There is no keel properly so called, but in its place a flat keel-plate of iron, about two feet wide and one inch thick, which runs the entire length from stem to stern. This is the base upon which all the rest is reared, plates and girders alike. The iron plates which form her planking are three-quarters of an inch thick. Up to the water-mark the hull is constructed with an inner and outer skin, two feet ten inches apart, both skins being made of three-quarter inch plates, except at the bottom, where the plates are an inch thick; and between these, at intervals of six feet, run horizontal webs of iron plates, which bind the two skins together, and thus it may be said that the lower part of the hull is two feet ten inches thick. This mode of construction adds materially to the safety of the vessel; for, in the event of a collision at sea, the outer skin might be pierced while the inner might remain intact. This space may also at any time be filled with water, and thus ballast, to the amount of 2500 tons, be obtained. Some idea of the magnitude and weight of the vessel may be formed from the fact that each iron plate weighs about the third of a ton, and is fastened with a hundred iron rivets. About thirty thousand of these plates were used in her construction, and three million rivets. The fastening of these rivets was one among the many curious operations performed in course of building. The riveting men were arranged in gangs, each gang consisting of two riveters, one holder-up, and three boys. Two boys were stationed at the fire or portable forge, and one with the holder-up. This boy's duty was to receive the red-hot rivet with his pincers from the boy at the forge, and insert it in the hole destined for its reception, the point protruding about an inch. The holder-up immediately placed his heavy hammer against the head of the rivet, and held it firmly there, while the two riveters assailed it in front with alternate blows, until the countersunk part of the hole was filled up, after which the protruding head was cut off smooth with the plate, the whole operation scarce occupying a minute. In riveting the double part of the ship the holder-up and his boy were necessarily in the interior part of the tubes, and passed the whole day in the narrow space between, (of two feet ten inches wide), in comparative darkness, having only the glimmer afforded by a single dip candle, and being immediately under the deafening blows of the riveters.