"This small room," said the professor, "is known as the chamber of egress, because, as is quite obvious, it is from here that one leaves the ship for the outer world. But it has also another purpose besides the mere furnishing of access to the trap-door, as I will endeavour to explain to your ladyship. You are, of course, aware that one of the objects with which the _Flying Fish_ was constructed was to enable her crew to explore the ocean depths, and to examine and, if necessary, operate upon the ocean's bed. Now, in order to leave the ship and walk out upon the sea floor, an aperture of some kind in the hull is clearly necessary, through which we may pass; and that aperture you see before you in the shape of the trap-door. But you will readily understand that, with the ship sunk to the bottom, the water will pour violently through that trap, if it is opened without the observance of proper precautions; and unless some special means are adopted to prevent such a catastrophe, the water will quickly invade and fill the entire hull. Hence this room. Its use, in actual practice, is this: having donned our diving-suits in the diving-room, we pass into this small chamber by means of the door of communication, which you see in that partition, close the door carefully behind us, and turn on this tap, which admits a small stream of water into the room from outside. The pressure of water being considerable, the room quickly fills; but the partition, with its water-tight door, effectually prevents the water from penetrating any farther into the hull of the ship--and we then throw open the trap-door, and walk forth on to the sea floor. Upon our return we close the trap-door behind us, thus, turn on this air tap, and immediately a stream of densely compressed air rushes into the chamber, expelling the water through this valve in the floor. And when the water is all out, we turn off the stream of compressed air, and open this valve, which allows the compressed air to pass into the habitable portion of the ship, quickly reducing the air-pressure in this room to what it is in the other habitable portion of the ship; then we open this door, and pass into the diving-room." The professor then threw open the door and, with a profound bow, stood aside to allow Lady Elphinstone to pass through. The room in which the party presently found themselves was an apartment about twenty feet square, one side of which was wholly occupied by four cupboards labelled respectively "Sir Reginald Elphinstone", "Colonel Lethbridge", "Captain Mildmay," and "Von Schalckenberg." "This," explained the professor, "is the room wherein we shall equip ourselves for our submarine rambles; and," throwing open the door of one of the cupboards and disclosing certain articles neatly arranged upon hooks fastened to the walls, "here is a suit of the clothing and armour that we shall wear upon such occasions."