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. [Sidenote: 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._ [Sidenote: Currents of the Tide.] [Sidenote: Tides at the Bell Rock.]