122 ELEMENTS OF GENERAL SCIENCE 



water in the well. The atmospheric pressure will force the 

 water up the pipe and against the piston, opposed only by 

 the pressure due to the depth and weight of the water. If 

 the suction pipe is too long, the water may not reach the 

 cylinder. The water will stand at such a height that the down- 

 ward pressure in the pipe, due to the weight of the water, will 

 be equal to the atmospheric pressure outside. Since, as noted 

 above, the height to which the atmospheric pressure will raise 

 a column of water is about 33 feet, the suction pipe should 

 not exceed this length. 



135. The problems of water pressure. In the discussion of 

 the pump and the siphon, as in all discussions dealing with 

 water, it is necessary to refer to water pressures. For this 

 reason a somewhat more exact knowledge of pressure in liquids 

 is needed to understand such problems as those suggested. 



Let us set ourselves the problem of discovering the pressure 

 upon the bottom of a rectangular aquarium tank, the bottom 

 of which is 6 inches wide and 24 inches long, and which is 

 filled with water to a depth of 1 foot (fig. 65). The bottom 

 will be pressed upon by the weight of the water in the tank, 

 and the total pressure on the bottom will be equal to the 

 weight of the water. If the bottom is 6 by 24 inches, its area 

 is 144 square inches, or 1 square foot, and the total amount 

 of water in the tank is 1 cubic foot. The pressure on the bot- 

 tom is therefore the weight of 1 cubic foot of water, and this 

 is 62.5 pounds. The pressure on each one of the 144 square 



62 5 



inches of the bottom is equal to -r-rr> or 0.434, pound. We 



144 



may say that the bottom of the box is under a pressure of 

 0.434 pound per square inch. 



We may think of our cubic foot of water as being divided 

 into vertical columns 1 inch square, in which case we should 

 have 144 such columns, one above each square inch of the 

 bottom. Each square inch of the bottom may be thought of as 

 supporting the weight of one of these columns, each of which 



