Every schoolboy is taught the principle of the diving bell, which can be illustrated by the use of a tumbler or glass. If a tumbler is turned upside down and forced into water, the water will not rise to fill the tumbler, owing to the fact that the air, being the lighter, will remain in the tumbler and the water will simply rise, compressing the air to the same pressure per square inch as the pressure surrounding it. Now if you push a tumbler down into the water a distance of thirty-four feet the tumbler would be about one-half full of water and one-half full of air, which corresponds to one atmosphere in pressure. Now if an additional tumbler full of air was compressed to the same pressure and released in that tumbler it would force the water out, and there would be a double volume, or two atmospheres of air, in the tumbler, or just twice what there would be on the surface and under normal atmospheric pressure. This is the principle on which the diving compartment in the Lake type boat operates, it being only necessary to admit air into the diving compartment until the pressure equals the outside water pressure; then a door opening outwardly from the bottom may be opened to permit ready egress or ingress, and so long as the air pressure is maintained no water will rise in the boat. A professor of physics in the University of Pennsylvania visited the _Argonaut_ in Baltimore during some early experiments with her, and in discussing the features of the diving compartment with which, from his position as a professor of natural physics, he should have been entirely familiar, expressed some doubt as to its practicability. He said he understood the theory of it all right, but thought there might be some difficulty in carrying it out in a practical way as I had explained. I invited him into the diving compartment and told him that I would submerge the boat and open the door for him for his benefit, so that he could explain to his students that he had actually seen it done. He turned pale and said, "Oh, no; I would not put you to that trouble for the world"; but by that time I had the heavy iron door closed between the diving compartment and the main hull, and had already started to raise the pressure of the air in the compartment, and assured him that it was not the least trouble in the world; on the contrary, it was a great pleasure. By this time beads of perspiration were standing on his face. When one undergoes air pressure for the first time considerable pain is ofttimes experienced in the ears, due to the pressure on the Eustachian tubes and ear-drums not becoming equalized. To equalize this pressure it is necessary for divers or those undergoing pressure to go through the movement of swallowing, which has a tendency to relieve the unequal pressure and stop the pain. I noticed that the professor was experiencing quite a little pain and consequently told him to swallow, and it was really amusing to see the