118 GENERAL SCIENCE 



*, 

 is forced out of the special compartments and air takes its 



place, making the boat lighter. 



Density of a Solid Heavier than Water. It is quite 

 easy to find the density of a regular-shaped solid by 

 measurement, the density of a body being its mass 

 divided by its volume. With irregular solids, however, 

 other means must be employed ; since, while it is always 

 possible to find the mass of a body by weighing, it is some- 

 times quite difficult to find the volume of it by measur- 

 ing. Archimedes' principle, however, furnishes an easy 

 and accurate method for determining the volume of any 

 solid, regular or irregular. Water has unit density, since 

 a cubic centimeter of water weighs a gram. Now since a 

 solid immersed in water is buoyed up with a force equal 

 to the weight of the water it displaces, the loss in weight 

 of the solid when weighed in water will just equal the 

 weight of an equal volume of water. 



The statement, the density = ma . ss , becomes numer- 



volume 



ically the same as density = mass . 



loss of weight in water. 



Density of Solids Lighter than Water. The density of 

 solids lighter than water may be found in a similar way by 

 using a heavy sinker to hold the lighter solid under water. 

 Arrange the apparatus as shown in Figure 106. Find the 

 weight with the body in the air and the sinker in water, 

 then weigh both under water. The difference in the two 

 weights will be the buoyant force on the body alone and 

 will be equal to the weight of the displaced water and 

 numerically equal to the volume of the body. The density 

 may then be obtained by the use of the same equation : 

 mass 



density 



volume or loss of weight in water. 



