368 Introduction to the Study of Science 



161. The submarine. The submarine has been rapidly 

 developed within a period of five years. It is constructed so 

 that its total weight is just a little less than the weight of its 

 own volume of water; hence by admitting water to air-tight 

 compartments or tanks, it is made to sink to any depth de- 

 sired. When under way the boat may be made to dive, like 

 a whale or porpoise, by a horizontal rudder. Tilting the rudder 

 causes the boat to rise or descend. When the water is ex- 

 pelled from the tanks by compressed air, the boat is lifted to 

 the surface by the buoyant force of the water below it. 



The Diesel oil motor or engine is generally employed on 

 submarines. When the boat is lying idle, this operates dynamos 

 which charge storage or secondary cells. When the boat is 

 under water, the motive power is the electricity generated by 

 these cells. On the surface the Diesel engine furnishes the 

 power directly for the propellers. The storage batteries furnish 

 electricity for lighting. 



Submarines are made to resist the pressure at depths of 100 

 to 200 feet, in very rare cases to withstand the pressure at 

 1000 to 5000 feet. The air necessary for the men is supplied 

 by tanks of compressed air or oxygen, and the exhaled carbon 

 dioxid is absorbed by sodium hydroxid. 



When the submarine is below the surface of the water, objects 

 like ships may be seen by means of the periscope. This is a 

 tube twenty or more feet long, with an arrangement of lenses 

 and mirrors, that transmit light rays to the observer in the 

 conning tower of the boat. The submarine illustrates the 

 principle discovered by Archimedes as to water's buoyant force 

 and displacement. 



162. General importance of travel and transportation. 

 Travel and transportation have become indispensable in 

 modern life, and they have been made easy, speedy, safe, and 

 economical by man's conquest of natural obstacles, by his 

 discovery and control of motive forces other than those inherent 

 in men and animals, and by his construction of vehicles to move 



