In connection with these observations on the tides, some experiments were also made with an instrument adapted for lifting water from considerable depths, without the possibility of its intermingling with the surface water. By means of this instrument, the water at the bottom of the Dee, at Aberdeen, was found to be salt, while that at the surface was quite fresh. These streams of fresh and salt water run in distinct currents, and in contrary directions, the salt water, from its greater specific gravity, flowing at the bottom of the river, and fluctuating with the level of the ocean, while the fresh water is actually lifted upwards, and continues all the while to flow seaward on the surface of the salt water. Towards the point of high-water, however, the flood-tide gains strength on the margin of the basin of the harbour, where the water becomes salt, and forms an eddy-tide in a contrary direction to the central stream, which is observed still to run toward the sea. Having made similar observations on the waters of the Thames, the Garonne, and other rivers, with nearly the same results, after making allowance for the more level state of the country, in the track of these great streams,—it is concluded, that the currents at the embouchure of rivers bear a strong resemblance to the operation of the in and off shore tides of the ocean.
Phenomena of in and off shore Tides accounted for.
We further observe, that the great wave of the German Ocean produces its tides in regular succession, and at stated periods, as it moves from north to south; but the tides of the more inland seas are subject to many irregularities, both in their periodic times, and in the direction of their currents. Let us suppose, then, that we have arrived at the instant of high-water on the shores at the entrance of the Firth of Forth, and that the tidal waters are then moving in a body, and with a certain pressure, towards the higher parts of the Firth , and even affecting the river above the bridge of Stirling. We find, that at the entrance of this estuary, on the days of new and full moon, it is high-water at a quarter past one o’clock; but at Alloa, situate 70 miles above the Bell Rock, it is not high-water till about two hours and a half later. The in-shore tidal waters having to encounter the shelving shores, islands, sunken rocks, and projecting points of land, which lie in their course up the Firth , acquire lateral as well as perpendicular motion, and being thus checked in their progress, are brought sooner to a maximum state than the off-shore stream, which flows in deeper water, and comparatively free of obstruction, till it reaches its ultimate limits; though it gradually diminishes in breadth, till the stream of the new tide gaining strength becomes general; and the central current, formerly running in a contrary direction, at length disappears, and takes the course of the new tide.
We would, therefore, be understood to ascribe this anomaly in the flowing and ebbing of the sea, to the obstruction which the current of its waters meets not only at the surface or margin, but at the bottom, which, from the variety of the soundings of the depths, appears to be as various as the face of the land. A striking proof of this is afforded at the Bell Rock: on the northern side of which there are 11 fathoms, at the distance of about three quarters of a mile; while, on the southern side, and at a similar distance, the water deepens to 35 fathoms; so that a perpendicular section of this rock under water forms a precipitous declivity, such as we are quite accustomed to see on the land. Now, if we apply this irregular conformation of the bottom of the sea, to the production of the in and off shore tides, and conceive that the tidal currents extend their motion to the bottom, it must be evident, that this obstruction presented to the stream will bring the tides to a maximum state sooner on the northern side of the rock, where the water is so much shallower, than on the southern side. This is also agreeable to observation; for, the tides upon the Bell Rock begin to flow and ebb one hour sooner than at the distance of about three quarters of a mile from it on the northern side, and about two hours and a half sooner than at the same distance on the southern side. The marginal current is thus checked by the shallowness of the water, and the projecting points of land; while the central stream, flowing comparatively without obstruction, continues to run till the most inland creeks are supplied with tidal water; and vice versâ, it continues its stream outwards, till these waters are again run off.
Progressive times of high-water.
The progressive times of high-water, at intermediate points between the Bell Rock and the port of Alloa in the Firth of Forth, appear to follow the same general law, as the great wave of the German Ocean in its progress from the Orkneys southward. These observations on the tides of the Forth apply equally to the Firth and River of Tay, and indeed to all the tributary streams and arms of the sea which communicate with the German Ocean, according to their local situations and magnitudes.
Tides of the Mediterranean and Baltic Seas.
If due allowance be made for the peculiar situation and circumstances of the Mediterranean and Baltic seas, it is apprehended, from all that we have been able to learn of the operation of the currents at the Strait of Gibraltar, and Sound of the Cattigate, that these seas are supplied and discharged by in and off shore tides or currents, under certain modifications, and making due allowance for local circumstances, in the same manner as on all other parts of the coast.
I have been thus particular relative to the in-shore and off-shore tides, because they appear in a very puzzling form to the mariner, while writers on the theory of the tides are almost silent on this subject.