74 



APPLIED SCIENCE 



reservoir to the point of discharge. The perpendicular height 

 is called the "head." Mechanics, engineers, etc., often 

 speak of a "head of water," meaning the pressure that 

 water exerts. "A head of 50 ft.," for instance, is the pres- 

 sure (due to its weight) of a column of water 50 ft. high. 



The pressure per square inch at any point in a body of water 

 equals the depth in feet below the surface, or the head times .434. 

 If P is pressure per square inch and H is head, 



Then 



P = H X .434 



and 



H = 



.434 



To find the head when pressure is given the rule is: Divide the 

 pressure by .434. 



74. Dams and Water Wheels. Look at a mill or factory 

 erected on the side of a stream. The water will usually be 



found confined by a wall of earth 

 or stone. The water runs from 

 the stream through an opening 

 called a canal and then to the 

 water wheel. The difference in 

 height between the canal and the 

 river represents the fall or pres- 

 sure of water which moves the 

 machinery in the mill. In case 

 of floods the water can run freely 

 over the dam, without affecting 

 the mill. 



Falling water is a source of energy that supplies power 

 to operate mills, factories, electric power plants, etc. Many 



FIG. 44. Overshot Water 

 Wheel. 



