The design of guns had by this time become subject to more scientific consideration than had hitherto been bestowed, and their manufacture had been improved by the Swiss invention of the boring machine, which enabled them to be cast solid instead of being cast hollow on a core. Iron guns came more and more into favour as the century progressed, especially for naval use. The cost of iron was only one-eighth that of brass. The art of casting iron in homogeneous masses had by this time made progress, and though hitherto it had been the custom to make iron ordnance of great thickness and weight, repeated trial proved that they could be made lighter, if required, without undue loss of strength, and that in action they outlasted brass ordnance, which cracked, bent at the muzzle, and wore out at the vent. A well-made iron gun was almost indestructible. At the siege of Belleisle, in the Seven Years’ War, the brass guns soon wore out, and had to be replaced by iron ship guns; and it was long, indeed, before a suitable brass was discovered, which would withstand the repeated fire of large charges without losing its tin-element and degenerating into a spongy and craterous material. Muller, in his _Treatise of Artillery_, of 1768, described how he had seen cast iron at the Carron works so tough that “it would flatten and tear like brass”; and advocated iron guns of a new and light construction to replace Schalk’s brass guns forming the armament of the _Royal George_, and give a saving in weight of over a hundred and sixty tons. [Illustration: FRENCH TWENTY-FOUR-POUNDER WITH SPHERICAL CHAMBER From St. Remy’s _Mémoires_] In respect of design, the newly acquired knowledge of the true principles governing internal ballistics began gradually, in the latter part of the century, to show its effect. Hitherto, ever since gunpowder had been in military use, pieces had been cast in masses of varying size and shape and ornamented to please the fancy of the founder. Cannon had been made with double or triple reinforces of metal, so that their exterior surface was stepped longitudinally from muzzle to breech. Experience probably pointed out on many occasions the bad design of a piece whose sections showed sudden alterations in shape; but it was not till after the middle of the eighteenth century that this consideration was discussed by a professional. “Since powder acts uniformly and not by starts it is hard to judge from whence this ridiculous custom has arisen.... There should be no breakings in the metal.” The piece, continues Muller, should be of cylindrical bore, and its outer contour should be a curve slightly concave, corresponding presumably to the curve of the powder pressure. But as this curve would be difficult to find, he recommends a sloping straight line from breech to muzzle as sufficiently exact for practical purposes.