178 



HYDRODYNAMICS. 



MEASUREMENT OF PRESSURE AT DIFFERENT HEIGHTS. 



The amount of pressure which any given height of 

 water 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 of fourteen inches ;* one pound will fill it 



twenty-eight inches ; two pounds, fifty-six inches ; ten 



Fig. 149. pounds, twenty -three feet; twenty 



2ibs. 5Gin. pounds, forty-six feet, and so on. 



Now, as the side pressure is the same 



as the pressure downward for the 



., ^ .„. same head ofwater, the same column 



• !/2lbs.42in. . ' 



will, of course, exert an equal press- 

 ure on a square inch of the side of 

 the tube. Or, if the tube be bent, as 

 lib 20 ill. shown in the annexed figure {Fig". 

 149), the pressure upward on the end 

 of the tube, at a, will be the same for 

 the various heights. 

 V2 lb. Id: in. Now, as the pressure of a column 

 fifty feet high is about twenty-two 

 pounds on a square inch, the pressure 

 on the four sides is equal to eighty- 

 j ll eight pounds for one inch in length. 



Hence the reason that considerable 

 strength is required in tubes which have much head 

 of water, to prevent their being torn open by its force. 



* This is nearly correct, for a cubic foot (or 1728 cubic inches) of 

 water weighs 62 lbs. Consequently, one pound will be 27.9 cubic 

 inches, and will fill the tube nearly 28 inches high. 



