Lieutenant Perry Scope, commanding the X-class flotilla, was sitting in his comfortable little office on the mother-ship Ozark, when I entered with a letter from the secretary of the navy, giving me permission to go on board a United States submarine. Without such authorization no civilian may set foot on the narrow decks of our undersea destroyers, though he may visit a battleship with no more formality than walking into a public park.

“We’re too small and full of machinery to hold a crowd,” explained the lieutenant, “and the crowd wouldn’t enjoy it if they came. No nice white decks for the girls to dance on or fourteen-inch guns for them to sit on while they have their pictures taken. Besides, everything’s oily—you’d better put on a suit of overalls instead of those white flannels.”

There were plenty of spare overalls on the Ozark, for she was the mother-ship of a family of six young submarines. Built as a coast defense monitor shortly after the Spanish War, she had long since been retired from the fighting-line, and was now the floating headquarters, dormitory, hospital, machine-shop, bakery, and general store for the six officers and the hundred and fifty men of the flotilla.

Moored alongside the parent-ship, the submarine X-4 was filling her fuel-tanks with oil through a pipe-line, in preparation for the day’s cruise and target-practice I was to be lucky enough to witness. Two hundred and fifty feet long, flat-decked and straight-stemmed, she looked, except for the lack of funnels, much more like a surface-going torpedo-boat than the landsman’s conventional idea of a submarine.

“I thought she would be cigar-shaped,” I said as we went on board.

“She is—underneath,” answered Lieutenant Scope. “What you see is only a light-weight superstructure or false hull built over the real one. See those holes in it, just above the water line? They are to flood the superstructure with whenever we submerge, otherwise the water pressure would crush in these thin steel plates like veneering. But it makes us much more seaworthy for surface work, gives us a certain amount of deckroom, and stowage-space for various useful articles, such as this.”

Part of the deck rose straight up into the air, like the top of a freight-elevator coming up through the sidewalk. Beneath the canopy thus formed was a short-barreled, three-inch gun.

“Fires a twelve-pound shell, like the field-pieces the landing-parties take ashore from the battleships,” explained the naval officer, as he trained the vicious-looking little cannon all around the compass. “Small enough to be handy, big enough to sink any merchant ship afloat, or smash anything that flies.” Here he pointed the muzzle straight up as if gunning for hostile aeroplanes.

“And please observe,” he concluded, as the gun sank down into its lair again, “how that armored hatch-cover protects the gun-crew from shrapnel or falling bombs.”

I followed him to the conning-tower, or, as he always spoke of it, the turret. The little round bandbox of the Holland has developed into a tall, tapering structure, sharply pointed fore and aft to lessen resistance when running submerged. Above the turret was a small navigating-bridge, screened and roofed with canvas, where a red-haired quartermaster stood by the steering-wheel, and saluted as we came up the ladder. The lieutenant put the engine-room telegraph over to “Start,” and a mighty motor throbbed underneath our feet. Then the mooring was cast off, the telegraph put over to “Slow Ahead,” and the X-4 put out to sea.

“How long a cruise could she make?” I asked.

“Four thousand miles is her radius,” answered her commander. “Back in 1915, ten American-designed submarines crossed from Canada to England under their own power.”

“Yet it is only a few years since we were told that submarines could only be used for coast defense, unless they were carried inside their mother-ships and launched near the scene of battle,” I remarked. “Or that each battleship should carry a dinky little submarine on deck and lower it over the side like a steam-launch.”

“People said the same thing about torpedo-boats,” agreed the lieutenant; “they began as launches—now look at the size of that destroyer smoking along over there. Ericsson thought that any ironclad bigger than a Civil War monitor would be an unwieldy monster. Even John P. Holland fought tooth and nail against increasing the length of his submarines. This boat of mine is five times the length of the old Holland, but she’s only a primitive ancestor of the perfect submarine of the future.”

“She isn’t a submarine at all,” I replied presently, as the X-4 swept on down the coast at a good twenty-two knots, her foredeck buried in foam and the sea-breeze singing through the antennÆ of her wireless. “She’s nothing but a big motor-boat.”

“And she’s got some big motors,” replied the lieutenant. “Better step below and have a look at them.”

I went down through the open hatchway to the interior of the boat and aft to the engine-room. There I found two long, many-cylindered oil-engines of strange design, presided over by a big blond engineer whose grease-spotted dungarees gave no hint as to his rating.

“What kind of machines are these?” I shouted above the roar they made. “And why do you need two of them?”

“Diesel heavy-oil engines,” he answered. “One for each propeller.”

“What is the difference between one of these and the gasoline engine of a motor-car? I know a little about that.”

“Do you know what the carburetor is?” asked the engineer.

“That’s where the gasoline is mixed with air, before it goes into the cylinder.”

The engineer nodded.

“The mixture is sucked into the cylinder by the down-stroke of the piston. The up-stroke compresses it, and then the mixture is exploded by an electric spark from the spark-plug. The force of the explosion drives the piston down, and the next stroke up drives out the refuse gases. That’s how an ordinary, four-cycle gasoline motor works.

“But the Diesel engine,” he continued, “doesn’t need any carburetor or spark-plug. When the piston makes its first upward or compression-stroke, there is nothing in the cylinder but pure air. This is compressed to a pressure of about 500 pounds a square inch—and when you squeeze anything as hard as that, you make it mighty hot—”

“Like a blacksmith pounding a piece of cold iron to a red heat?” I suggested. The engineer nodded again.

“That compressed air is so hot that the oil which has been spurted in through an injection-valve is exploded, and drives the piston down on the power-stroke. The waste gases are then blown out by compressed air. There are an air-compressor and a storage tank just for scavenging, or blowing the waste gases out of every three power-cylinders.”

“What are the advantages of the Diesel over the gasoline engine?”

“In the first place, it gives more power. You see, three out of every four strokes made by the piston of a gasoline engine—suction-stroke, compression-stroke, and scavenging-stroke—waste power instead of producing it. But the Diesel is what we call a two-cycle engine; its piston makes only two trips for each power-stroke. In the second place, it is cheaper, because instead of gasoline it uses heavy, low-price oil. And this makes it much safer, for the heavy oil does not vaporize so easily. The air in some of the old submarines that used gasoline motors would get so that it was like trying to breathe inside a carburetor, and there was always the chance of a spark from the electric motors exploding the whole business, and your waking up to find the trained nurse changing your bandages. The German navy refused to build a submarine as long as there was nothing better than gasoline to propel it on the surface. They didn’t launch their U-1 till 1906, after Dr. Diesel had got his motor into practicable shape. It cost him twenty years of hard work, but without his motor we couldn’t have the modern submarine. And they’re using it more and more in ocean freighters. There’s a line of motor-ships running to-day between Scandinavia and San Francisco, through the Panama Canal.

“Aft of the Diesel, here,” continued the engineer, “is our electric motor, for propelling her when submerged. Reverse it and have it driven by the Diesel engine, and the motor serves as a dynamo to generate electricity for charging the batteries. As long as we can get oil and come to the surface to use it, we can never run short of ‘juice.’14

“Besides turning the propeller, the electricity from the batteries lights the boat, and turns the ventilating fans, works the air-compressor for the torpedo-tubes, drives all the big and little pumps, runs a lot of auxiliary motors that haul up the anchor, turn the rudders, and do other odd jobs, it heats the boat in cold weather—”

“And cooks the grub all the year round, don’t forget that, Joe,” said another member of the crew. “Luncheon is served in the palm room.”

We ate from a swinging table let down from the ceiling of the main-or living-compartment of the submarine, that extended forward from the engine-room to the tiny officers’ cabin and the torpedo room in the bows. Tiers of canvas bunks folded up against the walls showed where the crew slept when on a cruise. For lunch that day we had bread baked on the mother-ship, butter out of a can, fried ham, fried potatoes, and coffee hot from a little electric stove such as you can see in the kitchenette of a light-housekeeping apartment on shore. The lieutenant’s lunch was carried up to him on the bridge. When the meal was over, most of the men went on deck, and my friend the engineer put a large cigar in his mouth. I took out a box of matches and was about to strike one for his benefit when he stopped me, saying,

“Don’t ever strike a light in a submarine or a dynamite factory. It’s unhealthy.”

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.”

“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.

“Turn a tumbler upside down and force it down into a basin of water,” he replied, “and you trap some air in the top of the tumbler, which prevents the water from rising beyond a certain point. The same thing takes place in our tanks, and to fill them we have to force in the water with powerful pumps that compress the air in the tanks to a very small part of its original bulk. This compressed air acts as a powerful spring to drive the water out of the tanks again when we wish to rise. By blowing out the tanks, a submarine can come to the surface in twenty seconds or one sixth the time it takes to submerge.”

“When are we going under?” I asked him. The lieutenant looked at his watch and answered,

“We have been submerged for the last four minutes.”

I experienced a feeling of the most profound disappointment. Ever since I had been a very small boy I had been looking forward to the time when I should go down in a submarine boat, and now that time had passed without my realizing it.

“But why didn’t I feel the boat tilt when she dived?” I demanded.

“Because she went down a very gentle slope, between two and three degrees at the steepest. The only way you could have noticed it would have been to watch these gages.”

Large dials on the wall of the turret indicated that the X-4 was running on what was practically an even keel at a depth of sixteen feet and under a consequent water-pressure of 1024 pounds on every square foot of her hull.

“How deep could she go?”

“One hundred and fifty feet—if she had to. The strong inner hull of a modern submarine is built up of three quarter inch plates of the best mild steel and well braced and strengthened from within. But as a rule there is no need of our diving below sixty feet at the deepest, or far enough to clear the keel of the largest ship. You will notice how the depth-control man is holding her steady by manipulating the forward horizontal rudders, just as an aviator steadies his aeroplane.”

“He must be a strong man to handle those two big horizontal rudders.”

“He has an electric motor to do the hard work for him, as has the quartermaster steering the course here with the vertical rudder.”

The same red-headed petty officer that I had noticed on the bridge now grasped the spokes of a smaller steering-wheel inside the conning-tower.

“What is that queer-looking thing whirling round and round in front of him?” I asked.

“A Sperry gyroscopic compass,” replied the lieutenant. “An ordinary magnetic compass could not be relied on to point in any particular direction if it was shut up in a steel box full of charged electric wires, like the turret of a submarine. We tried to remedy this by building conning-towers of copper, till Mr. Sperry perfected a compass that has no magnetic needle, but operates on the principle of the gyroscope. You know that a heavy, rapidly rotating wheel resists any tendency to being shifted relative to space?”

“Yes.”

“The earth, revolving on its axis, is nothing but a big gyroscope—that is why it stays put. The little gyroscope on this compass spins at right angles to the revolution of the earth and so keeps in a due north and south line. But the frame it is mounted on turns with the ship, so the relative positions of the frame and the gyro-axis show in what direction the submarine is heading.”

“And you can see what’s ahead of you through the periscope. Who invented that?”

“The idea is a very old one. Certain French and Dutch inventors designed submarines with periscopes as long ago as the eighteen-fifties. In the Civil War, the light-draft river-monitor Osage had attached to her turret a crude periscope made by her chief engineer, Thomas Doughty, out of a piece of three-inch steam-pipe with holes cut at each of its ends at opposite sides, and pieces of looking-glass inserted as reflectors. By means of this instrument her captain, now Rear-Admiral, Thomas O. Selfridge, was able to look over the high banks of the Red River when the Osage had run aground in a bend and was being attacked by three thousand dismounted Confederate cavalry, who were repulsed with the loss of four hundred killed or wounded by the fire of the monitor’s 11-inch guns, directed through the periscope.15

“But as late as 1900 the periscope was so crude and unsatisfactory an instrument that John P. Holland would have nothing to do with it. The credit for bringing it to its present efficiency belongs chiefly to the Germans, who kept many of their scientists working together on the solution of the difficult problems of optics that were involved.

“By turning this little crank,” the lieutenant continued, “I can revolve the reflector at the top of the tube. This reflector contains a prism which reflects the image of the object in view down through a system of lenses in the tube to another prism here at the bottom, where the observer sees it through an eyepiece and telescope lenses.”

I looked into the eyepiece, which was so much like that of an old-fashioned stereoscope that I felt that it, too, ought to work back and forth after the manner of a slide trombone. I found myself looking out over the broad blue waters of a sunlit bay. I noticed a squall blackening the surface of the water, a catboat running before it, and the gleam of the brass instruments of the band playing on the after deck of a big white excursion steamer half a mile away.

“I can almost imagine I can hear the music of that band,” I exclaimed. “The optical illusion is perfect.”

“It has to be,” rejoined the lieutenant. “If the image were in the least distorted or out of perspective, we couldn’t aim straight.”

“What do you do when the periscope is wet with spray?” I asked him.

“Wash the glass with a jet of alcohol and dry it from the inside with a current of warm air passing up and down the tube. A periscope-tube is double: the outer one passing through a stuffing-box in the hull, and the inner tube revolving inside it. The old-fashioned single tubes were too hard to revolve and the resistance of the water used to bend them aft and cause leakage. We can raise and lower the periscopes at will, and all our larger boats have two of them, so that they can keep a lookout in two directions at once, besides having a spare eye in case the first is put out.”

“What are those two little things that big naval tug is towing over there?” I inquired.

“The target for our torpedo practice,” replied Lieutenant Scope. “We shall try to put four Whiteheads between those two buoys as the tug tows them past at an unknown range and speed. If you step forward to the torpedo room you can see them loading the tubes.” As I walked forward it occurred to me that the twenty-odd men on board the X-4 seemed to be moving about inside her with perfect freedom, without disturbing her trim. I mentioned this to one of the crew.

“It’s the trimming-tanks that keep her level,” he explained. “As we’re walking forward, our weight in water is being automatically pumped from the trimming-tank in the bow to the one astern. A submarine is just one blamed tank after another. Stand clear of that chain-fall, sir; they’re loading No. 1 tube.”

Stripped to the waist like an old-time gun-crew, four beautifully muscled young gunner’s mates were hoisting, with an ingenious arrangement of chains and pulleys, a torpedo from the magazine. The breach of the tube was opened and the long Whitehead thrust in, two flanges on its sides being fitted into deep grooves in the sides of the tube, so that the torpedo would not spin like a rifle-bullet but be launched on an even keel. The breach was closed, and the men stood by expectantly.

“Skipper’s up in the conning-tower, taking aim through the periscope,” explained the man who had told me about trimming-tanks. “The tubes being fixed in the bow, he has to train the whole boat like a gun. Likewise he’s got to figure out how far it is to the target and how fast the tug is towing it, how many seconds it’s going to take the torpedo to get there, and how much he’s got to allow for its being carried off its course by tide and currents. When he gets good and ready, the lieutenant’ll press a little electric button and you’ll hear—”

“Thud!” went the compressed air in the tube, and the submarine shuddered slightly with the shock of the recoil. But that was all.

“There she goes!” said my friend the tank-expert. “As soon as the Whitehead was expelled, a compensation-tank just above the tube was flooded with enough water to make good the loss in weight.”

“What keeps the sea-water from rushing into the tube after the torpedo leaves it?” I asked.

“A conical-shaped cap on the bow of the boat keeps both tubes closed except when you want to fire one of them. Then the cap, which is pivoted on its upper edge, swings to port or starboard just long enough for the torpedo to get clear and swings back before the water can get in.”

Four of the ten torpedoes carried in the magazine were sped on their way to the unseen target. I returned to the turret as the wireless operator entered and handed a typewritten slip to Lieutenant Scope, who smiled happily and said to me,

“The captain of the tug reports that all four shots were hits and all four torpedoes have been safely recovered.”

I was too astonished to congratulate him on his marksmanship, as I should have done.

“How in the name of miracles!” I gasped. “Can you receive a wireless telegram under the sea?”

“By the Fessenden oscillator,” he replied, and added to the wireless man,

“Take this gentleman below and show him how it works.”

“Did you ever have another chap knock two stone together under water when you were taking a dive?” asked the operator. I nodded in vivid recollection.

“Then you have some idea how sounds are magnified under water. It is an old idea to put submarine bells down under lighthouses and fit ships with some kind of receiver so that the bells can be heard and warning given when it is too foggy to see the light. The advantage over the old-style bell-buoy lies in the fact that sound travels about four times as fast through water as through air,16 and goes further and straighter because it isn’t deflected by winds or what the aviators call ‘air-pockets.’ The man who knows most about these things is Professor Fessenden, of the Submarine Signal Company of Boston, who first realized the possibility of telegraphing through water.17

“Fastened outside the hull of this boat is one of the Fessenden oscillators: a steel disk eighteen inches in diameter, that can be vibrated very rapidly by electricity. These vibrations travel through the water, like wireless waves through the ether, till they strike the oscillator on another vessel and set it to vibrating in sympathy. To send a message, I start and stop the oscillator with this key so as to form the dots and dashes of the Morse code. To receive, I sit here with these receivers over my ears and ‘listen in,’ just like a wireless operator, till I pick up our call ‘X-4,’ ‘X-4.’”

“How far can you send a message under water?”

“Ten miles is the furthest I’ve ever sent one. Professor Fessenden has sent messages more than thirty miles. The invention only dates back to 1913 and what it will do in the future, there is no telling.”

“Even now, couldn’t a surface vessel act as eyes for a whole flotilla of submarines and tell them where to go and when to strike by coaching them through the Fessenden oscillator?”

The operator nodded.

“We’re doing it to-day, in practice. But don’t forget that an enemy’s ship carrying a pair of oscillators can hear a submarine coming two miles away. You can make out the beat of a propeller at that distance every time.”

“But how can you tell how far away and in what direction it is?”

“I can’t, with a single oscillator like ours. But a ship carries two of them, one on each side of the hull, like the ears on a man’s head. And just as a man knows whether a shout he hears comes from the right or left, because he hears it more with one ear than the other, so the skipper of a surface craft can look at the indicator that registers the relative intensity of the vibrations received by the port and starboard oscillators and say,

“‘There’s somebody three points off the starboard bow, mile and three quarters away, and heading for us. Nothing in sight, so it must be one of those blamed submarines.’

“And away he steams, full speed ahead and cutting zigzags. Or maybe he gets his rapid-fire guns ready and watches for Mr. Submarine to rise—like the X-4’s doing now.”

Freed of the dead weight of many tons of sea water blown from her ballast-tanks by compressed air, the submarine rose to the surface like a balloon. Ventilators and hatch-covers were thrown open and we swarmed up on deck to fill our grateful lungs with the good sea air. Three motor-boats from the tug throbbed up alongside with the four torpedoes we had discharged.

“Those boats wait, one this side of the target, one near it and the third over on the far side, to mark the shots and catch the torpedoes after they rise to the surface at the end of their run,” said Lieutenant Scope. “We very seldom lose a torpedo nowadays. They tell a story about one that dived to the bottom and was driven by the force of its own engines into forty feet of soft mud, where it stayed till it happened to be dug up by a dredger.”

The four torpedoes were hoisted aboard, drained of the sea water that had flooded their air-chambers, cleaned and lowered through the torpedo hatch forward down into the magazine. By this time the bridge and railing were again in place and the flags fluttering over the taffrail as the X-4, her day’s work done, sped swiftly up the coast to home and mother-ship.