MECHANICAL USES OF WATER AND AIR 137 



all objects which are immersed in water are buoyed up by a 

 force equal to the weight of the water they displace. 



In order to assist in explaining why objects are buoyed 

 up by the water, let us imagine a cube measuring 1 centi- 

 meter on each edge submerged 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 a 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 pres- 

 sure is 10 grams on the top and 

 11 grams on the bottom. (Why?) 

 These two do not balance each 

 other. The pressure upward is 

 greater by 1 gram than the pressure 

 downward. Plainly, such an object 

 as we have supposed, if its own 

 weight amounted to nothing, would 

 be pushed up to the surface. If it 

 weighed more than a gram its 



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 



weight would overcome the upward figure . Note that the pressure 



push and it would descend to the 



bottom. 



against the bottom is greater by 

 1 gram than that upon the top 



154. Floating objects. If an object weighs less than the 

 water it displaces, it will float or, if submerged, will rise to 

 the surface. It will not rest with its top even with the sur- 

 face of the water, but it will project far enough above the 

 surface so that the submerged part will displace an amount 

 of water the weight of which is equal to its own weight 

 (fig. 71). For example, if the 1-centimeter cube weighed 

 0.5 gram, it would rise until only one half of it was under 

 water, and it would therefore displace 0.5 gram of water. 

 An iron pail will float on the water because, though it is 



