UPWARD PRESSURE. 177 



with plugs ; then press this hox, empty, bottom down- 

 ward, into water, allowing none to run in at the top. 

 Now draw one of the side plugs, and the water will be 

 immediately driven into the box by the pressure out- 

 side. If a bottom plug be drawn, the water will im- 

 mediately spout up into the box, showing the pressure 

 upward against the bottom. Hence the pressure in 

 all directions^ upward, sideways, and downward, is 

 proved. 



The upward pressure of liquids may be shown by 

 pouring into one end of a tube, bent in the shape of the 

 letter U, enough water to partly fill it ; the upward 

 pressure will drive it up the other side till the two 

 sides are level. 



On tliis principle depends the art of conveying water 

 in pipes under ground, across valleys. The water will 

 rise as high on the opposite side the valley as the spring 

 which supplies it. The ancient Romans, who were 

 unacquainted with the manufacture of strong cast-iron 

 pipes, conveyed water on lofty aqueducts of costly ma- 

 sonry, built level across the valleys. Even at the pres- 

 ent day, it has been deemed safest to build level aque- 

 ducts for conveying great bodies of water, as in very 

 large pipes the pressure would be enormous, and might 

 result in violent explosions. 



If the valleys are deep, the pipes must be correspond- 

 ingly strong, because, the higher the head of water, the 

 greater is the pressure. For the same reason, dams 

 and large cisterns should be strongest at bottom. Res- 

 ervoirs made in the form of large tubs require the 

 lower hoops to be many times stronger or more numer- 

 ous than the upper. 



H2 



