[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.