382

HISTORY OF ENGINEERING.

Book E

single stone, lying upon the horizontal bearings furnished by the ledges, it would, while itremained entire, have no lateral pressure or tendency to thrust out the sides of the en-compassing walls, and the several pieces of which the floors were composed might have theproperty of a whole stone; thecentre stone was made largeenough to admit of a man hole,with dovetails on its four sides,like those of the entire, solid, bymeans of which the others wereconnected with it, consequentlythe whole, like a single stone,rested upon the ledge, withoutany tendency to spread the walls,which was further providedagainst by an iron hoop or cir-cular chain. Sir ChristopherWren , in the construction of thecupola of St. Paul’s, had alreadyadopted with success the sameconstruction, and to this Smeaton refers for his model.

Each of the four floors hadtwo endless chains, the bars ofwhich are composed of linksinch square, and the grooves thatreceived them were 4 inches indepth and width, and whenplaced each chain was run with11 cwt. of lead.

Ft. In.

The height of the sixfoundation courses tothe top of the rock - 8

The height of the eightcourses to the entrydoor - . - 12

The height of the tencourses of the well holeto the store-room floor 15The height of the fourrooms to the balconyfloor - - 34

Height of the main co-lumn, containing forty-six courses

70

The lantern is formed mostlyof copper; there are 16 frameswith 9 panes each, and the bal-cony is covered with thick platesof lead; there were 16 sheetsput together with strong ribbedjoints; the whole of this workwas very securely braced, and thecopper funnel adapted to let outthe smoke from the candles. Ametal conductor, to guard againstthe effects of lightning, was added.

a

Fig. 390. LANTERN OF EDDYSTONB LIGHTHOUSE.

The lower diameter, immediately above the rock, is 25 feet, and the upper 15 feet, in-cluding the thickness of the walls; the height of the lantern is about 24 feet, and thediameter 9.

The building was completed in October, 1759 ; and upon the rock the workmen wereemployed 2674 hours; the number of pieces of stones used was 1493, independent of 75large cubic joggles and centre plug-stones, 162 cubic joggles of G inches, used in the well-hole courses, 399 fiat joggles in the courses of the rooms and lantern, 399 joint stones inditto, 1800 oak. trenails, 1^ inch in diameter, used in the solid, 4570 pair of oak wedges for