Chap. VIII.
367
5 feet to 1 perpendicular, and drove all the rubble iroin the outer to the inner slope,making its area equal to the other. Time was requisite to give the whole its perfect con-solidation, which being obtained, and no movement in the masses l>eing apparent, theslope on the side towards the sea down to the foreshore was cased with regular coursesof masonry, which was dowelled, joggled, dovetailed, and cramped together, to lay thelower courses of which the diving bell was made use of. The three lower courses wereali granite, laid horizontally on their natural beds ; these were dovetailed, lewised, andbolted to each other. The mortar was compounded of one part of Italian puzzolana, onepart Aberthaw lime, mixed with two parts of line sharp freshwater sand: this com-position, before it was used, was well worked together in a mill, with as little water aspossible : this, when applied to the masonry, speedily became hard. Roman cement wasemployed for the outer joints and for some distance within, and to increase its setting.For the interior of the work, a concrete was made use of, composed as the first, but witha greater quantity of sand.
To transport the stone from the quarries to the breakwater, vessels of about 60 tons’burthen were employed ; these had two railways laid along them, parallel to each other,and towards the stern they formed an inclined plane, which prevented the trucks fromrunning too far. The blocks of stone were placed upon trucks, and then in parallel lineson the railways. After the vessel had arrived at its destination, the trucks were dis-charged of their load by heaving them up ; this was done by windlasses placed on thedeck; when the truck had arrived at the inclined plane, it was tilted over by its ownweight, and the stone was at once let go, and deposited into the sea; the truck then madeway for another, and after the whole was discharged, the vessel returned, to be againfreighted at the quarry : to economise time, steam-tugs were made use of, and consequentlymany voyages were performed in a day. To guide the vessels, and to enable them to shootout their load in the proper place, buoys were laid down in the line at every 30 feet; anaccount was kept of the quantity deposited, and the level it rose to, so that at all times theactual state of the work was known.
Tins important work is so situated, that vessels can enter the Sound with either aneasterly or westerly wind; and the two entrances are so set out, that the tidal andfresh water keep them at all times clear, and prevent any deposit or accumulation of sandfrom obstructing them.
The heaviest gales are those from the south and west, when the breakwater is exposedto the whole violence of the vast Atlantic ; hitherto it has been found to stand against thispower, and effectually to check the waves which uninterruptedly roll through the Bay ofBiscay . 3,369,261 tons of stone are computed to have been used in this stupendous un-dertaking from the year 1812 to March, 1841, when it was completed at a cost of nearly
million sterling. A calculation has also been made upon the entire cubical contents ascompared witli the quantity of stone deposited, and it is found that the interstices occupyabout 37 per cent, of the whole, which arises from the employment of large blocks of stones.
Messrs. Walker and Burgess have constructed at the western end a lighthouse, designedby the late Mr. Rennie. The dimension of this lighthouse, which is circular, is 14 feetclear diameter, and the centre of the light is placed 55 feet above the top of the breakwater.It is built of granite, and divided by floors into store, dwelling, bed, and watch rooms.The lantern is 12 feet in width, and 7 feet 6 inches high, in which is a dioptric fixed lightwith mirrors; the south side shows red lights, and the north white, which distinguishes itfrom the other lights on the coast.
Where this lighthouse is placed, the diameter of the head of the breakwater is 390 feet atthe level of low water, and at the top 75 feet. All the lower courses are secured with slatedowels, and vertical and horizontal dowels 18 inches long and 6 inches square at the centre,sunk 8 inches into the lower course of stone; both ends are also dovetailed, and secured intiieir places by plugs of copper, and by wedges driven into the lower stone.
Eddystone Lighthouse is built upon a rock, which lies nearly south-west from the middleof Plymouth Sound, according to the true meridian ; and the nearest point of land is thepromontory called Ramhead, which is distant about 10 miles, and bears from thence south,scarcely one point west, or, by the compass, south-west by south, allowing two points ofvariation of the north end of the magnetic needle to the westward. These rocks havederived their name from the set or current of the tides which are observed there. Aneddy of the tide is a current setting in a direction contrary to the main stream, and isoccasioned by some obstruction ; this eddy may be either a smoothness on the surface ofthe water, or a current iii the opposite direction to the tide, according to the velocity of thestream, or the size of the island or rock which interposes to produce it.
The rocks are situated in west longitude 4° 5', and north latitude 50" 10', and consistof three principal ridges, called the House, South, and East Reefs. They lie north andsouth, in which direction their greatest extent is 600 or 700 feet; there is also a smallrock, called the North-east, seen only in spring tides, which lies about 1000 feet from theHouse Rock. They arc either granite or gneiss, called in Cornwall moorstone; they