26 



FLUID PRESSURE 



level in both vessels. G-ravity makes the liquid flow from 

 a and causes liquid pressure which is great enough to 



force it upward into b. In 

 the same way these forces 

 are used to give cities a 

 supply of water. 



Pipes from a pond or 

 " reservoir, located on high 

 land, lead the water down 



into the city. Gravity of course causes it to flow down- 

 ward, giving it pressure enough to fill the pipes. In 

 these pipes the water may rise to the tops of buildings, 

 provided they are not as high as- the surface in the 

 reservoir (see Fig. 12). These pipes may be tapped at 

 any points by faucets, hydrants, or fountains, out of 

 which the water will run with some force. The force 

 with which the water runs is called its head; the head 

 of water at any point generally increases with the verti- 

 cal distance from the point to the surface in the reservoir, 



FIG. 12 



as If (Fig. 12). Some force is used up by the rubbing 

 of the water on the pipes as it flows, so that its head is 

 less as the distance away from the source increases. 



37. Buoyancy. We have doubtless noticed that 

 many bodies seem to weigh less when held in water. 



