26 Common Science 



The earth's gravity is pulling on the ship and on the 

 water. If the ship has displaced (pushed aside) its 

 own weight of water, gravity is pulling down on the 

 water as hard as it is on the ship ; so the ship cannot 

 push any more water aside, and if there is enough air 

 space in it, the ship floats. 



Perhaps the easiest way to say it is like this : Any- 

 thing that is lighter than the same volume of water will 

 float; since a cubic foot of wood weighs less than a 

 cubic foot of water, the wood will float; since a quart 

 of oil is lighter than a quart of water, the oil will float ; 

 since a pint of cream is lighter than a pint of milk, the 

 cream will rise. In the same way, anything that is 

 lighter than the same volume of air will be pushed up 

 by the air. When a balloon with its passengers weighs 

 less than the amount of air that it takes the place of 

 at any one time, it will go up. Since a quart of warm 

 air weighs less than a quart of cold air, the warm air 

 will rise. 



You can see how a heavy substance like water pushes 

 a lighter one, like oil, up out of its way, in the following 

 experiment : 



Experiment n. Fill one test tube to the brim with kero- 

 sene slightly colored with a little iodine. Fill another test 

 tube to the brim with water, colored with a little blueing. 

 Put a small square of cardboard over the test tube of water, 

 hold it in place, and turn the test tube upside down. You 

 can let go of the cardboard now, as the air pressure will 

 hold it up. Put the mouth of the test tube of water exactly 

 over the mouth of the test tube of kerosene. Pull the card- 

 board out from between the two tubes, or have some one 

 else do this while you hold the two tubes mouth to mouth. 



