HOW GUN-BARRELS ARE MADE. 225 



other words rust ; and here is the one great preser- 

 vative principle in good iron : ' it is the fibre of 

 dense cocoa-wood, compared with that of willow or 

 saugh.' In all situations and for all purposes, where 

 iron is liable to sudden changes of either heat or cold, 

 wet or dry, the very best of iron should be obtained ; as 

 it will be less affected by the changes of temperature, 

 and amply repay by its durability the extra cost in the 

 first instance 



" The frequent welding and re-rolling of iron is of 

 the most beneficial tendency, the elongation of the 

 fibres being highly advantageous ; for a fibrous piece 

 of iron may be compared to a wire rope, the more 

 strands the greater tenacity ; and the smaller the 

 strands, even up to a proximity of fineness to the 

 human hair, the greater the weight they will bear in 

 tension. One large wire which, when single, will sus- 

 pend 500 Ibs., will, when drawn down to six small ones, 

 suspend 600 Ibs. ; and so on to the greatest extreme. 

 Another great advantage received by the repeated 

 reworking of iron is obtaining an increased density ; 

 for when this is secured to a certain extent, you have 

 closed in proportion the pores of the metal ; and in 

 this state it is not liable to that degree of expansion 

 or contraction, or that fluctuation in strength, which 

 arises from softening the iron. Nor can you gain, save 

 t6 a limited extent, any improvement by hammering, 

 hammer-hardening, for instance, simply because it is 

 already improved to the utmost extent we are at pres- 

 ent acquainted with." 



Mr. Greener explains quite fully how the various 

 10* o 



