4.--It should be capable of carrying sufficient pure air to support its crew for a specified time, or of having the means of purifying the air within the boat, and exhausting the foul air. 5.--It should be able to rise and sink at will to the required depth, either when stationary or in motion. 6.--It should be so fitted that the crew possess the means of leaving the boat without requiring external assistance. 7.--It should carry a light sufficient to steer by, and to carry on the various operations. 8.--It should possess sufficient strength to prevent any chance of its collapsing at the greatest depth to which it may be required to manipulate it. The results of former experiments with such boats prove that manual power, which was the original mode of propulsion, is not the motive power best adapted to such a boat; compressed air, gas as used in the Lay torpedo boat, and steam, are all of them far preferable to the original method, but which of these modern ones is the most practicable has yet to be decided. The most difficult point to be overcome in connection with a submarine boat is that of steering it correctly when beneath the surface of the water. _Confederate Submarine Boat._--The Confederate submarine torpedo boat that sunk the Federal vessel of war _Housatonic_ on the 17th of February, 1864, was built of boiler iron, 35' long, 3' beam (extreme), 5' high in the centre. She carried a crew of nine men. She was propelled by means of a screw propeller worked by eight of the crew, her greatest speed being four knots an hour in smooth water. She carried a sufficient quantity of air to enable the crew to remain submerged for the space of two to three hours. Two fins were fitted on the outside for rising and falling at will, when in motion. There were two manholes provided, fitted with bull's-eyes. This boat was intended to pass under a vessel's bottom, towing a torpedo after her, which was arranged to explode on contact. She was the means of drowning fourteen men before she made her last attempt, when nine others were added to the above list. In her successful attack on the _Housatonic_, she was armed with the bow spar torpedo, and was sunk, owing to her running into the hole formed by the explosion of her torpedo. About three years after the American civil war was over, this submarine boat was recovered. Divers went down, and found her lying alongside the hull of the _Housatonic_, with the remains of the nine men in her.