposite currents was also known to the author, prior to the commencement of the Bell Rock works, but they had not struck him so forcibly till that period. For here, even after the flood-tide had overflowed the rock, and put a stop to the operations, the boats in carrying the artificers on board of the Tender, had still to row against the current of a strong ebb-tide.

Currents along Shore considered as Eddy-tides.

It would be foreign to this work to enter into the theory of the tides generally; all, therefore, that is here proposed, is to endeavour to account for these in and off shore currents. In doing this, however, it will be necessary to observe, that the great wave or “theoretical tide,” as it has been termed by the late eminent Professor Robison, is produced by the united attraction of the sun and moon, which, between the Tropics, has been calculated by philosophers to raise the water from 8 to 14 feet perpendicularly. It is observable, that the attractions of these heavenly bodies elevate the parts of the ocean to which they are vertical, without having any direct tendency to progressive or lateral motion. The currents along our coasts, may therefore be considered merely as Eddy-tides, occasioned by the interposition of the land, which obstructs the undulating motion incident to the rise or fall of a fluid. In this manner the land may be said to displace a portion of the tidal waters which have been elevated above the medium level of the sea; and were it not for such obstruction, the great waves of the tide might be supposed to undulate indefinitely over the expanse of the ocean. To compare small things with great, these effects may be conceived as in some degree exemplified, by the disturbing effect of a vessel passing along a navigable canal, or the undulations which are observable in the wake of a ship or wheels of a steam-boat in motion on a smooth sea.

Progress of the Great Waves of the Tides.

The great wave which supplies the British tides, appears to be propagated between the coast of Labrador and Greenland, on the one hand, and the European shores on the other; and this great wave seems to be divided into two lesser waves. One of these flows between Ireland and the coast of France, into the British and St George’s Channels; while the other enters by the North Sea into the German Ocean; and in its course from north to south, supplies all the friths, rivers, and bays connected with it, invariably in the form of In and Off shore tides, which are every where observable along the margin of this great basin. This northern wave is found to occupy about 12 hours in flowing southward from the 58th to the 52d degree of latitude, or from the Orkney Islands to the numerous Sand-banks which pervade and encumber the apex of the German Ocean, where the currents become extremely desultory and irregular. The coast of the British Isles, accordingly, may be said to, displace a portion of this northern wave, and thus to produce the irregularities which we are endeavouring to account for.

Progressive periods of High-Water in the Firth of Forth.

At present, we shall confine our attention to the tides of the Firth of Forth. Here, as on other parts of the coast, the tidal waters have a tendency to flow towards the shores and higher parts of the Forth, till the instant of high-water upon the shore, when the tide begins to ebb, and run in a contrary direction. A central stream, however, continues to run with unabated force, as flood-tide, during two or even three hours longer, as before noticed, according to the situation and local circumstances of the coast. It appears, from a comparison of the several periods of high-water on the shores of the Firth of Forth, as nearly as some of them could be ascertained, that the precise time of high-water becomes later and later in the same tide, as we proceed westward; at the Bell Rock, for example, it is high-water on the days of new and full moon, as before noticed, at ½ past 1 o’clock; at the Carr Rock, at ¾ past 1; at Elie, still further up the Firth , at 2; Kinghornness, at ¼ past 2; Queensferry, at ¾ past 2; and at Alloa, at ¾ past 3 o’clock. The off-shore stream of the tide continues to flow proportionally longer till it has supplied the higher parts of this estuary with its portion of tidal waters; and in like manner, the central stream of ebb-tide continues its course till these waters are again run off.

Currents at the Mouth of the Dee and other Rivers.

In many points, it is found, that the operation of the tidal waters of extensive arms of the sea, bears a close resemblance to what is observable upon the small scale in the currents of rivers, especially at their junction with the ocean. An interesting example of this occurs on the river Dee, which falls rapidly into the harbour of Aberdeen. Here the author having occasion to make some observations on the tides in the summer of 1812, stationed several assistants at low-water mark, between the entrance of the harbour and the bridge, about two miles up the river. The waters of the Dee, even at the entrance of the harbour, have almost a constant current seaward, notwithstanding the opposite direction of the flood-tide of the Ocean. On the occasion alluded to, one of his assistants, a very intelligent shipmaster, continued at his post while the water flowed up to his middle; and, when accosted about his situation, he significantly observed, “That it was rather extraordinary, as the stream had never ceased to indicate the continuance of an ebb-tide, while the water was still rising upon his body.”

Water salt at bottom and fresh at top.