[2] Allies: Governments united by a treaty or having common interests. England, France, Belgium, Russia, Italy, the United States, and Japan are called the Allies.

[3] U-Boat: So called because it is an undersea boat, or Unterseeboote, as the Germans call their submarines.

[4] Paleozoic.—The next to the lowest geological series of strata.

[5] The Pterodactyl was one of these and it was the largest living thing that ever flew. It is pronounced Ter-o-dak´-til.

[6] A knot is the speed of a boat when she is making 1 nautical mile in 1 hour. A nautical mile is 6,080 feet.

[7] The design, construction and operation of submarine torpedoes will be found in Chapter V.

[8] The storage battery was invented by Gaston PlantÉ in 1860. The electric motor was discovered in 1876, by whom nobody knows.

[9] These pipes can be bought cut to length and threaded to suit of The Chicago Model Works, 166 West Madison Street, Chicago, Ills., or of Luther H. Wightman, 132 Milk Street, Boston, Mass.

[10] The L. E. Knott Apparatus Company, of Boston, Mass., sell a standard motor, as they call it in their catalogue, for $3.75. It weighs 1½ pounds and takes up a space of about 3½ inches square. Powerful little motors can be bought at almost any electrical supply house, and you can use one of these by building up the base.

[11] Cells of this kind that measure 2 x 2½ x 6 inches on the sides can be bought of the Manhattan Electrical Supply Company, of 17 Park Place, New York City.

[12] A machinist’s saw for sawing metal.

[13] The pulley can be bought of dealers in model makers supplies. See footnote on page 32.

[14] You can certainly get it of the F. W. Devoe and C. T. Raynolds Company, 101 Fulton Street, New York City.

[15] The top of the conning tower is called the bridge.

[16] The hull is the body or shell of a boat or ship.

[17] For a description of the gyroscopic compass, see Inventing for Boys, by the present author and published by the Frederick A. Stokes Company, New York.

[18] Any figure formed by two straight lines which meet is an angle, also the space between them.

[19] For a simple theory of how the steam engine works, see Engine Building for Boys, by the present author and published by Small, Maynard & Co., Boston.

[20] Any kind of machine which develops its power at first hand is a prime mover, as, for instance, the water wheel, steam engine, and gas engine. An electric motor energized by a storage battery, or a compressed air motor are only subsidiary movers.

[21] A dynamo is an electric machine, which generates a direct current of electricity, like a battery.

[22] It can be bought of the E. I. Horsman Co., Toy Dealers, Union Square, New York.

[23] The guns and other munitions on a boat are called her armament.

[24] The millimeter is the 1-1000 part of a meter. The meter is the fundamental unit of length used in the metric system of measurement. It is 39.37 inches.

[25] Lenticular (pronounced len-tik´-u-lar) means having lenses.

[26] Telescopes and their construction are fully explained in The Magic of Science, by the present author, and published by Fleming H. Revell Co., New York.

[27] 360 degrees = a complete circle.

[28] For a complete description of how to make and use wireless apparatus, and how it works, see The Book of Wireless, by the present author, and published by D. Appleton & Co., New York.

[29] A floating dock is built with a water compartment on each side and a platform on the bottom, thus leaving a large open space in between to hold a submarine or other vessel. By partly filling the compartments with water the platform sinks into the water deep enough so that the vessel can pass over and on top of it; then by pumping the water out of the compartments the floating dock is raised and the platform with the vessel on it is lifted out of the water where she can be examined and repaired if necessary.

[30] Integral means a part of a whole thing which is needed to make it complete.

[31] A cipher code is one in which the message is telegraphed in the Morse alphabet of dots and dashes as usual, but the words which form the message are given other meanings which have been previously agreed upon, and this prevents the enemy from knowing what is sent.

[32] The word auxiliary in this sense means other smaller and less important craft which give aid to and supports the larger and more powerful ships of the fleet.

[33] A squadron is one of the divisions of a fleet.

[34] A country that neither helps nor hinders the countries that are at war is said to be neutral.

[35] July, 1917.

[36] A boat’s capacity for cargo.

[37] The caliber of a gun is the gage, or diameter of the bore of its barrel. Thus a gun of 3-inch caliber means that the bore is 3 inches in diameter; a gun that has a caliber of 45 centimeters means that its bore is 45 centimeters across. A centimeter is ¹/₁₀₀ of a meter and a meter is 39.37 inches.

[38] For a full description of the trajectory of bullets, see Shooting for Boys, by the present authors, and published by Moffat, Yard & Co., New York

[39] A full description of the Lewis machine gun for airplane work will be found in How to Fly by the present author and published by D. Appleton and Co., of New York.

[40] This means she sinks into the water 17 feet.

[41] These are real rubies, sapphires, and other gems made of chemicals in an oxyhydrogen furnace.