124 ELEMENTS OF GENERAL SCIENCE 



the water to a certain depth and turn it so that it faces in 

 various directions, we find that so long as it remains at the 

 same depth, it registers the same pressure, no matter in which 

 direction it faces. We thus come to the conclusion that in a 

 liquid at rest, and at a given depth, the pressure is the same in 

 all directions. 



If we apply this to the case of determining the pressure on 

 a square inch of the side of the tank, we may use the same 

 method that we should use for a square inch in a horizontal 

 position, but with this difference : the top and the bottom of our 

 square inch on the side are not at the same distance from the 

 surface, and we must therefore take the average depth. The 

 pressure against 1 square inch of the wall of the tank, with 

 the upper edge of the square inch 10 inches below the surface 

 of the water, would be 1 X 10.5 x 0.0362 pound. Solve this 

 and explain just why each number is used. 



138. Upward pressure in a liquid. Our statement that pres- 

 sure is equal in all directions should indicate that there are 

 upward pressures in a liquid. This may be made plain if we 

 place a card across the end of a lamp chimney and force the 

 chimney downward into the water. The card is held by the 

 water against the end of the chimney with considerable force. 

 In fact, if we attempt to force it away by pouring in water as 

 weight, before the card will be forced away we shall have to 

 pour in water until it stands at the same level inside the 

 chimney as outside. Can you show that this corresponds with 

 the facts we have learned in preceding sections ? 



139. Relation to size of a tank. It is common for people to 

 suppose that the pressure in a system of water pipes will be 

 affected by the size of the tank that supplies water to the 

 pipes. Let us imagine that our aquarium tank has a hole in 

 the bottom, to which is attached a pipe having an opening 

 1 square inch in area (fig. 66). If that opening were closed by 

 a stopper and the water were 1 foot deep, the pressure upon 

 the stopper would be 0.434 pound. If the stopper were 



