THE EDDYSTONE LIGHTHOUSE. 85 



Fig. 4. 



Fig. 5. 



Q D 



Three stones of a cramped course. 



In the courses supporting the floors, still greater 

 security was used. The floors are arches, dovetailed 

 together so as to form almost a solid piece each ; par- 

 ticularly as the centre piece, through which is the ladder 

 hole, dovetails into four others which confine all the 

 rest together. This floor is made to rest upon a double 

 ledge cut all round the interior of the building, and 

 occupying two courses of stone. In these, instead of 

 each stone being cramped to its neighbour, a channel 

 is cut all round in which is laid a chain ring of 

 great strength, answering the purpose of an iron hoop : 

 the cavities of the channels are filled up, like those of 

 the cramps, with melted lead ; so that the iron is secure 

 against rust.* These chains, of which there are two 

 round each floor, are made of iron bar, \\ inch square. 

 This chain course and floor are seen in Fig. 1 . 



The stone work was completed on the 23rd of Au- 

 gust, 1759. 



The lanthorn, the framework of which (as well as the 



*The fitting of the chains into these cavities is founded on the 

 property which metals possess of expanding on being heated. The 

 chain itself is made of too small a circumference to fit in the channel, 

 but being made red hot enlarges its dimensions considerably, and is 

 then forced in by levers and the rest of the cavity is immediately 

 filled with melted lead, which together with the iron contracts 

 on cooling, thus the whole circle is bound together with amaz- 

 ing power. The same principle is taken advantage of by coach- 

 makers in shoeing wheels of carriages, the iron rim which surrounds 

 the wheel is made so as to fit it very tightly when heated, it is driven 

 on red hot and cooled immediately by water, this causes it to con- 

 tract and bind the parts of the wheel firmly together. 



