Breech-loading cannon cannot be constructed for bullets of larger diameter than that of the rifle bore, without a ductile bullet be used; for, as is usual in breech-loading small-arms, the bullet rifles itself as it is forced up the grooves. The projectiles for rifled cannon have hitherto been cast with corresponding grooves and lands to fit the internal form of the cannon. A compound shot, composed of iron, and covered externally with ductile metal, has been tried in a few instances; but, unfortunately, the difficulty of combining two metals so dissimilar as iron and lead has been found so great as invariably to end in a failure; therefore no prospect exists of bringing into play this, the best point existing in breech-loading arms. Lastly, the tendency of all guns to absorb the heat, developed during explosion, puts a limit to all extreme rapidity of fire; even if this was not already limited by the more essential point of taking aim. At Sweaborg it was found necessary to allow an interval of five minutes between each discharge of a mortar, and yet the whole of them burst after an average of 120 shots. Time and ingenuity spent in planning and constructing breech-loading cannon will always end in disappointment and failure. Many are the plans extant, evincing great skill, perseverance, and everything needful in point of mechanical experience, but betraying a total ignorance of the metallurgic science and of practical results from the use of the engine. The study of these points will save money, time, and what is of more value, brain-work, which might be better employed. Striving to produce perfect breech-loading cannon is like striving to square the circle. CHAPTER IV. ON THE MANUFACTURE OF IRON FOR GUN BARRELS. A considerable progress in improvement has taken place in manufacturing the higher quality of iron since my last publication. Not that I arrogate to myself any credit on that score, but it is evident that good frequently comes of flagellations, whether on the body or the mind. One part of human nature will ever fear the exposure of bad qualities, while another is emboldened to advance in improvement if the slightest chance exist of success or encouragement. Thus we often see men striving to produce one invention on the back of another, with wonderful perseverance, finding many blanks and rarely a prize; for truly in this competing age, the mind must be strong that can fight long. Bitter is the disappointment of the truly ingenious mind, to see the produce of his brains thrown as lumber into the _herring barrel_,--as the printer terms the receptacle for what he sets no value upon; while the valueless contrivances of the mean and sordid are preferred and rewarded, because they enable the manufacturer to produce cheaper, by foisting on the public a deceptive or a spurious article. All inventions for purposes of deception, are readily, aye, eagerly, patronised; for they return gold to the coffers sooner.