[Sidenote: 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.
[Sidenote: 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.
