At the time of high-water of spring-tides, the south-western reef is about 16 feet, or nearly the whole rise of the tide, under the surface of the water; while the part of the rock on which the light-house is built, is about 12 feet below high water-mark of spring-tides: At low-water of neap-tides, hardly any part of the rock is visible: But at low-water of spring-tides, the general level of the north-eastern end where the light-house is built, is about four feet perpendicular above the level of the sea, though particular points measure six or even seven feet in height above the low-water mark of spring-tides.
Depth at the distance of 100 yards, and upwards from it.
At the distance of about 100 yards from the rock in all directions, excepting on the south-western reef, there is a depth of water varying from two to three fathoms at low water of spring tides. On the north-west side, or in the direction of the shores of Forfar and Fife, the greatest depth is 23 fathoms; but on the south-eastern or seaward side, in the direction of the dip or inclination of the strata, the water deepens more suddenly to 35 fathoms; in the same direction from the rock, however, the soundings again become less, being only 22 fathoms upon Mars Bank, distant about 33 miles; this bank appears to be a deposition formed by the joint operation of the waters of the Friths of Forth and Tay, influenced by the great tidal wave of the German Ocean. It may here be noticed as a fact connected with the depth of the German Ocean, that at Queensferry passage, in the Firth of Forth, the depth of the water is about 35 fathoms, while the greatest depth of the sea across to Denmark, does not exceed 45 fathoms. The depths of the German Ocean will be seen, by inspecting Chart No. [3.], where sectional lines are delineated between various points of Great Britain and the opposite Continent, on which the reader will see the relative depths marked by shaded lines, in a new and it is hoped a perspicuous manner.
Current of the Tides.
Currents of the Tide.
Tides at the Bell Rock.
The tides at the Bell Rock, are observed to follow the same laws as on the opposite shores of the Firth of Forth. The currents along the coast take their direction from the figure of the land, and, in their course, they are therefore occasionally deflected from, and inclined towards it. At the Bell Rock the flood-tide sets south-west, and the ebb-tide north-east, being nearly in the direction of the shores of Forfar and Kincardine. The velocity of the water in spring-tides, or when the sun and moon are in conjunction, and in opposition, is about three miles per hour, but in neap-tides, or at the quadratures of the moon, the current is only at the rate of about one mile per hour. On the days of new and full moon it is high-water upon the Bell Rock at half past one o’clock, being about the same periodic time as at the harbour of Arbroath, or nearest point of the mainland. In the ordinary state of the weather, the perpendicular rise and fall of the sea at the Bell Rock, in spring-tides, is 15 feet, and in neap-tides, 8 feet; but so much depends upon the direction of a prevailing tract of winds, that the tides are often found to vary from 1 to 3 feet above and below these numbers. This irregularity in the tides of the German Ocean and its subsidiary friths or inland seas will be easily accounted for, by considering the effect of westerly winds passing for a length of time over the Atlantic, which must naturally force an undue quantity of water from thence into the entrance of the North Sea, between the coasts of Scotland and Norway; while the Strait of Dover, to the southward, from the same cause, is gorged by the surplus waters of the British Channel flowing in an opposite direction, and checking the tide of the German Ocean. When the winds blow from southerly and easterly directions, the reverse of this happens, and the waters are then proportionally low.
In and Off shore Currents.
A curious anomaly connected with the flowing and ebbing of the sea, in the early part of each tide, is observable in the contrary currents which take place along the shore, and at a distance from it. For example, the flood-tide begins to flow in many situations two or even three hours sooner on the shore than at the distance of from one mile to four miles in the offing. The same thing also happens with regard to the ebb-tide, which begins to fall and run in a contrary direction to the flood-tide, two or three hours sooner on the shore than at a distance. These effects are very different from the state of things three or four leagues from the land, or in the open sea, where the lateral motion of the tide-waters is scarcely sensible. That an extensive tract of coast should produce changes on the current of the tides, is perhaps what we should expect; but it is somewhat curious to find the same appearances connected with small islets, and, as in the case of the Bell Rock, even with an insulated reef, situate at the distance of 12 miles from the land, and sunk to the depth of from 2 to 3 fathoms under the surface of high-water. So strikingly observable is this, that the tide begins to ebb about two hours sooner on the Bell Rock than at the short distance of one mile from it. In the course of the light-house operations this was rendered sufficiently obvious, by the swinging round of the several vessels at their moorings according to the flood and ebb tides. For example, the Floating Light-ship was moored about three miles in a north-west direction from the Bell Rock, and the moorings of the Tender and two Stone Lighters, were respectively laid down at the distance of ¾ and ½ mile from it, as will be seen from [Plate V.] Now, these vessels were all found to swing to the tide at periods proportional to their distance from the rock. Although it may, therefore, seem strange, that this comparatively small object should affect the tides in its vicinity in a manner similar to the shores of an extensive coast; yet, as the rock shelves outwards, with an extensive base, especially on the northern side, it must impede the under current of the tide, and indeed forcibly proves the existence of such under currents.
Not accounted for by Writers on the subject.