A lifeboat must be buoyant, and firmly ballasted, self-righting, containing plenty of space for the rescued, strength to battle with the heavy seas, and power to resist the many strikings against rocks and wrecks.  The buoyancy is obtained by having air chambers formed along the sides of the boat, and a watertight deck, the space between which and the boat's floor is filled by air chambers.  Beside this, at each end there are air cases built across, and reaching to the high gunwales of the bow and stern.  The power of discharging water is obtained by forming a watertight deck at the load-water-line.  In this deck there are several large open tubes, having their upper openings on the surface of the deck, and the lower ones in the boat's floor, thus passing through the space between the deck and the floor, and, of course, hermetically closed to it.  In some boats the tubes are kept open, but in the self-righting boats they are fitted with self-acting valves, which open downwards only, so that they will allow any water shipped to pass through them, whilst none can pass upwards.  Papa explained that, as the deck is placed above the water-line, any water resting on it will be above the outside level of the sea, and will run out through the valves and tubes into the sea.  As fluids always gain their level by specific gravity, the water passes through the valves until none remains above the surface of the deck.  In the smaller lifeboats, which have no decks, the only way to relieve the boat is by bailing.  It is important that a lifeboat should be well ballasted, especially the larger sailing-boats.  These are now ballasted with water, which is let in after the boat is off the beach, and is allowed to fill every available space to a certain height.  By being thus heavily ballasted, they can make their way through the most tremendous seas, which would drive back any ordinary boat.  Only once has a boat of this description been upset.