“Don’t ever strike a light in a submarine or a dynamite factory. It’s unhealthy.” [Illustration: Courtesy of the Electric Boat Company. Forward deck of a U. S. Submarine, in cruising trim.] I apologized profusely. “The air is so much better than I had expected that I forgot where I was.” “Yes,” said the engineer, chewing his unlighted cigar, “there is plenty of good air in a big modern boat like this, running on the surface in calm weather and with the main hatch and all ventilators open. But come with us when we’re bucking high seas or running submerged on a breathing-diet of canned air flavored with oil, and you’ll understand why so many good men have been invalided out of the flotilla with lung-trouble. We’re the only warships without any dogs or parrots or other mascots on board, for no animal could endure the air in a submarine.” [Illustration: Courtesy of the Electric Boat Company. Same, preparing to submerge. Railing stowed away and bow-rudders extended.] “I thought every submarine carried a cage of white mice, because they began to squeak as soon as the air began to get bad and so warned the crew.” “That was a crude device of the early days,” replied the engineer. “We don’t carry white mice any more, though I believe they still use them in the British navy.” I went up on deck, to find that the _X-4_ had reached the practice-grounds and was being made ready for a dive. Her crew were busy dismantling and stowing away the bridge and the light deck-railing, hauling down the flag, and closing all ventilators and other openings. “How long has it taken you to get ready?” I asked Lieutenant Scope. “Twenty minutes,” he answered. “But the real diving takes only two minutes. We’ll go below now, sink her to condition, and run her under with the diving rudders.” “What are those things unfolding themselves on either side of the bows?” I asked. “I thought the diving rudders were carried astern.” “Modern submarines are so long that they need them both fore and aft,” replied the lieutenant. “As you see, the diving rudders fold flat against the side of the boat where they will be out of harm’s way when we are running on the surface or lying alongside the mother-ship. Better come below now, for we’re going to dive.” We descended into the turret and the hatch was closed. The Diesel engines had already been stopped and the electric motors were now turning the propellers. “Why are those big electric pumps working down there?” I asked. “Pumping water into the ballast-tanks.” “But doesn’t the water run into the tanks anyhow, as soon as you open the valves?” I asked the lieutenant.