vessel at the desired depth. If it is desired to lie at rest off the entrance of the enemy's harbor to wait for her ships to come out, the submarine proceeds to her station submerged with a small amount of buoyancy,—which is the usual method of navigating submerged. When she arrives at the desired station the speed is reduced and an additional amount of water is gradually admitted to give her a small amount of negative buoyancy. At the same time her anchoring weights are paid out until they touch bottom. As soon as they do so water is forced out of the ballast tanks by compressed air until positive buoyancy is restored and the vessel stops sinking and remains at rest anchored between the surface and the bottom, like an anchored buoyant mine. By winding in on the anchor cables a submarine may then be hauled down nearer the bottom, and by paying out the cables she may rise nearer the surface. On picket duty off harbor entrances she remains sufficiently near the surface to project her telescoping periscope occasionally above the crest of the waves to keep watch and see that an enemy ship does not enter or clear. In this condition there is no necessity to have any machinery running on board the submarine, therefore she can remain for weeks at a time on station without exhausting her fuel supply. It is only necessary for her to renew the air supply now and then, which can be done at night. Another method for holding a vessel at rest is by taking in and forcing out alternately small quantities of water so as to keep her in equilibrium between positive and negative buoyancy. Another method is to use vertical propellers operating in wells extended from the sides, and by running these it is possible to exert an upward or downward pressure and so hold her at a depth. Neither of these methods is as satisfactory, however, as the anchor weights, because the vessel will not hold a definite position on station, but will drift off with the current. They also make a drain on the storage battery and require constant attention on the part of the members of the crew. By the anchor weights scheme the vessel may stay on station as long as the food and fuel supply holds out.

SHOWING VARIOUS CONDITIONS IN WHICH A SUBMARINE OF THE LEVEL KEEL TYPE FITTED WITH BOTTOM WHEELS, MAY NAVIGATE

1, running light on surface; 2, awash, ready for submergence; 3, submerged, depth controlled by hydroplanes; 4, running on bottom.

The above facts set forth simply the outstanding mechanical principles upon which the operation of the submarine is based. The submarine of to-day, however, has many auxiliaries, to describe which in detail would require several volumes of technical description.

I will briefly enumerate a few of the more important of these devices and describe their function as applied to the war submarine.

THE LOWER PORTION OF GALILEO PERISCOPE

THE PERISCOPE IS THE EYE OF THE SUBMARINE.
(See description.)

The Periscope.—The periscope is the eye of the submarine. In its simpler form it consists of a stiff metallic tube, from fifteen to twenty feet in length and about four inches in diameter. Referring to Figure 1, on page 23, it is made up of an object glass, A, which "views" or takes an impression of all objects within its range or field of vision, and transmits an image of such object through the right-angle prism, B, which turns the image so that it appears some distance down the tube, say, for purposes of description, at C. If a piece of ground glass were held at the focus of the objective lens at C, the image could be seen. The lens D, located farther down the tube, in turn now "views" the image and transmits it still farther down the tube, where it is turned through the right-angle prism, E, and where the image is again turned into an erect position. A piece of ground glass located at F would show the image in the same manner as an image is shown on the ground glass of a camera. The magnifying eyepiece G magnifies the image so that distant objects appear of natural size.

Other figures show a periscope as made by the Officina Galileo in Florence, Italy. This firm makes periscopes with binocular eyepieces. The success of any periscope depends upon the character of the material used in the lenses and prisms and the accuracy of the workmanship. This firm, which is probably the oldest optical manufacturing house in the world, said to have been founded by Galileo himself, turns out instruments of the most beautiful workmanship. The flange of the instrument is bolted to the top of the conning tower, or deck, and a gate valve is arranged between the deck and the eyepiece so that in case the tube should be carried away the gate valve can be closed and thus prevent water from entering the vessel. A hand wheel arranged below the binocular eyepiece permits of easy rotation of the instrument. Provision is made for introducing dry air; this prevents condensation forming on the lenses or prisms within the tube.

Owing to the fact that there is a certain loss of light in transmitting the image through the various prisms and lenses, it is customary to magnify the image so that it appears to be about one-quarter larger than when viewed by the natural eye. This has been found by experience to give, when viewed through the periscope alone from a submerged vessel, the impression of correct distance.

Previous to 1900 there was no instrument which would give through a long tube normal vision and a correct idea as to distance. At this time I took up with various opticians the question of producing such an instrument. They all contended that it was impossible to produce an instrument that would give through a long tube a field of vision equal to the natural eye or that would convey a correct idea as to the distance of an object when viewed through a long tube. The camera lucida which Mr. Holland and others had used in the earlier submarines simply threw a picture of the object on a bit of white paper, usually located on a table. This did not give to the observer any more idea of the correct distance of an object than a photograph would. Believing, however, that a solution could be found, I then purchased a variety of lenses and started making experiments.

Without any special knowledge of optical science, one day quite by accident I secured the desired result and found that it was possible to secure practically normal vision through a tube of considerable length. About the same time, Sir Howard Grubb, of England, brought out an instrument in which he accomplished the same result. I then continued in my experimental work and brought out an instrument which was designed to give a simultaneous view of the entire horizon.

This instrument was called an "omniscope." It was first called a "skalomniscope," which was a word coined with the idea of describing the function of the instrument and which, translated, means "to view and measure everything." A scale was used in connection with this instrument which would convert it into a range finder by measuring the image of an abject of known dimensions, such as the length of a ship or the height of its smokestack, and give simultaneous reading as to its distance.

For a time it was necessary for us to manufacture our own sighting instruments, but later, when the optical houses understood the principle of the periscope, they took up the matter of manufacture and have so greatly improved them that it is now possible to secure instruments of great accuracy and fine definition.

The periscope, however, is faulty, in that it is only an instrument for day use. As soon as dusk comes on the periscope becomes useless. The passing of the image down the tube and through the various lenses and prisms reduces the brilliancy of the image to such an extent that, even though it is finally magnified to above normal, the image is so thin at night that it cannot be seen. This forces the submarine to become vulnerable in making an attack at night, as it is necessary for the conning tower to be brought a sufficient distance above the surface of the water to permit the commanding officer to secure natural vision.

With the powerful searchlights and rapid-fire guns, the submarine would have little opportunity to approach a surface war vessel at night without great danger of being discovered and destroyed.

THE VOICE AND EAR OF THE SUBMARINE

A Fessenden oscillator, before being installed. The flange of the oscillator is riveted to the shell of the ship and its diaphragm is caused to vibrate by the sound waves, which pass through water more distinctly than they do through the air. To send out signals it is caused to vibrate mechanically by electrical apparatus.

Invisible Conning Tower.—For night observation it has been proposed to use transparent conning towers built of clear glass, in which the commander takes his station and just sticks his head above the crest of the waves in order to direct his vessel against the enemy. This has not as yet come into general use because of the difficulty of securing sufficiently clear glass in the desired form. Experiments have been made, however, which show that quite a large transparent conning tower cannot be seen on a submarine at rest even when within a couple of hundred yards; the application of these conning towers will greatly increase the submarine's efficiency for night work.

Submarine Sound Receivers.—All modern submarines are fitted with devices which enable the commanders of submarines to communicate with each other when running under water even when considerable distances apart. One of these outfits consists of a signal bell and a powerful receiver with which sounds may be transmitted and heard. Conversations may be carried on by the Morse and other codes for distances of ten or twelve miles.

TORPEDO TUBES ASSEMBLED READY FOR INSTALLATION IN A SUBMARINE BOAT

Left view, the breech end of the tube. Right view, the outboard doors, which must first be opened before the torpedo is expelled from the tube by compressed air. When the torpedo is expelled it starts a compressed-air engine supplied with air stored at high pressure within the torpedo, and will run several thousand yards under its own power.

A later device, called the Fessenden oscillator, will transmit or receive sounds a distance of twenty miles. The principle of its operation is that of setting up wave vibrations by very large transmitters; these vibrations are carried by the water and taken up by receivers on other submarines. It has been found that the human voice will set up vibrations in the Fessenden transmitter so clearly that wireless conversation may be carried on under water for several hundred yards. I discovered in my earlier experiments that when a submarine was lying submerged, with all machinery shut down, the noise of the machinery in an approaching ship could be detected quite a distance off without the use of any special kind of receivers. In this way the commander of a submarine can always note the approach of an enemy simply by shutting down his own machinery. The warning thus given him comes long before he could sight the enemy ship were he on the surface. After a little experience one can tell the type of ship approaching from the sound, as every type of ship has sounds peculiar to her class. The smash of paddle wheels, the deep, slow pound of the heavy merchant ships or battleships, the clack and the whir of the higher speed machinery on destroyers or torpedo boats, are all easily recognizable when one becomes familiar with them. At the present time all the larger submarines are fitted with wireless outfits on their decks which they may use when on the surface to communicate with other submarines or with their base.

Torpedo Tubes.—These are used to start the automobile torpedo on its course toward the enemy. In simple form they are tubes about eighteen inches in diameter and seventeen feet long, placed in line with the axis of the vessel. They are fitted with doors both internal and external to the submarine. The inboard door of the tube opens into the interior of the vessel and permits the loading of the torpedo. When the torpedo is to be discharged the inboard door is closed and securely fastened. The outer door is then opened, and through the operation of quick-opening valves compressed air is admitted back of the torpedo and the torpedo is driven out of the tube in the same manner that the bullet is driven out of an air rifle or the cork out of a pop-gun. Some of the larger modern submarines carry several torpedo tubes firing in line with the axis of the vessel both forward and aft. Some carry torpedo tubes on their decks which may be made to train to fire broadside on either side of the vessel.

A WHITEHEAD TORPEDO

Courtesy of the Scientific American

The forward end of the torpedo is the war head filled with guncotton or trinitrotoluol. A detonator is screwed into the end of the war head to set off the main charge on contact. An air flask forms the middle portion of the torpedo. Aft of this is the depth-control mechanism, in which a diaphragm controls the diving rudder by the pressure of the water against a spring set for the desired depth. A pendulum controls the levelling mechanism and a gyroscope its direction in the horizontal plane, tending to keep it on the course by its control of the vertical rudder.

REAR END OF THE WHITEHEAD TORPEDO

Courtesy of the Scientific American

Showing compressed air engine and twin propeller with their control gear.

Automobile Torpedoes.—These are the projectiles which are used to destroy the enemy's ship. They are called automobile torpedoes because they will, on being ejected from the torpedo tubes, continue running in the