Transportation Water and Air 275 



is often painted on the ship's hull. A second line above it often 

 shows how much more the vessel can be lowered in the water 

 with safety when loaded. Then the amount of cargo weight can 

 be determined. Our greatest modern liner, 950 feet long, can 

 carry 30 thousand tons of load and can cross the Atlantic in 

 about five days. 



Experiment to Show Why a Metal Ship Floats. Procure 

 a tin biscuit box. Weigh the box. Place it on the surface of 

 water and notice that it floats. Now hammer the sides of the 

 box out flat until it is merely a straight piece of tin. Weigh it 

 again. Does it weigh as much as it did when it was in the shape 

 of a box? Place it on the water. Does it float? Is the volume 

 of the tin as great as was the volume of the box? How does 

 this explain the floating of a metal ship? 



Popular Mechanics Magazine. 



A MODERN SUBMARINE 



The Submarine. Man has not been content simply to sail on 

 the surface of the water. Early in the history of ship building 

 he began to experiment with craft that would travel under water. 

 The first practical submarine, however, was not invented until 

 1775, when one was built by David Bushnell of Maine. The 

 modern submarines are developed from inventions of John Hol- 

 land, another American. His first submarine was 60 feet long. 

 Today, there are submarines of approximately 400 feet in length 

 that approach in size and power many a modern powerful surface 

 steamship. The submarine is built to weigh less than an equal 

 volume of water. Therefore, under normal conditions, the vessel 

 rises somewhat above the surface of the water. It is equipped 



