MECHANICS AND USEFUL ARTS. 81 



of the structure of a ship iron bolts should never be driven at all, 

 except in situations where they can be removed and replaced. 



** Covering iron bolts with zinc (called galvanizing) does not 

 protect the iron from rusting, as the acid of the oak surroiihding 

 the bolt soon dissolves off the zinc cover, and corrosion proceeds 

 with all its concomitant evils. 



" A large quantity of copper metal fastening is now required in 

 first-class ships. It is expensive. Let us inquire how the great- 

 est strength, at the lowest cost, can be got from it. 



" The screw form, I believe, will be found the strongest and 

 cheapest method for the use of copper metal. This form gives a 

 secure hold, and does not injure the wood if the pitch of the 

 screw be kept high ; that is, the threads of the screw be kept far 

 apart. I have been accustomed to use screws 7 inches long in- 

 stead of trenails. 



" The bolt is moulded in threads, 3 turns in an inch, cut in a 

 three-quarters inch bolt of Prince's metal, weighing 13 oz., and 

 costing 9d. This screwed through a 3-inch plank penetrates the 

 timbers 4 inches and requires no rivet, as I have tried to start a 

 deal end from a 4-inch thick piece of oak secured in this manner, 

 with a strain of 36 cwt. suspended, without having been able to 

 produce the least separation of the deal from the oak. The 

 necessity of a through fastening does not exist, as the timber can 

 be secured to the ceiling by a similar screw to keep it exactly in 

 place ; thus a long length of metal bolt is saved, the timber but 

 slightly wounded, and the strength of the frame immeasurably 

 increased. 



" For larger fastenings, such as those for securing timbers and 

 floors to iron riders, I have used a thread one-eighth inch in 

 height, placed round outside a seven-eighths inch Prince's metal 

 bolt, instead of cutting into the body of the bolt, in order to pre- 

 serve its lateral strength and rigidity. The turns of the screw 

 are 3 in 2 inches ; a length of 14 inches weighs 3 pounds, and 

 costs 2s. 6d. I found a strain of 49 cwt. was barely sufficient to 

 tear this screw through a 3-inch deck deal end, and of course a 

 lono-er length screwed into oak would require a heavier strain 



O O 



for its removal. 



"Pure copper cannot be cast into a screw for any strength, 

 and therefore I have used Prince's metal (a mixture of 16 oz. 

 copper, 3 oz. zinc, and k oz. tin.) This mixture runs into every 

 sinuosity of the casting-mould, is so tough that it will bend more 

 than double cold, and I believe will not crystallize and break 

 when it has grown old." 



THE " CAPTAIN." 



The following are the particulars and dimensions of the " Cap- 

 tain," whose recent loss has startled the American as well as the 

 British public. H.B.M.S. "Captain;" designer, Capt. Cowper 

 Coles, R.N. ; builders, Messrs. Laird Brothers, Birkenhead, G. 

 B., 1870. 



