What was known as the square or 'box' type, that had been in use nearly fifty years, had to be discarded, in view of this increase of pressure, for a circular form, resembling a Gloucester cheese set on edge, with the furnaces on the flat side of it. The thickness of plates had to be greatly increased, the excellence of the metal to be more rigidly insisted on, while the difficulty of manufacture was considerably enhanced. Machines that had been devised for punching out the rivet holes became of no value, as thick plates suffered in strength by the process, and the operation of drilling had to be substituted for it. This was, of course, more expensive, but amply repaid its cost in the long run.
The 'Penelope,' completed in 1868, was the first important ironclad fitted with twin screws, and is noteworthy as having been the forerunner of a long line of twin-screw battleships. When the 'Audacious' class came to be built, immediately following on the 'Penelope,' the twin screw system was not so generally accepted, as two ships of the same type, the 'Swiftsure' and 'Triumph,' were only allowed one screw apiece. Nowadays all vessels we build of any size above second-class gunboats are fitted with two sets of engines. Of the advantages of the plan there can be no doubt, but in this place, where the aim is to be rather historical than critical, any disquisition on the subject would be impertinent — in the original sense of the word.
In August 1869 was commissioned a single-screw frigate that for speed and other good qualities excelled everything the world had hitherto seen, and indeed she
