the vessel, and let out as required, while the vitiated air was allowed to escape into space. As the air in the interior was under a pressure of 15 pounds to the square inch, great care had to be taken to prevent it from leaking through the outer sheathing. If a hole happened to be made through the latter, the air rushed out in a second, and the unfortunate men inside died immediately.
The cannon slid in and out through tubes fitted with telescopic joints, which rotated in gimbals, so that they could be worked without allowing air to escape, and the muzzle of the gun was closed by a slide while the shot was introduced at the breech. The torpedoes were worked from torpedo-tubes in the same manner. The cannon varied in size from 200 tons to 10-pounders, the armament of each ship being selected according to the duties which it was expected to perform in an engagement. Besides the holes for the cannon, the hull was pierced in many
