UPWARD PRESSURE OF LIQUIDS. 199 



box by the pressure outside. If a bottom plug be drawn, 

 the water will immediately spout up into the box, show- 

 ing the pressure upward against th.e bottom. Hgnce the 

 pressure in all directions^ upward, sideways, and down- 

 ward, is proved. 



The upward pressure of liquids may be shown by pour- 

 ing 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 the water up the other side until the two sides are 

 level. 



On this 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 unac- 

 quainted with the manufacture of strong cast-iron pipes, 

 conveyed water on lofty aqueducts of costly masonry, 

 built level across the valleys. Even at the present day, 

 it has been deemed safest to build level aqueducts for con- 

 veying great bodies of water, as in very large pipes the 

 })ressure 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. Reservoirs 

 made in the form of large tubs require the lower hoops to 

 be many times stronger or more numerous than the upper. 



MEASUBEMENT OF PRESSURE AT DIFFERENT HEIGHTS. 



The amount of pressure which any given height of wa- 

 ter exerts upon a surface below may be understood by 

 the following simple calculation : 



If there be a tube one inch square (with a closed end), 

 half a pound of water poured into it will fill it to a height 



