MECHANICAL USES OF WATEE AND AIE 135 



faucet depends on its distance below the level of the water 

 in the tank and not at all upon its position directly beneath 

 the tank or at one side. 



152. Dams. The building of dams in order to confine 

 water or raise the surface to a higher level is very common 

 (fig. 69). They need to be constructed with a great deal 

 of care in order to be able to withstand the weight of water 

 to which they are subjected. The pressure to which a dam 

 will be subjected depends only upon the depth of the water 

 and the area of the submerged part of the dam. A great 

 many lives and much property have been lost by reason of 

 the floods which have been released by breaking dams. One 

 of the greatest catastrophes of this kind was the breaking of 

 the dam near Johnstown, Pennsylvania, in 1889. About two 

 thousand lives were lost in the flood which resulted. 



The pressure of the air, like that of water, is due to its 

 weight, but we cannot measure the depth of the air and cal- 

 culate its pressure as we do with water. We therefore rely 

 upon the barometer, which is, in fact, a pressure gauge, to 

 give us our information about air pressures (see Chapter I). 



153. Buoyancy. We are all familiar with the fact that 

 objects appear to be lighter in the water than out of it 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 cen- 

 timeter 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 differences 

 there may be in the substances, if the pieces are of the same 

 volume they will be buoyed up by the same force. The 

 objects are also buoyed up by a force (1 gram) which is pre- 

 cisely equal to the weight of the volume of water they dis- 

 place when immersed (1 cubic centimeter). It is found that 



