The tenacity of the different metals is determined by ascertaining the weight required to break wires of the same length and gauge. Iron appears to possess the property of tenacity in the greatest, and lead in the least degree. (Fig. 77.) [Illustration: Fig. 77. B. Pan supported by leaden wire broken by a weight which the iron wire at A easily supports.] [Page 63] The tenacity of iron is taken advantage of in the most scientific manner by the great engineers who have constructed the Britannia Tube, and that eighth wonder of the world, the _Leviathan_, or _Great Eastern_ steam-ship. In both of these sublime embodiments of the genius and industrial skill of Great Britain the advantage of the cellular principle is fully recognised. The magnitude of this colossal ship is better realized when it is remembered that the _Great Eastern_ is six times the size of the _Duke of Wellington_ line-of-battle ship, that her length is more than three times that of the height of the Monument, while in breadth it is equal to the width of Pall Mall, and that a promenade round the deck will afford a walk of more than a quarter of a mile. Up to the water-mark the hull is constructed with an inner and outer shell, two feet ten inches apart, each of three-quarter-inch plate; and between them, at intervals of six feet, run horizontal webs of iron plates, which convert the whole into a series of continuous cells or iron boxes. (Fig. 78.) [Illustration: Fig. 78. Transverse section of _Great Eastern_, showing the cellular construction from keel to water-line, A A.] This double ship is useful in various ways; in the first place, the danger arising from collision is diminished, as it is supposed that the outer web only would be broken through or damaged; so that the water would not then rush into the steam-ship, but merely fill the space between the shells. In the second place, if there should be any difficulty in procuring ballast, the space can be filled with 2500 tons of water, or again pumped out, according to the requirements of the vessel. The strength of a continued cellular construction can be easily imagined, and may be well illustrated by a thin sheet of common tin plate. If the ends be rested on blocks of wood, so as to lap over the wood about one inch, they are easily displaced, and the mimic bridge broken down from its [Page 64] supports by the addition to the centre of a few ounce weights; whilst the same tin plate rolled up in the figure of a tube, and again rested on the same blocks, will now support many pounds weight without bending or breaking down. (Fig. 79.) [Illustration: Fig. 79. A. Flat tin plate, breaking down with a few ounce weights. B. Same tin plate rolled up supports a very heavy weight.]