WATER PRESSURE 129 



In the case of the faucets at the various places in the house, 

 the pressure on each square inch of the interior of the pipes 

 or faucets depends upon the depth of the water in the pipes 

 and tank, and is exerted equally on every square inch of the 

 interior of the pipe and faucet at any particular level. The 

 violence with which the water will spurt from an open 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. 



145. Air pressure. The pressure of the air is due to its 

 weight and follows the same laws as those we have been 

 dealing with in the case of water, but we cannot measure 

 the depth of the air and calculate 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 sect. 5). 



It will be recalled that the mercurial barometer consists 

 essentially of a glass tube held in a vertical position, with the 

 lower end immersed in a dish of mercury, the upper end 

 closed, and the air wholly removed from the tube. The mer- 

 cury fills such a tube to a point about 30 inches above the 

 level of the mercury in the dish. The force which sustains 

 the column of mercury at this height is the pressure of the 

 air on the surface of the mercury in the dish. The downward 

 pressure of the air, due to the weight of the air, is transmitted 

 to the mercury in the tube and causes it to rise until the 

 height of the column is such that the downward pressure 

 which it exerts, due to its weight, is equal to the pressure 

 of the air. 



That the pressure of the air is responsible for the height 

 of the column of mercury in a barometer is shown in a gen- 

 eral way by the fact that the column rises and falls in corre- 

 spondence with changes in air pressure. It is shown still more 

 clearly when a barometer is placed in an air-tight chamber and 

 the air is exhausted by an air pump. As the removal of the 



