80 ANNUAL OF SCIENTIFIC DISCOVERY. 



drive in trenails, and thus take at least half the strength out of 

 the timber. 



"About seaports, where old ships are broken up, many old 

 timbers are met with in the fields spotted with two large holes in 

 about every foot of their length ; decay will be observed in all 

 those holes, caused by the woody fibre being bruised by trenail- 

 drivinf, for bruised fibre gives nourishment to drv-rot fungus. 



^^ ' a */ ^j 



Trenails having been squeezed in driving become rotten and 

 weak, cease to hold the planks to the timbers with firmness, get 

 bent, and allow a ship to bend and yield throughout its whole 

 frame ; this is called hogging and sagging. 



" Iron bolts and spikes are the cheapest strength that can be 

 put into a ship. They are the handiest fastenings that a workman 

 can use ; and a little rusting allows a very small fastening to take 

 a very strong hold ; in fact, it is everything that could be wished 

 did it but last without decay. 



" In a ship iron bolts are always damp and always rust; rust 

 frets away woody fibre. Iron bolts, too, always contain a por- 

 tion of sulphur, which gets converted into sulphuric acid, which 

 decomposes both the salts always found in oak, and also salt water, 

 never absent at sea. A ring of decomposed wood surrounds 

 every bolt; and as the salts and oxide of iron are not prejudicial 

 to fungus growth, dry-rot fungus takes possession of the ring of 

 decomposed wood. 



"Iron bolts are inadmissible in the bottoms of ships sheathed 

 with copper ; the salt water acting on so large an extent of cop- 

 per sends such quantities of electricity through the iron bolt that 

 the substance of the bolt is carried away, and a vacancy, which 

 lets in leaking- water, is left in its place. 



" Copper bolts and cupreous metal bolts are more expensive 

 and less strong than iron, but, unlike iron bolts, instead of fretting 

 the wood in which they are inserted, actually preserve it, for the 

 verdigris which is formed on the ^copper bolt poisons the dry-rot 

 fungus. But the copper bolt has the serious disadvantage of 

 having little hold on the wood through which it passes, and this 

 little holdfast becomes less after the wood has shrunk with age, 

 so that the only value of the fastening power of copper metal 

 bolts is left in the riveted ends of the bolt, and when this end 

 breaks off, as it frequently does in 9 or 10 years, by getting crys- 

 tallized, the fastening is of no value at all. 



"Trenails are too cheap and useful, as plugs for keeping out 

 leaking-water, to be given up in wooden-ship construction ; but 

 the disadvantage of their unwieldy size, boring through and de- 

 stroying everything, should be reduced as much as possible. 

 Trenails should be always of the best materials, creosoted to pre- 

 vent the introduction of dry-rot, kept small in size to prevent 

 their doing immoderate harm to the worthier parts of the ship, 

 and driven short to obviate the destruction of timbers, and 

 floors. 



" It is agreed on all sides that iron bolts must never be used in 

 the wake of copper sheathing. Indeed, to insure the durability 



