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 rapidity with which he worked his "Adam's apple" up and down. He then asked if there was any danger. I answered him that there was none, except to those who were troubled with heart-disease. He immediately put his hand up over his heart and said, "Well, my heart is quite seriously affected," but by that time we had secured the necessary pressure to enable me to open the diving door at the bottom, so I released the "locking dogs" and allowed the door to open, and when he saw the water did not come in, his face cleared and he said, "Well, you know I never would have believed it if I had not seen it," and then he added that he would not have missed seeing it for the world.