EDDY STONE Rocics. These rocks are chiefly co vered by the tide, and are well known to mariners as highly dangerous, particularly to the navigation of Ply mouth Sound. They have been noted as the cause of numerous shipwrecks, and have been much celebrated on account of the lighthouses which have been erected upon them. The first of these was hardly completed, in 1698, when it was unfortunately swept away in a storm, the engineer and all his people losing their lives. The second had the misfortune to be demolished by ac cidental fire in 1755. In 1759, Mr Smeaton completed the present Eddystone lighthouse entirely of stone, which remains a proud monument of the fame of that illustri ous engineer, who has, besides, left a most complete and luminous narrative of the progress of the work, where our readers will find a distinct and accurate ac count of every particular connected with this arduous undertaking. We shall here give some general de scription of the situation and circumstances connected with these dangerous rocks, referring the further ac count of the several buildings to our article LIGHT HOUSE.
The Eddystone rocks have evidently derived their name from their situation, relatively to the set or cur rent of the tides. An eddy of the tide, is understood to be a current setting in a direction contrary to the main stream, and is occasioned by an island, or rock oppo sed immediately to the tide ; and therefore, according to the velocity of the stream, and the magnitude of the interposed body, the eddy is found to produce either a smoothness on the surface of the water, or a current in the opposite direction to the tide. Thus, it is not uncom mon in the rapid tides and eddies of the Pentland Frith, between the shores of Caithness alai the islands of Ork ney, to see ships carried along under similar circum stances, in opposite directions, merely by the impulse of these currents. Although no such c ffect is to be looked for at the Eddystone, where the rocks are comparatively small, and the tides less rapid, yet in moderate weather they cause a smoothness in the water, and produce va ions currents, according to the lie of the rocks.
The position of the Eddystone rocks is such, as to render the entrance of Plymouth Sound extremely dif ficult. They are situated in NV. Long. 4° 5', and N. Lat 50° 10', bearing about 14 miles south-west front Plymouth, 10 miles south-west by south from the Ramhcad in Cornwall, and 21 miles east, one-half north, from the Bolthead in Devonshire.
The Eddystone rocks consist of three principal ridges, which have been distinguished by the relalive names of House Reef, South Reef, and East Reef These rocks, in their greatest extent, lie north and south, and in this direction measure about 600 or 700 feet in length ; be sides a small rock, seen only in spring tides, called the north east rock, which lies about one thousand feet from the house rock. From the general aspect and appear ance of the Eddystone rocks, there is reason tO infer that the whole consists of the same kind of stone. The state of the weather did not admit of the writer of this article touching at any of the rocks excepting that on which the lighthouse is built, which lie the more re grets, as, from the smallness of the house rock, it would be a kind of sacrilege to mutilate it by breaking off spe cimens, so that he was left to judge imperfectly of its composition from parts which were worn by the feet of the light keepers. It seems to be either granite or gneiss, called moonstone in Cornwall. The felspar is most abun dant in it, and is chiefly of a brownish colour, containing large irregularly shaped white specks. It clips towards the north west, at the rate of about one perpendicular to two horizontal, is extremely hard, and has a very rug ged appearance. Its greatest horizontal dimensions at low water is about 65 feet, and its least about 35 feet. On the eastern side it is perpendicular to the surface of the water, and at one place may be said rather to over hang, where the top of the rock is elevated about 18 feet above low water mark of spring tides, and is there fore seldom covered by the rise of the tide, but on the western side it slopes towards low water mark. The lighthouse is qf difficult access on all sides, and can only be approaaed in moderate weather. The gut, or landing place, is formed by the house reef, and the south reef, which afford some shelter for a boat at low water. In this gut there is about one fathom of water at the low est tides, but in all directions from the rocks the water suddenly deepens to 15 and 20 fathoms, and at greater distances to 45 and 50 fathoms.