Such cannon were all built up of long rectangular bars of iron upon which heavy rings of the same material were shrunk, the whole weapon, on completion, forming a heavy and extremely tough cylinder of wrought iron. The chambers, or breech-pieces, for the reception of the powder-charge, were built separately, with much thicker sides and smaller bores than the rest of the gun, into which they were screwed. The guns must not, I think, be therefore considered breech-loaders; for though it may be possible that they were screwed in and out at each discharge, I think it more probable that, as they were such heavy masses of metal, the breech-pieces were left screwed up and the charges inserted at the muzzle. But when cannon came to be made of more moderate dimensions--big enough to be effective against walls and the sides of ships, and small enough to be transported with reasonable facility--some system of breech-loading was almost universal. I say "almost", because guns began to be cast in brass in Germany at a comparatively early date, and such guns were probably often muzzle-loaders, since cast brass would not have been strong enough for the breech-closing methods in vogue. These were comparatively simple. The breech of the gun, which was built up much in the same way as Mons Meg and others of the same kidney, terminated in a species of trough. Into this trough fitted an iron cylinder which contained the charge of powder and was called a "chamber". The muzzle of the chamber was bevelled off or turned down so as to fit into the breech end of the bore of the gun itself, and was held in position by iron wedges, generally at the rear end, but sometimes across the top. In some of the larger types the trough was made in the huge block of tough oak to which the gun was fastened. In the Tower of London you can see a gun of this kind that was fished up from the wreck of the _Mary Rose_. As most guns were provided with at least two "chambers", one would imagine that a fairly rapid fire could have been kept up, at any rate with the smaller guns. This, however, would not seem to have been the case, for the French account of the battle off St. Helens (when the _Mary Rose_ capsized), which lasted for two hours, and in which a considerable number of ships were engaged, mentions that 300 rounds were fired as a fact indicating the uncommon fierceness of the fighting. And yet the _Henri Grace à Dieu_ alone carried over 100 guns of various sizes!