132 



ELEMENTS OF GENERAL SCIENCE 



148. Buoyancy. We are all familiar with the fact that objects 

 appear to be lighter in the water than out of the water, and 

 that some things float on water. No one has failed to notice 

 this effect when bathing. The floating effect of water may be 

 determined by weighing objects while suspended in the air and 

 again while suspended in water. If we take a cubic centi- 

 meter of each of several different kinds of substances that are 

 heavy enough to sink in water, and weigh each in this way, 



we shall find that each appears to 

 lose 1 gram of its weight when put 

 into water. No matter what differ- 

 ences there may be in the sub- 

 stances, if the pieces are of the same 

 volume they will be buoyed up by 

 the same force. Also, the objects 

 are buoyed up by a force (1 gram) 

 which is precisely equal to the 

 weight of the volume of water they 

 displace when immersed (1 cubic 

 centimeter). It is found that all 

 objects which are immersed in water 

 are buoyed up by a force equal to 

 the weight of water they displace. 

 In order to assist in explaining 

 why objects are buoyed up by the 

 water, let us imagine a cube measur- 

 ing 1 centimeter on each edge sub- 

 merged beneath the water so that the top is 10 centimeters 

 from the surface (fig. 70). The pressure on each of the four 

 sides will be 10.5 grams, since the average depth of the side 

 is 10.5 centimeters. The pressures on the sides exactly 

 counterbalance each other, and the object is not moved 

 either to right or left The pressure on the top is 10 grams, 

 and upon the bottom it is 11 grams. (Why?) These two 

 do not balance each other. The pressure upward is greater 



FIG. 70. Buoyancy 



If a cube measuring 1 centimeter 

 on each edge is submerged with 

 the top 10 centimeters below the 

 surface of the water, the pres- 

 sures against the surfaces, in 

 grams, will be as shown in the 

 figure. Note that the pressure 

 against the bottom is greater by 

 1 gram than that upon the top 



