THE STEAMBOAT 293 



different bells can be distinguished by the varying number 

 of strokes and the interval between. The receiving apparatus 

 consists of water tanks about two feet square fastened to 

 the ship below the water line. Each tank contains a micro- 

 phone which is connected by means of wires with a tele' 

 phone in the pilot house. By this means the pilot can hear 

 the strokes of the bell and can tell from their number and 

 intervals the exact locality. These signals operate up to 

 fifteen or twenty miles. They operate regardless of fog or 

 any other conditions of weather. They have already been 

 the means of saving many lives and thousands of dollars' 

 worth of property. 



Why a boat floats. When a boat is made of wood it is not 

 difficult to understand why it floats ; but when boats are 

 made of a heavy substance like steel, it is not so easy to 

 understand. Whether a body will float or sink depends on 

 its specific gravity, which means the weight of a body com- 

 pared with water. A body that weighs the same as an equal 

 volume of water is said to have a specific gravity of i . If it 

 weighs twice as much, it has a specific gravity of 2 ; if half 

 as much, a specific gravity of .5. 



A body that has a specific gravity of less than i floats; 

 one that has a specific gravity of more than i sinks, when 

 placed in water. Now when a piece of steel is so much 

 heavier than water, how is it that a boat made of steel 

 floats? The explanation is that a large amount of air is 

 inclosed within the steel sides of the boat, and the specific 

 gravity of the steel and air combined is less than i ; that is 

 to say, the whole volume is lighter than an equal volume of 

 water. 



LABORATORY EXERCISE 30 



Purpose. To learn why a boat floats. 



Apparatus. Spring balance, stone, overflow can, catch 

 bucket, wooden cylinder te fit overflow can, egg, salt, shot, 

 test tube. 



